PLANT HORMONES

PLANT HORMONES

10.    PLANT HORMONES

Growth and development in plants are influenced externally by environmental factors and internally by various signalling molecules that translate various inputs and cues and mediate cell to cell communication in order to generate the appropriate response. Plant hormones are chemical messengers that regulate co-ordination among many cells and generally induce complex growth and developmental processes. Alteration in the physiology of plants due to hormonal contribution enhances the rate of survival in a plant as they determine morphological and anatomical features from germination of the seed to plant senescence. It includes mobilization of food reserves from seed to plantlet, root and stems elongation, shedding of leaves, flower formation, fruit ripening, defending against pathogen and maintain homeostasis under unfavourable environment. 
Hormones are the signal that translates the internal development and external environment input into the appropriate response. Regulation of hormones has also occurred at the level of synthesis, transport, uptake and turnover of hormones.
They regulate by spatially and temporally. Now LET’S TALK About Plant hormones, their expression regulation and function.
Plant Hormones
Multicellular plants require communication between cells to coordinate their activities. This coordination is carried out by some messengers. These messengers were later found to be similar to animal hormones. In 1905, the British physician E.H. Starling introduced the term ‘Hormone’ to describe these chemical messengers. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit.
Plant development was regulated by only five types of hormones. The main phytohormones we’ll discuss are auxins, gibberellins, cytokinins, abscisic acid and ethylene. Besides them, we shall have look on brassinosteroids and jasmonic acid that also play important role in the life cycle of plants.  
They all have a wide range of morphological effect on plant development.
A variety of their signalling molecule they play role in resistance to the pathogen is Jasmonic acid, salicylic acid and polypeptide systemin.
10.1.     AUXIN
It is the first discovered plant hormone that generally elongates plant body. 
The tendency of curvature towards sunlight in Canary grass attracted Darwins to study plant movements. They observed the movement of coleoptile towards sunlight but when the coleoptiles were decapitated or covered by foil, it failed to show curvature. It was concluded that some signal is produced in the tip, travels towards the growth zone and causes the shaded side to grow faster than the illuminated side.


After that Boysen-Jenson observed that the growth stimulus was diffusible from apical parts via gelatin but not through water impermeable barriers like mica. Consequently, curvature was observed only when the tip of coleoptiles was separated by gelatin. 

Paal contributed by arranging the tip on either side of stump showing growth even in darkness.


Finally, it was F.W. Went (1926), who isolated auxin from apical tips of coleoptiles from Avena sativa. He decapitated the coleoptiles and the tips were placed on the agar plate, after that when auxin was diffused completely from tip to agar, they were divided into agar blocks. Went asymmetrically placed these blocks on stumps and observed differential growth in coleoptiles i.e. increase in auxin on one side stimulated cell elongation and absence of auxin on the other side decreases the growth rate. Moreover, the curvature proved to be proportional to the amount of active substance in the agar. Due to the utilization of coleoptiles from Avena seedlings, his experiment got famous as Avena Curvature Test.


Naturally occurring auxin is in the form of indole-3-acetic acid (IAA). The similarity in structures of tryptophan amino acid and IAA indicated that tryptophan is the probable precursor. Major sources of auxin are apical shoot, pollen, embryo and developing buds. Indole acetic acid is generated by tryptophan amino acid by several pathways :
Auxin was the First plant hormone to be discovered and it has a principal role in the most fundamental of plant responses—the enlargement of plant cells. The primary auxin in most plants is indole-3-acetic acid (IAA). Although other compounds with auxin activity, such as indole-3-acetic acid, phenylacetic acid, and 4-chloro-IAA, are also found in plants.
10.1.1.     Distribution of Auxin
Auxin is synthesized in meristematic regions and other actively growing organs such as coleoptiles apices, root tips,  germinating seeds, and the apical buds of growing stems. Young and rapidly growing leaves, developing inflorescences and embryos following pollination and fertilization are also significant sites of auxin synthesis. Auxin distributed throughout the entire plant.
2,4-D or 2,4,5-T are used as herbicides. Broadleaf Plant( dicot) are more susceptible than narrow-leaf plants( Monocot).
According to went the coleoptiles curved because the concentration of auxin is increased in one side than another side. This stimulated cell elongations where auxin concentration is high. This phenomenon is called differential growth.
IAA is synthesized in Meristem, young leaves and developing fruits and seed. IAA is associated with rapidly dividing and growing tissue, especially in the shoot. Plant tissue is capable of producing a low level of IAA. Went called the hormone auxin (auxin: to grow). The chemical auxin is indole-3-acetic acid. 
Auxin support cell elongation up to a particular concentration. Too much concentration can kill the plant. This launched a huge biochemistry search for other herbicidal auxins in the chemistry laboratories worldwide. Some of the resulting auxins are: 


10.1.2.    Bioassay on auxin
Went used Avena sativa (oat) coleoptiles in a technique called the Avena coleoptile curvature test. Youngest leaves are covered by a protective organ that is called coleoptiles. Coleoptiles are very sensitive to blue light. The apex of coleoptiles was removed and the apical pieces were placed on the agar blocks. Allowing a period of time for the substance to diffuse from the tissue into the agar block, he then placed each agar block asymmetrically on a freshly decapitated coleoptile. The substance then diffused from the block into the coleoptiles and preferentially stimulated the elongation of the cells on the side of the coleoptile below the agar block. The curvature of the coleoptile was due to differential cell elongation on the two sides. Moreover, the curvature proved to be proportional to the amount of active substance in the agar. Went’s work was particularly significant in two respects: first, he confirmed the existence of regulatory substances in the coleoptile apex, and second, he developed a means for isolation and quantitative analysis of the active substance.
Now LET’S TALK about how IAA is synthesized by plant meristem
10.2.     Two pathways for Biosynthesis of IAA
10.2.1.     Tryptophan dependent biosynthesis
IAA is structurally related to the amino acid tryptophan, and early studies on auxin biosynthesis suggest that tryptophan is the probable precursor. Tryptophan converts to IAA by several pathways:
10.2.1.1. The Indole 3 pyruvic acid (IPA) pathway
It involves deamination of tryptophan, followed by decarboxylation reaction to form indole-3-acetaldehyde. Indole- 3-acetaldehyde is then oxidized to IAA by IAA dehydrogenase.
10.2.1.2 The TAM pathway
The tryptamine (TAM) pathway is similar to the IPA pathway, except that the order of deamination and decarboxylation reaction is reversed, and different enzymes are involved. In Lycopersicon (Tomato) evidence of both IPA and TAM pathway have been found.
10.2.1.3.The Indole 3 acetonitrile (IAN) pathway
In the indole-3-acetonitrile (IAN) pathway tryptophan is first converted to indole-3-acetaldoxime and the to indole-3- acetonitrile. The enzyme that converts IAN to IAA is called nitrilase. This pathway is important in 3 families: Brassicaceae, Poaceae and Musaceae.


