For growth and development of plants, light is an essential component gifted by nature. Primarily we have studied that light is used only for photosynthesis, i.e. production of food from CO2, water and chlorophyll. Apart from photosynthesis, it regulates the metabolic activities of the plant that alters physiological and even morphological features. For example, let's say Vishnu, a student of LTA observed that pea seedlings were grown in darkness (etiolation) exhibited a tall and spindly appearance. They were colourless, pale due to the presence of etioplasts in them along with apical hooks, but on the application of light flashes to etiolated seedlings initiates changes like the rate of stem elongation was decreased, beginning of apical hook straightening one of the major observation was the appearance of green colour that resulted from chloroplast synthesis.
It was concluded that driving force of these changes is apart from photosynthesis because chlorophyll was not present during etiolation of seedlings. This kind of development in plants during etiolation (dark period) is known as skotomorphogenesis. Whereas in presence of light it is switched to photomorphogenesis.
Photomorphogenesis is a process that denotes the influence of light on morphological and physiological changes with respect to plant development.
14.1.    Photoreceptors :
Photoreceptors are pigment molecules that play fundamental roles in photoreception and subsequent adaptation of plant growth and development. They perceive, interpret and transduce light signals via distinct intracellular signalling pathways, to modulate photoresponsive nuclear gene expression that ultimately leads to adaptive changes at the cell and whole organism level.
Three types of photoreceptors : 
14.1.1.    Phytochromes :
Using spectrophotometer, red/far-red light absorbing pigments were discovered by biophysicist Warren Butler and biochemist Harold Siegelman. Butler coined the term phytochrome. Phytochrome absorbs red (R)/far red light and involves in regulation of seedlings and time of flowering in adult plants. They are widely expressed across many tissues and developmental stages. Structurally, phytochromes are soluble proteins that exist as homodimers weighing 250 KDa.
It is made up of two equivalent subunits with each monomer of 125 daa. The pigment is covalently bound to light sensory chromophore molecules that initiate phytochrome signalling. The apoprotein part of each monomer is composed of the N-terminal domain (70KDa) and C-terminal domain (55 K Da), that are connected by a flexible hinge region. The N-terminal domain can be further divided into four consecutive sub-domains; i.e.; N-terminal extension (NTE), PAS, GAF and PHY. The PAS subdomain provides a platform for protein-protein interactions, responding to the alteration in light conditions. The chromophore is attached to a conserved cysteine residue in GAF subdomain. The C-terminal can also be divided into two sub-domains, i.e., PAS related domain (PRD) containing two PAS repeats [PAS-A and PAS-B] and the histidine kinase-related domain. PRD sub-domain is important for nuclear translocation of phytochrome as it contains nucleus localizing sequence (NLS). HKRD domain lacks a critical histidine residue and thus may be an evolutionary remnant rather than an active histidine kinase. The phytochrome chromophore is usually phytochromobilin (PfB). It is synthesized inside plastids from precursor; 5-aminolevulinic acid. Bilins are derived from the closed tetrapyrrole ring of haem by an oxidative reaction catalysed by haem oxygenase to yield their characteristic opens chain. It is attached to GAF sub-domain via a thioether linkage.

Phytochrome exists in two forms; Pr and Pfr. Pr is a blue coloured pigment that absorbs light of wavelength 660 nm. Pr biosynthesis occurs in the cytosol and is generally consider of them as inactive form whereas Pfr, a bluish green coloured pigment that absorbs maximally at 730 nm is biologically active because in latter the NLS is exposed that provides nuclear translocation for transmitting light signals to the nucleus.
This unique feature of phytochrome that dictates interconversion between two Phytochrome isoforms is termed as photoreversibility. The principal difference between the Pr chromophore and Pfr chromophore appears to be cis-trans isomerization of the methane-bridge between its structural rings.