Another tryptophan-dependent biosynthetic pathway that uses indole-3-acetamide as an intermediate-is used by various pathogenic bacteria such as Pseudomonas savastanoi and Agrobacterium tumefaciens. This pathway involves the two enzymes tryptophan monooxygenase and indole-3-acetamide (IAM) hydrolase. The auxin produced by this pathway in the bacteria is responsible for the morphological changes in the plant host. IAA can be conjugated to other molecules and retrieve later.
10.2.2.     Tryptophan independent pathway
Evidence for the biosynthesis of IAA via a tryptophan-independent pathway has been obtained from mutants of both maize and Arabidopsis. Two e.g. of these mutants, trp2 and trp3 of Arabidopsis lack tryptophan synthase and are unable to convert indole-3-glycerol phosphate to tryptophan. Arabidopsis mutants accumulate indole-3-acetonitrile. Arabidopsis also contains the nitrilase enzymes necessary for converting indole-3-acetonitrile to IAA, thus implicating indole-3-acetonitrile as an intermediate. The source of indole-3-acetonitrile is not known, although its accumulation in tryptophan mutants suggests a tryptophan-independent pathway for the biosynthesis of indole-3-acetonitrile as well. It is known that indole-3-acetonitrile can be derived from glucobrassicin, the principal glucosinolate present in members of the family Cruciferae.
In certain bacteria like Pseudomonas savastoni and Agrobacterium tumefaciens, tryptophan is utilized for production of indole-3-acetic acid by using indole-3-acetamide as intermediate. It involves two enzymes i.e. tryptophan monooxygenase and indole-3-acetamide hydrolase. Auxin produced by this pathway in bacteria is responsible for morphological changes in the host plant. Among members of family Brassicaceae, the compound glucobrassicin act as the source for indole-3-acetonitrile, which is then converted into indole-3-acetic acid thus these plants do not require tryptophan for auxin synthesis. Mutants with the gene for tryptophan synthase can follow this mode of auxin production independent of tryptophan amino acid.
10.2.2.1.Storage of Auxin
Auxin is stored in the cell in the bound form as conjugates with sugars forming glycosyl esters. Glycosyl conjugates are inactive and release free in the cell by alkaline or enzymatic hydrolysis.
Availability of auxin in plants is restricted when synthesis occurs more than need. It is performed by conjugation with different molecules, else bioactive auxins can lead to harmful effects. Auxin is stored in cellular components with sugars forming glycosyl esters thus becomes inactive but can be retrieved by enzymatic hydrolysis. Effects of UV-light and ionizing radiation in presence of riboflavin degrade IAA. Inactivation of this hormone can be achieved by the action of peroxidase enzymes along with flavoproteins.


IAA is degraded by ultraviolet and ionizing radiation and visible light in the presence of riboflavin and also IAA oxidase and peroxidase along with flavoprotein inactivate IAA.

10.3.     Distribution of Auxin
IAA distribution is regulated by pH. About one-third of the IAA is located in the chloroplast.
10.3.1.     Polar transport of Auxin
IAA moves mainly from the apical to the basal end (basically). This type of unidirectional transport is termed polar transport. Auxin is the only plant growth hormone known to be transported polarly. Auxin leaves the cell through the plasma membrane, diffuses across the compound middle lamella and enters the cell below through its plasma membrane. This process is an active process and thus requires metabolic energy.
These experiments showed that auxin is transported preferentially in the basipetal direction. Acropetal transport is minimal. The rate of auxin movement is about 6.4-20 mm/hr, which is many times faster than the rate of diffusion. This is a clear indication that it involves the carrier protein for transportation. For auxin transport there are two driving forces, one is proton motive force and other is membrane potential.
10.3.2.     Auxin uptake
The undissociated form of IAA i.e. IAAH in which carboxylic group is protonated can easily diffuse across the plasma membrane. Normally the environment of the cell wall is maintained at pH 5 thus most of the IAA remains in the form of IAAH and diffuse passively across the plasma membrane. AUX 1 is the permease type carriers that carry IAAH and are distributed uniformly around the cells. 
10.3.3.     Auxin efflux
pH of the cytosol is about 7.2 thus IAAH dissociates into the anionic form (IAA-) & H+.  Movement of auxins towards outside the cell is influenced by negative membrane potential. Then IAA- is released out by PIN transporters. These PIN transporters are 10-12 transmembrane proteins which are localized at the basal ends of conducting cells.

1.     When Auxin is in the acidic cell wall its trends to gain a proton(H+)
2.     It is then transported across the cell membrane through a simple diffusion as now the auxin molecule is neutral. A  specific Auxin influx transporter known as Aux Transporter are present at the top of the cell. This transporter transport the remaining IAA- the form of Auxin
3.     Inside the cell the pH is high. Auxin trends to lose a proton and now auxin exist in IAA- forms
4.     The Anionic Auxin is transported across the cell membrane by a specific efflux carrier located at the base of the cell. 
These transporters are called as PIN transporter. Mutation in PIN protein results in an embryo with poorly formed meristems.
Steps 1-4 are repeated many times causing transport of auxin from Apex to top base.
A significant amount of auxin transport also occurs in the phloem. By phloem, Auxin has transported acropetally (towards the tip) in the root. Polar transport is also specific for both natural and synthetic auxin. Polar transport involves specific protein carriers on the plasma membrane that can recognize the hormone and its active analogues. The major site of basipetal polar auxin transport in stem and leaves in the vascular parenchyma tissue.
Acropetal polar transport in the root is specifically associated with the xylem parenchyma of the stele.
10.4.     Recycle of PIN Transporter 
PIN protein is not stable permanently on the plasma membrane. But are rapidly cycled to an endosomal compartment via endocytic vesicle and then recycled back to the plasma membrane. PIN protein is localized at the basal ends of root cortical parenchyma cells.
Auxin Transport Inhibitors
Certain compounds bock the polar transport of auxin hormone in plants, preventing auxin efflux and thus named as auxin transport inhibitors (ATIs). For example, 2,3,5-Tri iodo benzoic acid (TIBA) competes with auxins for the binding site on PIN transporters. 1-N-naphthylphthalamic acid (NPA) interfere with transport by binding to protein complex associated with efflux carrier. Auxin uptake is inhibited by 1-naphthoxyacetic acid (1-NOA) as it binds to AUX-1 influx carrier and reconfigures it.
10.5.     Physiological and developmental roles of Auxin.
10.5.1.     Cell elongation: Acid growth hypothesis
Auxin activates H+ ATPase on the plasma membrane.
pH of the cell wall thus reduces to 4.5
Lowered pH activates expansin proteins.
The expansion is responsible for the breakdown of hydrogen bonding of cell wall polysaccharides.