14.1.2.     The efficiency of photoconversion: During day time red light treatment of Pr results in production about 85% Pfr + 15% Pr that is, red light absorbed by same Pfr converts it back to Pr form. Similarly, during the night, far-red results in production about 97% Pr + 3% Pfr because far-red light absorbed by same Pr molecules converts it back to Pfr form. At saturation, not all phytochrome is interconverted because the absorption spectra of the two forms overlap in the red region of the spectrum, it leads to absorption of some red light by Pfr and absorption of some for - red light by Pr. This point of equilibrium is referred as the photostationary state. Observation of light-induced response in plants after photoperception includes a time period, called as lag time. It is variable in plant species and indicates the complexity of response in them.
Interconversion of photochromatic forms is restricted to a limited time period. If this time gap is missed then far-red light can not influence Pfr photoconversion back to Pr. This release of time-gap indicates escape from photo reversibility.
Phytochromes in plants are encoded by PHY gene family. Products of PHY-gene family members can be classified as 
TYPE I Phytochromes 
TYPE II phytochromes
Type I includes phytochrome-A (PHY-A).
It is primary photoreceptor responsible for mediating photomorphogenic responses in far-red light. It is light labile. It is generally found in dark growth per-seedlings.
Type II includes phytochrome B, C, D and E [PHY-B, PHY-C, HY-D and PHY-E]. They are light stable and regulates de-etiolation
responses in presence of red light. These are equally present in dark and light-grown seedlings.
14.1.3.    'The law of reciprocity': It states that magnate itude of light-induced response is a function of total photons impinging on leaf surface, i.e., fluence rate X irradiation time. It is independent of fluence rate alone; Phytochromes are induced by a specific amount of light, even if this total amount of light is absorbed in the partition.
Each phytochrome response has a characteristic range of light fluences over the magnitude of the response is proportional to the fluence. These responses fall into three major categories based on the amount of light required, VLFRs, LFRs, HIRs 

14.1.4.    Phytochrome signalling
Plant phytochrome is an autophosphorylating serine/threonine kinase that phosphorylates itself and other proteins that mediates cellular signalling. On perceiving light, the serine residue of N-terminal is phosphorylated by kinase activity of phytochrome that results in conformational change and exposure of nuclear localization signal for nuclear transport of phytochrome or it may activate enzymes like phytochrome kinase substrate-1 (PKS1) and nucleoside diphosphate kinase-2 (NDPK2).
Plant development is regulated not only by the difference between light and darkness but also by light quality and quantity.
The amount of light is majorly captured by the tall trees for photosynthesis. So, the plants growing under the canopy are devoid of red light but far-red light is easily available to them. Absorbed far-red light induces auxin biosynthesis in shoot apex and young leaves resulting elongation of stems and leaves-petioles towards unfiltered light. The phytochrome participates in directing the lower plants to elongate and enables them to overtop competitors. This phenomenon is called shade avoidance syndrome (SAS). 
Some seeds often germinate in spaces receiving sunlight through gaps in the canopy than in densely shaded space because the red light coming from sun ensures the seeds to be self-sustaining before their endosperm food reserves were exhausted. Recent studies on light-dependent lettuce seed germination, have shown that red-light induced seed germination is the result of an increase in the level of the biologically active form of the hormone gibberellin. This hormone encourages the formation of proteases that digest reserve food and supply them to developing an embryo. Phytochromes are thus involved in signalling the biosynthesis of gibberellin precursors that generate active form.
In some plants, a rapid response to phytochrome-mediated ion fluxes across membranes results in turgor changes around ventral motor cells and dorsal motors cells, located on opposite sides of pulvinus in plant Albizia pulvini. In dark, K+ and Cl ions moves into dorsal motor cells (DMCs) and moves out of ventral motor cells (VMCs) resulting in increase of osmotic potential that causes entry of water in DMCs and turgid DMCs with help of flaccid VMCs results from leaflet to close whereas during day time efflux of K+ and Cl ions from DMCs and their influx in VMCs results in turgid VMCs due to water entry and exosmosis, it results from flaccid DMCs. Those results in the opening of the leaflet.
Phytochromes regulate K+ & Cl ion channels by phosphorylated protein signals. 