Acid growth refers to the ability of plant cells and plant cell wall to elongate or expand quickly at low (acidic) pH. This form of growth does not involve an increase in cell number.  
According to acid growth hypothesis, hydrogen ions act as the intermediate between auxin and cell wall loosening.
Auxin promotes the expression of H+ATPase. H+ATPase increase the rate of proton extrusion (wall acidification).
Expansion enzymes work in an acidic environment.
The plant hormone auxin triggers complex growth and developmental processes. Its underlying molecular mechanism of action facilitates rapid switching between transcriptional repression and gene activation through the auxin-dependent degradation of transcriptional repressors. On perception in the nucleus, auxin triggers broad and specific transcriptional responses. The core components of the auxin signalling machinery belong to three protein families: the F-box TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX PROTEIN (TIR1/AFB) auxin coreceptors, the Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) transcriptional repressors, and the AUXIN RESPONSE FACTOR (ARF) transcription factors. Auxin promotes interaction between TIR1/AFB and Aux/IAA proteins, resulting in degradation of the Aux/IAAs and the release of ARF repression.  Gene expression associated with ARF activation has been implicated in diverse processes in land plants, including tropic responses and the establishment of polarity, as well as embryogenesis and organogenesis in flowering plants, and both gametophyte and sporophyte development in nonflowering plants. At the cellular level, auxin affects all aspects of cellular growth, cell elongation, cell division and differentiation. 


The repression of auxin-induced genes
Auxin functions by triggering genome-wide transcriptional responses via its effects on ARF activity. At low auxin levels, Aux/IAA transcriptional repressors interact with ARFs and repress their activity. In the presence of auxin, the TIR1/AFB proteins bind to Aux/IAA transcriptional repressors and mediate their polyubiquitylation and subsequent proteasomal degradation. The Aux/IAA transcriptional repressors interact with proteins in the auxin signalling pathway through a number of protein domains. The Aux/IAAs consists of 3 functional domains: a leucine repeat EAR motif within domain I, an internal domain II that contains a GWPP-core degron motif, and a C-terminal region that forms a type I/II Phox and Bem1(PB1) domain. The PB1 domain facilitates interactions with ARF proteins as well as self-dimerization, but the degron motif is required for interaction with TIR1/AFB proteins and thus determines Aux/IAA stability. Domain I functions as a repression motif by recruiting transcriptional co-repressors. 
Auxin Perception
Polyubiquitylation of the Aux/IAA transcriptional repressors requires an E3 ubiquitin ligase SCFTIR/AFB complex. SCF complexes consist of an F-box protein that provides substrate recognition, an ARABIDOPSIS SKP1 HOMOLOG1 (ASK1) adaptor (SKP1 in animals and fungi), the scaffold protein CULLIN1 (CUL1), and RING-BOX PROTEIN1 (RBX1) that promotes a transfer of ubiquitin molecules to the substrate. The SCF-TIR/AFB complex binds the Aux/IAA substrate in an auxin-dependent manner through the TIR1 or AFB F-box protein. TIR1/AFB proteins are composed of an F-box motif and a leucine-rich repeat (LRR) domain. An auxin molecule is anchored to the bottom of a hydrophobic binding pocket in TIR1 formed by the LRR domain, providing a binding surface for an Aux/ IAA protein. In turn, the Aux/IAA protein binds to the upper part of the auxin-binding pocket through its degron motif. Thus, auxin functions as a molecular ‘glue’ that stabilizes the TIR1/AFB-Aux/ IAA interaction.

Auxin-dependent regulation of transcription
Auxin perception by the auxin receptor complex and the subsequent degradation of the Aux/IAAs allow ARF-mediated transcriptional responses. ARF transcription factors bind to the promoters of auxin-responsive genes through cis-regulatory auxin response elements (AuxREs). The TGTCTC sequence was first identified in the promoter of the soybean GH3 gene as a functional AuxRE and has been commonly used in auxin-responsive reporters. The core element TGTC appears to be required for the recruitment of ARFs. ARFs contain an N-terminal DNA-binding domain (DBD), a variable middle region (MR) and, similar to Aux/IAAs, a C-terminal type I/II PB1 dimerization domain. ARFs can dimerize through their PB1 domain, as well as through an N-terminal motif formed by the flanking regions of their designated DBDs. Different ARF monomers bind to similar elements, but that as homodimers, as represented by ARF1 and ARF5, their preferred spacing between AuxREs varies. This variability of ARF dimers forming on different promoters can contribute to complex transcriptional responses. The formation of favoured ARF-Aux/IAA dimers may destabilize ARF interactions with DNA and contribute to ARF repression in the absence of auxin. 
Auxin-regulated chromatin switches
The recruitment of Aux/IAAs to the promoters of auxin-responsive genes by activating ARFs results in gene repression. This repression mechanism involves chromatin modifications that result in decreased accessibility of target genes. Aux/IAA proteins can interact with the TOPLESS (TPL) and TPL-related (TPR) corepressor proteins through their EAR motif. In turn, TPL and TPRs interact with histone deacetylases that catalyze the removal of acetyl groups from histone proteins, leading to DNA condensation and transcriptional repression. In flower primordia, ARF5 interacts with BRAHMA (BRM) and SPLAYED (SYD), both of which are chromatin-remodelling ATPase subunits of the SWI/SNF complex. Through their interaction with ARF5, BRM and SYD are recruited to promoters of auxin-responsive genes involved in flower formation, resulting increment in the accessibility of DNA to additional transcription factors that induce corresponding target genes. Aux/ IAAs prevent the association of BRM and SYD with gene promoters. Thus, the switching between gene repression and gene activation is enabled by auxin and Aux/IAA degradation. 

10.5.2.     Effects on general metabolism:
Isolated tissues, hypocotyls and epicotyl segments, leaves, excised roots and even whole plants have been used to monitor various biochemical and physiological responses to hormone treatment. In response to  Auxin respiratory rate amino acid metabolism, nucleic acid synthesis, protein synthesis and photosynthetic activity are increased.
10.5.3.     Auxin-induced new root formation:
1.     Application of IAA to stem cutting ends induces new root formation and facilitates lateral root development.
2.     Auxins stimulate initiation of roots and formation of lateral roots. It induces the pericycle cells of roots to divide and thus arise the lateral roots.
Hormonal and genetic control of lateral root formation in Arabidopsis.
lateral root formation is a three-stage process consisting of 
10.5.4.     Auxin promotes fruit growth after fruit setting in various plants. eg. achenes of strawberry.
Apical dominance is regulated by levels of auxin. Apical bud inhibits the growth of lateral buds. This phenomenon is called apical dominance. Removal of apical bud induces the growth of lateral buds because of the basipetal transport (PIN) of auxin.
Phototropic movement is the growth curvature in stem apex in response to light.  When light rays fall on the stem from one side, it induces the movement of auxin from the illuminated region towards the darker region. This is because the light activates the phototropin protein. Phototropin block the PID protein which is crucial for the polarization of PIN transporter. Activated phototropin block the PID hence the PIN polarization is blocked. This leads to the asymmetrical distribution of Auxin. Using monochromatic light, it is proved that the most effective spectrum is 445 mm. 
10.6.     Gibberellins
Gibberellins (GAs) are a large family tetracyclic diterpenoid plant growth substances associated with various growth and development processes such as seed germination, stem and hypocotyls elongation, leaf expansion, floral initiation, floral organ development, and induction of some hydrolytic enzymes in aleurone layer of cereal grains. Gibberellins were first of all isolated from the fungus Gibberella fujikorai, that was found to be responsible for “foolish seedling” or “bakanae” disease of rice.
10.6.1.     Chemical nature of Gibberellins
Gibberellins are tetracyclic diterpene carboxylic acid.
All gibberellins are derived from the ent- kaurene ring structure and characterized according to the order in which they were discovered (GAx) where x is the number, in order of their discovery.