Greening of leaves is one of the important survival need of plants. This green coloured pigment is present in the chloroplast. Formation of clothes roplast is supported by the presence of light. Generally, phytochromes find a competitive environment in the nucleus, where negative regulators like phytochrome-interacting factors (PIFs) suppress the phytochrome signalling for chloroplast development, but on photoperception, Pr form of phytochrome is converted into Pfr form and gets phosphorylated. With the help of NLS, the phosphorylated Pfr enters the nucleus and binds with PIF-3 and derepress the promotor of MYB gene. MYB protein produced via central dogma, act as a transcription factor for the promoter of LHC gene. Expression of LHC genes results in LHC mRNA then by translation it is converted into LHC protein (light-harvesting complex) that contributes to chloroplast development during the night, far-red light converts the phosphorylated Pfr into Pr, thus negatively regulates the process.
14.2.    Cryptochromes :
This class of flavoproteins mediates effects of blue-light on plant development like inhibition of stem elongation. The HY4 plants showed an elongated hypocotyl when irradiated with blue light. Isolation of the 'HY4 gene' showed that it encodes 75 kDa protein with significant sequence homology to microbial DNA photolyase, a blue light activated enzyme that repairs pyrimidine dimers in DNA formed as a result of exposure to ultraviolet radiation. In view of this sequence similarity, the hy4 protein later renamed cryptochrome 1. CRY gene encodes for CRY 1 & CRY 2 protein.

14.2.1.    Structure of Crypotochrome :
Cryptochrome transduces light signal into a chemical signal by a photo-induced negative charge within the protein either on FAD co-factor or on the neighbouring aspartic acid. This negative charge would electrostatically repel the protein-bound ATP molecule on the C-terminal domain that covers the ATP-binding pocket prior to photon absorption. The resulting change in protein conformation could lead to phosphorylation of sites that were previously inaccessible. Then the phosphorylated segment liberates the transcription factor HY-5 by competing for the same binding site at the negative regulator of photomorphogenesis, i.e., COP-1.
14.2.2.    Opening of stomata 
Blue light triggers opening of stomata by activating electrogenic pumps such as proton pumping H+ ATPases on the walls of guard cells. Cryptochrome activates the enzyme protein kinase A by phosphorylation that leads to activation of 
H+ATPases and K+, Cl ion channels. H+ moves outside the cell and K+ ions come inside the cells, to balance charge Cl ions also enters guard cells, increasing osmotic potential. Entry of water through endosmosis makes the guard cells turgid and stomata open up.
14.2.3.    Inhibition of stem elongation
A rapid visible response to photo-activation of cryptochrome is the inhibition of internode extension of growing seedlings Ca+2 and CaM-dependent cellular events can be linked to red light-induced inhibition of internode extension, CRY induces CaM activation, then Ca+2 - CaM complex stimulates plasma membrane-bound Ca2+ - ATPase which pumps Ca+2 out of cytosol into the walls when calcium concentration in cell wall increases, the cell wall loosening process is inhibited leading to wall stiffness, which in turn stops elongation.
Induction of flowering - Cryptochromes mediated light control of flowering time. Activated cryptochromes interact with CIB-1 (CRY2– interacting bHLH1) in response to blue light. CIB along with CRY stimulates expression of FT-gene. The product of the FT gene act as a transcription factor for floral identity genes like LFY, APL, AP2 resulting anthesis.
14.2.4.    Circadian Rhythm – 
A circadian rhythm is a biological process that includes the changes in plant metabolism alternating through the high activity and low activity phases with a regular periodicity of 24 hrs.
In Arabidopsis thaliana; three major clock genes have been identified, i.e., Cab-1, LHY and CCA-1. The protein products of these genes are all regulatory proteins. According to a recent model light and the TOC-1 regulatory protein activate LHY and CCA expression at dawn. The increase in LHY and CCA represses the expression of TOC-1 gene. Because TOC-1 is a positive regulator of LHY and CCA-1 genes, the repression of TOC 1 expression causes a progressive reduction in the levels of LHY and CCA-1, which reach their minimum levels at the end of the day. As LHY and CCA 1 level decline, TOC-1 gene expression is released from inhibition. At the end of the day levels of LHY and CCA-1 are at their minimum thus TOC-1 reaches its maximum. TOC then either directly or indirectly stimulates the expression of other genes, such as LHY and CCA-1 resulting induction of other "morning genes" during the day time, they repress the genes that are expressed at night, during the day. Light acts to influence the effect of TOC-1 gene in promoting LHY and CCA1 expression. Phytochrome and the blue-light photoreceptor CRY2 mediate the effects of red and blue light, respectively.
Apart from TOC-1 gene pathway plants regulates the process of photomorphogenesis by inducing rapid phosphorylation and repression of COP-1 ( constitutive photomorphogenesis-1). COP-1bis E3 ubiquitin ligase. Generally, it directs degradation of various photo-morphogenetic transcription factors like via 26S proteasome pathway during the dark period. At morning, Pr form of phytochrome absorbs red light and converts in Pfr. This activated Pfr translocates into the nucleus and activates SPA-1 (Suppressor of Phytochrome). It binds to COP-1 & HY-5 complex and releases the latter by directing COP-1 towards the cytoplasm. Recent studies suggest that COP-1 is inactivated by phosphorylation at 20th serine residue through a protein kinase namely PINOID (PIN) at and prevents from degradation. When COP-1 gets inactivated, PIF-3 allows expression of LHC-B genes.  In the afternoon, HY-5 involves in hypocotyl elongation. Soon the game changes on arrival of the evening when Pfr after absorbing far-red light and becomes Pr. Now, this activated PR weakens the SPA-1 & PIN and therefore COP-1 gets activated for the degradation of photo-morphogenetic factors like HY-5. At night, the active COP-1 stabilizes that inhibits phytochrome signalling and suppresses morning genes.

14.3.    Phototropism: They are blue light receptors controlling a range of responses that serve to optimize the photosynthetic efficiency of plants. They include phototropism, photoinduced stomatal opening and chloroplast movements response to changes in light intensity. Arabidopsis contains two photo tropins, phot 1 & phot 2, Phototropins are light activated serine/threonine protein kinases. Light sensing by the phototropin is mediated by a repeated motif at N-terminal region of the protein termed as LOV domain L for light O for oxygen and V for Voltage. Photoexcitation of the LOV domain results in receptor out phosphorylation and initiation of phototropic signalling. Phototropism, on stimulated by light alters the configuration of cytoskeletal arrangement & mediated organelle shift in plant cell resulting response of phototropism by altering the locating of PIN transporters in plant cells, chloroplast movement according to light intensity and differential apical growth in plants else to disruption of auxin polarity.

14.4.    Photoperiodism : 
Many temperate plants flower during the spring season when days are moderately short others flower during the short days of late summer and early fall. This mechanism that enables plants to respond to day length so that they flower at a specific time of the year is known as photoperiodism. The length of the daily period of light to which a plant is exposed is called photoperiod. Photoperiodism is the physiological reaction of the organism to the length of day or night. It can also be defined as developmental responses of plants to the relative lengths of light and dark periods. The concept of critical photoperiod states that a plant does not flower in response to a definite day length i.e, some plant needs exposure to light below critical photoperiod to flower such plants are called as short day plants. These plants are grown vegetatively; if exposed to day length in excess of critical photoperiod. They normally flower in the early spring or autumn. For example tobacco, soybean, chrysanthemum, sugarcane, etc. But some plants flowers, when they receive light period above critical photoperiod. These plants are long day plants. Below critical photoperiod, continuous vegetative growth is observed in them. These plants bloom in later spring or early summer. For example, barley, radish, spinach, onion, carrot, etc. 
Apart from the above two types of plants, some plants flower after a period of vegetative growth regardless of photoperiod. such plants are categorized under Day Neutral plants. They are independent of day or night lengths and flower around the year. For example tomato, cucumber, cotton, sunflower, maize, etc.
Pr induces flowering in short day plants and Pfr favours flowering in long day plants. 