10.6.2.    Biosynthesis of Gibberellins
Gibberellins are synthesized by the terpenoid pathway. Terpenoids are the compounds made up of isoprene units i.e. a five-carbon compound.

The GAs are diterpenoids. Terpenoids are compounds made up of five-carbon isoprenoid building blocks. Structurally GAs are formed from four isoprenoid units each consisting of five carbons. They possess a tetracyclic ent-gibberellane skeleton that contains 20 carbon atoms. The GA biosynthetic pathway can be divided into three stages, each residing in a different cellular compartment: plastid, ER, or cytosol. IPP (Isopentyl pyrophosphate) is the basic unit used by the cells to synthesize Gibberellic acids.  It is generally synthesized in green plants from glyceraldehyde-3-phosphate and pyruvate in the plastids. Although it can be synthesized in different organelles in different plants. Isoprene units through different intermediates are converted to geranylgeranyl diphosphate (GGPP) i.e. a 20-C compound. GGPP then cyclize to form ent-kaurene. Compounds such as AMO-1618, Cycocel, and Phosphon D  inhibits cyclization of GGPP to ent- kaurene and thus impedes gibberellins biosynthesis, therefore, they are used as growth height reducers. 
Oxidation of methyl group on ent- kaurene leads to the formation of GA12  i.e. the first Gibberellins synthesized. The process occurs in endoplasmic reticulum where kaurene is transported and oxidized by monooxygenases that use cytochrome P450 during reactions. This step is affected by Paclobutrazol/ Triazole fungicide and other inhibitors of P450 mono-oxygenases by stopping the synthesis of GA12.

The third step involves a group of soluble dioxygenases in the cytosol. Hydroxylation of C-13 or C-3 and successive oxidation at C-20 and finally loss of C-20 as CO2. The reaction is catalyzed by GA20 oxidase and leads to the formation of GA20.  GA20 is ultimately converted to GA1 by hydroxylation of C-3. GA1 is the biologically active form of gibberellins.
The reaction is catalyzed by the enzyme GA3-oxidase i.e. encoded by the Le gene. An enzyme GA2 oxidase i.e encoded by sln gene inactivates GA1 by converting it to GA8. Inhibitors of the third step interfere enzymes that utilize 2-oxoglutarate as co-substrates. The compound prohexadione (BX-112) inhibits GA 3-oxidase, the enzyme that converts inactive GA20 to growth-active GA1.


10.6.3.    Signal transduction of gibberellins
Key components include the GA receptor GIBBERELLIN INSENSITIVE DWARF1 (GID1), the DELLA growth inhibitors (DELLAs) and the F-box proteins like SLEEPY1 (SLY1) and SNEEZY (SNZ) for  Arabidopsis and GIBBERELLIN INSENSITIVE DWARF2 (GID2) for rice. The current model of GA action proposes that DELLA proteins restrain plant growth whereas the GA signal promotes growth by overcoming DELLA-mediated growth restraint.
10.6.4.    DELLA proteins: central repressors of GA-dependent processes
DELLAs are intracellular repressors of GA responses as they inhibit seed germination, growth and flowering, whereas  GA relieves their repressive activity. DELLAs contains conserved C-terminal GRAS domain that is involved in transcriptional regulation and characterized by two leucine heptad repeats (LHRI and LHRII) along with three conserved motifs, VHIID, PFYRE and SAW. DELLAs are distinguished from the rest of the GRAS family by a specific N-terminal sequence containing two conserved domains: the DELLA domain and the TVHYNP domain. The Arabidopsis genome encodes five DELLAs (GA-INSENSITIVE, GAI; REPRESSOR OF GA1-3, RGA; RGA-LIKE1, RGL1; RGL2 and RGL3). RGA and GAI repress vegetative growth and floral induction, RGL2 inhibits seed germination. 
RGA, RGL1 and RGL2 together modulate floral development and RGL3 contribute to plant fitness during environmental stress.

Perception of the GA signal: formation of the GA-GID1-DELLA complex
Recently, the characterization of the GA-insensitive dwarfism gid1-1 mutant allele in rice led to the discovery of the  GA receptor, GID1. Unexpectedly, GID1 encodes a soluble nuclear GA receptor.  GID1 contains a GA-binding pocket and a flexible N-terminal extension. Upon the binding of bioactive GA, the C3-hydroxyl group of the GA molecule becomes hydrogen-bound to the Tyr31 residue of GID1, which induces a conformational change in the N-terminal extension to cover the GA pocket. Once the pocket is closed, the upper surface of the lid binds with the DELLA and TVHYNP regions of DELLAs to form the GA-GID1- DELLA complex. It is noteworthy that DELLA and TVHYNP regions are essential for the interaction because their deletion results in an inability of DELLAs to interact with GID1, despite the presence of GA.


10.6.5.    GA promotes proteasome-dependent degradation of DELLAs
GA binding to GID1 stimulates the formation of the GA-GID1-DELLA complex. The formation of the GA-GID1-DELLA  complex stimulates the degradation of the DELLAs. F-box proteins are components of the SCF (SKP1, CULLIN, F-BOX) E3 ubiquitin-ligase complexes that catalyze the attachment of polyubiquitin chains to target proteins for their subsequent degradation by the 26S proteasome. GA-GID1-DELLA complex induces conformational changes in the GRAS domain of DELLA that enhance recognition between the VHIID and LHRII motifs of DELLA and the F-box protein SLY1/GID2. In turn, the SCF-SLY1/GID2 complex promotes the ubiquitylation and subsequent destruction of DELLAs by the 26S proteasome, thereby relieving their growth-restraining effects. Thus, GA promotes growth by mediating the proteasome-dependent destabilization of DELLA proteins. 
10.6.6.    DELLAs interact with key regulatory proteins to modulate plant development
An important function of DELLAs relies on their ability to interact with diverse classes of regulatory proteins. For example, DELLAs regulates hypocotyl elongation by interacting with PHYTOCHROME INTERACTING FACTORS (PIFs) and BRASSINAZOLE RESISTANT1 (BZR1). They control floral transition and fruit patterning by interacting with SQUAMOSA PROMOTER BINDING-LIKE (SPL) and ALCATRAZ (ALC) factors respectively). They contribute to plant defence by interacting with JASMONATE ZIM-DOMAIN (JAZ) proteins. Through these interactions, DELLAs block the DNA binding capacity of transcription factors (such as with PIFs) or inhibit the activity of transcriptional regulators (such as with JAZs). Meanwhile, GA relieves the repression of the DELLAs by promoting their degradation via the 26S proteasome pathway.

10.7.     Physiological and developmental effects of Gibberellic acids
Gibberellins promote seed germination via nutrient mobilization and breaking dormancy Many seeds, particularly those of wild plant species, do not germinate immediately after dispersal from the mother plant, and may experience a period of dormancy. Dormant seeds will not germinate even if provided with water. Abscisic acid (ABA) and bioactive GA act in an antagonistic manner, and the relative amounts of the two hormones within the seed can, in many species, determine the degree of dormancy. Light or cold treatments of dormant seeds have been shown to lower the amount of ABA and increase the concentration of bioactive GA, ending dormancy and promoting germination. Treatment of seeds with bioactive GA can often substitute for the light or cold treatment needed to break dormancy. 
During germination, GAs induce the synthesis of hydrolytic enzymes, such as amylases and proteases in cereal grains. These enzymes degrade the stored food reserves accumulated in the endosperm or embryo as the seed matured. This degradation of carbohydrates and storage proteins provides nourishment and energy to support seedling growth.
10.7.1.    GAs can stimulate stem and root growth
GA induces internode elongation, leaf expansion and used in sugarcane cultivation. Gibberellins induce stem elongation in ‘rosette’ plants like cabbage i.e. exhibits bolting effect. Gibberellins are also important for root growth. Extreme dwarf mutants of pea and Arabidopsis, in which GA biosynthesis is blocked, have shorter roots than wild-type plants, and GA application to the shoot enhances both shoot and root elongation.
10.7.2.    They have an influence on floral initiation and sex determination
GAs can substitute for the long-day requirement for flowering in LDPs. Sex determination is genetically regulated plants with unisexual flowers rather than hermaphroditic flowers. However, it is also influenced by environmental factors such as photoperiod and nutritional levels. In dicots such as cucumber (Cucumis sativus), hemp (Cannabis sativa), and spinach, GAs promote the formation of staminate (male) flowers, and inhibitors of GA biosynthesis promote the formation of pistillate (female) flowers.
10.7.3.    GAs promote pollen development and tube growth
Gibberellin-deficient dwarf mutants (e.g., in Arabidopsis and rice) have impaired anther development and pollen formation, and both these defects, which lead to male sterility, can be reversed by treatment with bioactive GA. In other mutants in which GA response (rather than GA biosynthesis) is blocked, the defects in anther and pollen development cannot be reversed by GA treatment, so these mutants are male-sterile. In addition, reducing the level of bioactive GA in Arabidopsis by overexpressing a GA deactivating enzyme severely inhibits pollen tube growth. Thus GAs seem to be required for both the development of the pollen grain and the formation of the pollen tube.
10.7.4.    Gibberellins promote fruit set and parthenocarpy
Gibberellin application initiates fruit growth following pollination and growth of some fruits. For example,  stimulation of fruit set by GA has been observed in pear (Pyrus communis). GA-induced fruit set in the absence of pollination results in parthenocarpic fruit. For example, the “Thompson Seedless” variety of grapes (Vitis vinifera),  normally produces small fruits because of early seed abortion. Fruits can be stimulated to enlarge by treatment with GA3. Both these effects of GAs on grapes are exploited commercially to produce large, seedless fruits. 
10.7.5.    Substitution of cold treatment or vernalization
Many plants which require cold treatment also require proper photoperiodic treatment for the induction of flowers,  without which vernalization does not have any effect.  If such plants are treated with gibberellins (GA), they produce flowers without subjecting the plants to cold and photoperiodic treatments.
The ‘green revolution’ dwarfing genes
The introduction of dwarfing genes into cereal crops was a major factor in breeding higher-yielding varieties during the ‘green revolution’, as they allowed more nitrogen fertilizer to be applied without leading to excessive stem elongation and subsequent lodging. For example, the introduction of wheat mutant dwarfing alleles at Reduced height-1 (Rht-B1 and Rht-D1) loci led to large increases in worldwide grain yields and improvements in both harvest index and lodging resistance was observed. 
10.7.6.     Cell Elongation
•     GA activates an enzyme XET (Xyloglucan endotransglycosylase)
•     XET is responsible for penetration of expansion and extension proteins in the cell wall.
•     It is active at low pH
•     Extension and expansion proteins lead to the loosening of the cell wall and thus cell elongation
10.7.7.     Cell Division
Molecular mechanism for cell cycle regulation. Four phases of cell cycle (G1, S, G2, and M) are operated by successive activation and deactivation of cyclin-dependent kinases (CDKs). During the cell cycle, these kinases bind with cyclins and get activated through phosphorylation by CDK activating kinases (CDKD and CDKF) whereas KRPs inhibit the complexes. G1 to S transition is controlled by CDKA–CYCD which phosphorylates the RBR proteins and releases the E2F transcription factor, which activates S phase related genes. The G2–M transition is dependent on CDKA/B and CYCA/B/D. The CDK complex is inactivated by phosphorylation through WEE1.
The exit from mitosis requires proteolytic degradation of CYCs which as mediated by the Anaphase-Promoting Complex/Cyclosome (APC/C) bind with CCS52 and CDC20. Phytohormones like auxin, cytokinin, gibberellins (GA), brassinosteroids, abscisic acid (ABA) and methyl jasmonate (MeJA) impact cell cycle regulation at different points (pointed and T shaped arrows indicate positive and negative regulation and the question mark indicates unknown regulation, respectively).
•     GA activates CDKs (Cyclin-dependent kinases)
•     This leads to G1 to S transition and also from G2 to M transition
10.7.8.     Mobilization of nutrients
•     GA induces mobilization of nutrients stored in the cereal endosperm of cereal grains.
•     GA is transported from the embryo to the aleurone layer.
•     Protease activates ß-amylase to a- and ß-amylase which is then mobilized for metabolism.
10.8.     Cytokinins
Cytokinin is a class of phytohormones with the structure resembling adenine which promotes cell division. They are primarily involved in cell growth and differentiation but also have an impact on axillary bud growth apical dominance and greening of leaves. 
The cytokinins were discovered as a consequence of the effort to reveal the factors that would stimulate plant cell to divide. F. Skoog and coworkers investigated the nutritional requirements for growth in tissue culture of plants and they reported the activity of specific cell division factors in vascular tissue. After some time, Miller isolated kinetin from autoclaving Herring sperms DNA. Letham in 1963 isolated and crystallized cytokinin from corn kernels and termed zeatin.
Cytokinin is the plant hormones that are derivatives of adenine and are found to be responsible for cell division, shoot and root differentiation, leaf expansion etc. Kinetin was the first synthetic analogue of cytokinin discovered from the endosperm of coconut (coconut milk) that was used in a culture medium. Later a natural cytokinin i.e. zeatin was discovered from the immature endosperm of corn. Zeatin is the most abundant natural cytokinin found in the plants.


Cytokinin can occur as free as well as in bound form. The bound form that is common in bacteria and plants are dihydrozeatin (DZ) and isopentenyl adenine(iP).
10.8.1.     Biosynthesis of cytokinins
The enzyme involved in the biosynthesis of cytokinin is IPT (Isopentenyl transferase ) and was first isolated from a  slime mould i.e. Dictyostelium discoideum and the gene encoding the enzyme was isolated from Agrobacterium. 
It involves the transfer of dimethylallyl diphosphate (DMAPP) to the 6th position of adenosine -5’-monophosphate thus forming the basic unit of cytokinin.

10.8.2.     Transport of cytokinin
Studies from the xylem exudates revealed that major sites for cytokinin synthesis are root apical meristem. This cytokinin moves through xylem into the shoot, with the minerals and water that are taken up by the roots from the soil.
10.8.3.     Signal transduction of cytokinin
Cytokinin signalling is mediated by a two-component signalling pathway similar to the two-component signalling systems (TCSs) found in bacteria. Cytokinin induces autophosphorylation of a histidine kinase (HK) protein, which results in the transfer of a phosphoryl group from a phosphoaccepting histidine residue in the kinase domain to an aspartate residue. The phosphoryl is then transferred to a conserved histidine on a histidine phosphotransferase (HP) protein. From there, it is finally transferred to an aspartate in the receiver domain of a  response regulator (RR). 
Histidine kinases: the cytokinin receptors 
The first indication of a link between cytokinin and the bacterial  TCS was the discovery that overexpression of CYTOKININ INDEPENDENT (CKI), induced a cytokinin response in plants. CYTOKININ RESPONSE 1 (CRE1) was discovered to be a cytokinin receptor, as loss-of-function mutants showed a reduced cytokinin response. Simultaneously, researchers working on WOL/CRE1 under the name ARABIDOPSIS HISTIDINE KINASE 4 (AHK4) act as a cytokinin sensor in bacteria and its histidine kinase activity was cytokinin-dependent. AHK2 and AHK3 were identified as cytokinin receptors. Arabidopsis mutants lacking all three receptors (AHK2, AHK3 and AHK4) show no cytokinin response in assays and produced small infertile plants.
Histidine phosphotransferase proteins
The Arabidopsis genome encodes five HP proteins (AHP1-AHP5) that exhibits phosphorelay activity. A sixth HP  protein, AHP6, lacks the conserved histidine residue and so acts as a pseudo-phosphotransferase and competes with the true HPs. AHP6 acts as an inhibitor of cytokinin signalling. The AHP proteins are known to shuffle between the cytosol and the nucleus. The quintuple AHP mutant has a reduced cytokinin phenotype, though not as severe as the triple receptor mutant. ahp2 allele in this mutant was not a null allele; replacing it with a null allele produced a quintuple AHP mutant that was seedling lethal.
Response regulators
The 23 functional response regulators (RRs) in Arabidopsis are divided into three groups, two of which (type A and type B) are involved in cytokinin signalling. Phosphorylation of the type A ARRs acts to stabilize, whereas phosphorylation of the type B ARRs enables them to bind to DNA and initiate transcription of downstream targets, including the type A ARRs. The type A ARRs are transcriptionally up-regulated by cytokinin. Type A ARRs only have a receiver domain and are generally thought to act as inhibitors of cytokinin signalling. Although genetic analysis has confirmed the negative activity of several of the type A ARRs, ARR4 has interacted positively with phytochrome B.


Fig. : The cytokinin receptors Arabidopsis histidine kinases (AHKs) are primarily localized on the endoplasmic reticulum, as well as on the plasma membrane. Cytokinin binds to AHK proteins, inducing conformational changes that trigger a phosphorelay. A phosphoryl group (P) is first transferred from a conserved His (H) to an Asp (D) residue within the receptor and is then relayed to five Arabidopsis histidine phosphotransferase proteins (AHP1-AHP5). The pseudo-HP AHP6 inhibits cytokinin signalling by competing with AHP1-5 for phosphotransfer. The AHPs continuously translocate between the cytosol and the nucleus, where the Arabidopsis response regulators (ARRs) are in turn phosphorylated. Phosphorylation of the type A ARRs stabilizes them. The phosphorylated type B ARRs can bind DNA and initiate transcription of cytokinin-responsive genes, including the type A ARRs, which act as inhibitors of cytokinin signalling. Although type A ARRs are generally considered negative regulators, ARR4 has been shown to upregulate phytochrome B. }
10.8.4.     Physiological and developmental effects of cytokinins
1.     Cytokinins Regulate Cell Division in Shoots and Roots
2.     Cytokinin is the determining factor regulating the cell division in roots and shoots. Development of ectopic meristem in tobacco leaves by the localized expression of the ipt gene gives evidence of the role of cytokinin to initiate the cell division. Both auxin and cytokinin regulate the cell cycle in plants.
Auxin binds to Auxin response element in the nucleus and activates Cdc 25- like phosphatase.
Cytokinin through signal transduction phosphorylates Cdc 2.
Cdc 25-like phosphatase dephosphorylates the Cdc-2 that enters the nucleus.
This enhances the transcription of cyclins and CDKs responsible for cell division.
3.     Auxin and cytokinin in the appropriate ratio lead to morphogenesis in the callus tissue. High auxin; cytokinin leads to root formation while low auxin; cytokinin form shoot. Any mutation causing a decrease in auxin; cytokinin ratio leads to formation tumours known as teratomas.
4.     Cytokinins are found to be responsible for lateral bud growth. Thus overproduction of cytokinin in the plants gives them bushy appearance.
5.     Leaf senescence is delayed by cytokinins mainly by zeatin riboside and dihydrozeatin riboside.
6.     Cytokinin also influences the mobilization of nutrients in the phloem from leaves to all the sites of utilization.
7.     Etiolated seedlings (Dark-grown seedlings) develop etioplasts which are unable to synthesize chlorophyll and enzymes. Cytokinins when applied to etioplasts, they develop into normal chloroplasts with proper grana, synthesize chlorophyll and photosynthetic enzymes.
10.9.     Ethylene
It is volatile phytohormone that contributes to the development of plants including fruit ripening, the formation of aerenchyma during the flood, adaptive responses to stress. Chemically, it is a simple hydrocarbon. Its response to plant is either desirable or can be harmful depending on developmental stages and concentration of ethylene.
D. Neljubow, while working in his lab observed peculiar growth in pea seedling due to the leaking of illuminating gas. At that time coal gas (ethylene) was used for lightening, its leakage from gas lines was known to cause defoliation of trees. This gaseous pollutant was reported as a fruiting hormone by Burg and Praat Goescl recognized ethylene to be a natural plant growth regulator.
Ethylene is a gaseous plant hormone. It was first identified as a natural product of plants by H.H. Cousins. They found that emanations of oranges stored in a chamber were responsible for premature ripening of bananas. Ethylene is responsible for a few symptoms in dark-grown seedlings of dicots like reduced stem elongation, increased lateral growth and abnormal, horizontal growth. These symptoms are termed as the triple response.
10.9.1.     Biosynthesis of ethylene and Yang Cycle
Methionine is the precursor of ethylene biosynthesis.
1.     The first step of biosynthesis involves the transfer of the adenosine group from ATP to methionine thus forming SAM (S- adenosylmethionine). This reaction is catalysed by SAM synthetase or methionine adenosyltransferase.
2.     SAM is cleaved by an enzyme ACC synthase to yield 5’- methyl thioadenosine (MTA) and ACC (1-aminocyclopropane1-carboxylic
3.     ACC is oxidized to ethylene by ACC oxidase. The sulfur i.e. released during ethylene formation is recycled back to methionine. This is known as Yang Cycle.

10.9.2.     Signal Transduction of Ethylene
The ethylene-signalling cascade starts with ethylene binding to its receptors. These receptors work as negative regulators of the pathway, actively repressing the ethylene response in the absence of the hormone. The receptors predominantly reside in the ER membrane. the ethylene gas can diffuse freely both in aqueous and lipid environments of the cell, this localization of the receptors might facilitate interactions with other cellular components. Ethylene is more soluble in hydrophobic environments, consistent with the location of the ethylene-binding pocket within the membrane. The receptors have been divided into two subfamilies but they share a modular structure composed of an N-terminal transmembrane domain responsible for ethylene binding, a GAF domain involved in protein-protein interactions between different receptor types, and a C-terminal domain required for the interaction with the downstream components of the pathway.  The basic functional unit of the ethylene receptor is a homodimer capable of binding ethylene. Copper is supplied by the intracellular copper transporter RAN1, is required for both ethylene binding and the receptor functionality. Plants carrying loss-of-function (LOF) mutations in ran1 lack ethylene-binding activity and display phenotypes similar to that of the LOF receptor.
RTE1 is a negative regulator of ethylene responses,  that co-localizes with the receptors at the ER and is also detected in the membrane of the Golgi apparatus. RTE1 functions by specifically activating ETR1 by promoting its transition from the inactive (in the presence of ethylene) to the active (in the absence of ethylene) signalling state.
The kinase activity of CTR1 is absolutely necessary for the downstream signalling to occur. The activated CTR1 kinase dimers engage in interactions that might enable affinity between ethylene receptor clusters. Downstream of CTR1 is EIN2 protein that consists of an N-terminal hydrophobic region made of 12 predicted transmembrane domains and a hydrophilic C-terminus to contain a conserved nuclear localization sequence.


The current model of the ethylene signalling pathway in Arabidopsis.
Ethylene is perceived by the receptor proteins ETR1, ERS1, ETR2, ERS2 and EIN4 binds to ethylene with high affinity.  These receptors are grouped into two classes based on the presence or absence of the receiver domain. The receptors work as homodimers but form higher order complexes in the ER membrane by interacting with other receptors through their GAF domains. Copper serves as a cofactor for ethylene binding and is delivered to the receptors by the copper transporter RAN1. RTE1 is associated with ETR1 and mediates the receptor signal output. The receptors are negative regulators of ethylene signalling.
In the absence of the hormone, the receptors activate CTR1, a Ser/Thr kinase that dimerizes when active and suppresses the ethylene response. CTR1 inactivates EIN2 by directly phosphorylating its C-terminal end. EIN2 can directly interact with the kinase domain of the receptors. The levels of EIN2 are negatively regulated by the F-box proteins ETP1 and ETP2  via the 26S proteasome. In the nucleus, the transcription factors EIN3/EIL1  are being degraded by two other F-box proteins, EBF1/2. In the absence of EIN3/EIL1, transcription of the ethylene response genes is shut off. 
In the presence of ethylene, the receptors bind the hormone and become inactivated, which in turn, switches off CTR1. This inactivation prevents the phosphorylation of the positive regulator EIN2. The C-terminal end of EIN2 is cleaved off by an unknown mechanism and moves to the nucleus where it stabilizes EIN3/EIL1 and induces degradation of EBF1/2. The transcription factors EIN3/EIL1 dimerize and activate the expression of ethylene target genes, including the F-box gene EBF2 or the transcription factor gene. Among the ethylene-responsive genes is the receptor gene ETR2, whose mRNA is upregulated by ethylene and is translated into the new batch of ethylene-free receptor molecules which then activate the negative regulator CTR1, thus providing the means of tuning down ethylene signalling in the absence of additional ethylene. Other regulatory nodes in the pathway are the exoribonuclease EIN5, which controls the levels of EBF2 mRNA, and the F-box proteins ETP1 and ETP2 that are degraded in the presence of ethylene leading to the stabilization of EIN2.
Ethylene receptors are two- component Histidine kinase receptors.
Ethylene binds to ETR1 (Ethylene Resistance 1) histidine kinase receptors
Copper co-factor is assembled into the ethylene receptors by RAN1 protein.
The ETR1 dimer is inactivated resulting in the inactivation of CTR1 ( Constitutive Triple Response 1) protein.
[In the absence of ethylene, ETR1 activates CTR1 protein that represses the ethylene triple response pathway through  MAP Kinase cascade]
Inactivation of CTR1 activates EIN2 (Ethylene insensitive 2) protein i.e. a transmembrane protein.
This leads to activation of ERF1 expression that further activates a cascade of transcriptional factors leading to the transcription of ethylene response genes.
10.9.3.     Physiological and Developmental effects of Ethylene
1.     Ethylene is responsible for ripening of fruits by the breakdown of cell walls and leads to a respiratory rise before the ripening phase in some fruits called climacteric fruits. Although some fruits do not possess respiration before ripening and are called non-climacteric fruits.
2.     In some dicots like tomato, anaerobic conditions around roots produce ethylene in shoots. It leads to epinastic response (downward curvature of leaves). In anaerobic conditions, ACC is accumulated in roots which are transported to shoots by transpiration and is converted to ethylene thus leading to an epinastic response of leaves.
3.     Ethylene can break seed and bud dormancy in several species and initiate their germination.
4.     Ethylene stimulates flowering in pineapple and mango. It may change the sex of developing flowers in many monoecious species.
5.     Ethylene is responsible for various responses like accelerating leaf senescence, chlorophyll loss and color fading of flowers.
6.     Ethylene along with jasmonic acid is responsible for the activation of several plant defence genes.
7.     Abscission of leaves is regulated by auxin and ethylene.
During leaf senescence, the auxin gradient is reduced.
It enhances ethylene production.
Leads to hydrolyzing enzymes of cell wall polysaccharides thus shedding the leaf.
Interactions among Ethylene, JA, and SA in Abiotic Stresses. (A) Ozone stress. (B) Wound response in tomato suspension cell culture. (C) Wounded tissues (local responses). (D) UV-B stress. ACS, ACC synthase; ETO1 (ETO3), ethylene overproducer; RCD1, radical-indu...
10.10.     Abscisic Acid
In 1963, during a project work on compounds responsible for abscission of cotton fruits, Frederick Addicott along with his co-workers identified abscisic acid. He was supported by two different groups. One was led by Philip Wereing that worked on bud dormancy in woody plants and another group was headed by V. Stevenick which was investigating abscission in leaves and fruits from lupine.
Abscisic acid is the stress hormone of plants. It was initially isolated from many parts of different plants like Avena coleoptiles, leaves of bean and lupin fruits. It was first found to promote abscission and exhibit a limited range of effects than auxin. ABA is synthesized in mesophyll cells, vascular tissues and guard cells.
10.10.1.    Biosynthesis of ABA
Biosynthesis of ABA begins in the chloroplast. Glucose-3-phosphate and pyruvate form IPP (Isopentenyl pyrophosphate)  i.e. the precursor of ABA.
1.     Glucose-3-phosphate and pyruvate form IPP through MEP (Methylerythritol-4-phosphate) pathway.
2.     Several terpenoids of C10, C20 and C40 are formed along with ß-carotene.
3.     Β-carotene forms violaxanthin.
4.     NCED ( Nine- cis- epoxycarotenoid) dehydrogenase cleaves violaxanthin yielding xanthoxin i.e. a 15-C precursor of  ABA.

10.10.2.    Signal transduction of ABA in the guard cell of stomata
On plasma membrane of guard cells, abscisic acid molecules bind to GCR2, a G-protein receptor that transduces signalling further. It stimulates enzyme phospholipase C (PLC) that catalyzes digestion of PIP in the cell membrane to IP2, which is subjected to kinase activity resulting formation of IP3. ABA induces production of cyclic ADP ribose (cADPR) sugar. IP3 and cADPR activate calcium channels on the walls of cellular vacuoles. Formation of reactive oxygen species by ABA molecules also activates calcium in channels on plasmalemma. Augmentation in intracellular calcium impedes importing potassium ion channels in the cells along with chlorine ion exporting channels that results in depolarization of the membrane. The increase in cytosolic pH inhibits the proton pumps localized on the plasma membrane that contributes to membrane depolarization. Increase in activity of membrane channels leads to the export of potassium ions and other anions towards outside of the guard cell plasma membrane. That ultimately causes a reduction in turgor pressure and the cells then becomes flaccid due to exo-osmosis with subsequent stomatal closure.


10.10.3.    Physiological and developmental effects of ABA 
Abscisic acid stimulates stomatal closure to reduce transportation loss during stress conditions. It promotes desiccation tolerance in embryo by encoding Late Embryogenesis Abundant (LEA) proteins. It ena bubbles precocious germination and induces the synthesis with deposition of new polypeptides mainly storage proteins. Hydrolytic enzymes that are institutional for the breakdown of storage reserves in seeds by impeding the transcription of alpha-amylase mRNA. Hate accounts for senescence in leaves and fruits.
1.     ABA controls the synthesis of late- embryogenesis-abundant (LEA) proteins that are involved in the desiccation tolerance of the embryo.
2.     ABA regulates the protein storage during embryogenesis and also maintains seed dormancy till the appropriate environmental conditions are achieved.
    Seed dormancy is regulated by ABA to GA ratio.
3.     ABA inhibits vivipary i.e. a condition in which seeds germinate while still attached to the plant.
4.     ABA inhibits the production of hydrolytic enzymes (a-amylase) for the breakdown of storage reserves in seeds.
5.     During water stress, ABA closes stomata. Thus reduces the transpirational loss.
6.     ABA enhances the growth of roots and inhibits shoot growth under limiting water conditions and also found to play a role in promoting leaf senescence.
BRASSINOSTEROIDS (BRs) are a class of polyhydroxylated steroidal phytohormones in plants with similar structures to animals’ steroid hormones. Brassinosteroids regulate a wide range of physiological processes including plant growth, development and immunity.
The BR signalling pathway is a complex phosphor-relay system that mediates plant growth and development. 
BR binding induces BRI1 phosphorylation of BRI1-KINASE INHIBITOR 1 (BKI1), thus relieving BKI1 inhibition of BRI1  and fully activating BRI1 through mutual phosphorylation between BRI1 and BAK1 or other SERK members. Once activated, BRI1 phosphorylates the BR-SIGNALING KINASE (BSK1) and CONSTITUTIVE DIFFERENTIAL GROWTH 1 (CDG1) kinase, which in turn activate a PP1-type phosphatase named BRI1-SUPPRESSOR 1 (BSU1). The activated BSU1 and PROTEIN PHOSPHATASE 2A (PP2A) de-phosphorylate and inactivate the GSK3-like kinase named BRASSINOSTEROID INSENSITIVE 2 (BIN2), allowing BR response transcription factors BRASSINAZOLE RESISTANT 1/2 (BZR1/2) to accumulate in the nucleus and bind to DNA. Additionally, many other transcription factors also contribute to global BR responses.

Jasmonates (JAs) are a class of plant hormones that play essential roles in response to tissue wounding. They act on gene expression to slow down growth and to redirect metabolism towards producing defence molecules and repairing the damage. They interact with many other plant hormones and therefore also have essential functions throughout development, notably during plant reproduction, leaf senescence and in response to many biotic and abiotic stresses.
JA-induced gene expression
When the levels of JA is low in the cell, the binding of MYC2 to a G-box within the promoter of a JA-responsive gene does not activate transcription due to binding of the repressors Jasmonate ZIM domain proteins (JAZs) to MYC2. The co-repressor like Novel Interactor of JAZ (NINJA) bound to JAZs, and TOPLESS (TPL) repress transcription via HISTONE DEACETYLASE 6 (HDA6) and HDA19. Upon stimulation by JA in the cell, JAZs are recruited by COI1 and subjected to ubiquitinylation and subsequent degradation by the 26S proteasome. Subsequently, MYC2 can activate transcription of early JA-responsive genes such as those encoding JAZ and MYC2. Transcription is mediated by the subunit 25 of Mediator complex (MED25) resulting functions of this signalling.


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