Short day plants require a more dark period for flowering, so also known as long night plants. Such plants in dark period absorb far-red light and their phytochrome is changing from Pr to Pfr form. Pfr transport to muscles and initiate floral gene expression and during light period Pfr converts back to Pr form, which inhibits flowering in SDPs. 
This can be easily explained by night break experiments. The dark period can be made ineffective by interruption with a short exposure to light. In many SDPs/LNPs, which needs Pr for the flowering introduction of red light influences Pr to Pfr. Now fuere is no Pr. so flowering is inhibited but the introduction of far-red light directs the conversion of Pfr to Pr form and results flowering in SDP/LNP.
But in the case of long-day plants / short night plants Pfr form induces flowering therefore it requires longer light period so that all phytochrome converts from Pr to Pfr.

14.5.    Flowering 
During the life cycle of angiosperms, a stage comes when vegetative growth in apical shoots transits to reproductive phase and initiate formation of flower for perpetual of their species. Flowering is a major developmental transition in plants from vegetative to reproductive state during which the plant acquires reproductive competence by producing an inflorescence. The timing of this change is influenced by endogenous and environmental signals such as hormones, day length and temperature.
14.6.    Genetic control of floral development -
The flower arises from the activity of three classes of genes which regulate floral development. Group of genes that encode transcription factors needed to initiate the induction of the floral meristem are kept under meristem identity genes. They are positive regulators of organ identity during floral development.
The second class of genes are organ identity genes. They encode for regulators that control directly expertise session of genes that contribute to the formation or function of floral organs. Third class of genes act as spatial regulators for the organ identity genes as they define boundaries for the interaction of meristem identity genes and organ identity genes by regulating whether they act in the same place at the same time. These are known as cadastral genes.
A model was developed to study the interactions of the different genes that control floral organ identity. All the genes are classified as expressing one of three activities A, B or C. The 'A' gene activities control development and 'C' gene activities control stamen and carpel development. 'A' gene function and are defined by two genes, namely, APETELA 1 & APETELA 2. Mutation in gene 'A' results in the development of carpels in the first whorl and second whorl develops as the stamen. Type 'B' gene functions are defined by genes APETELA 3 and PISTILLATA. The cumulative effect of 'B' gene mutations is that the second whorl develops as sepal rather than petal and third whorl develops as sepal rather than petal and third whorl develops as capel, not a stamen. Finally 'C' gene functions are defined by gene AGAMOUS. Mutation in gene-C have third whorl stamen is replaced by petal and fourth whorl replaced by sepal.

According to the new concept, A, B, C genes are functional when supported by active gene 'E'. Gene 'E' is present in all four whorls. Its normal function is antisepalous but on mutation, it directs formation of sepals. The function of gene 'E' is defined by genes SP1, SP2, SP3, SP4. Mutation in SP-1/2 results in sepal formation in all four whorls as it is present in all of them. Mutation in SP-3 also result in formation of sepal in 4 whorls because in first whole function of normal 'E' and normal 'A' gene arises sepal but mutant SP-3 is present in whorl two, three and four gene SP-4 is present in first & fourth whorl so mutation in SP-4 arises sepals in whorl 1 and whorl four.
The structure of the flower can be divided into four whorls. The outermost whorl consists of the green sepal. The second whorl comprises of petals. The third whorl is called as androecium and unit of this whorl is stamen. The fourth whorl is termed as gynoecium and its unit is carpel. Whorl three and four are reproductive part of the flower and whorl one and two serve as accessory organs supporting flower. The floral organs formed as a result of interactions of organ identity genes can be studied with the ABC model of flower development.
14.7.    Flowering Pathways 
Photoperiodic pathway \rightarrow 
Photoperiod is one of the most important factors affecting floral transition. The day length & light intensity is detected by photoreceptors influences floral expression. It includes initiation of signalling by phytochromes, cryptochromes that activate CONSTANS (CO) gene expression. The CO protein induces FT-mRNA by acting as a transcription factor of FT-gene. FT mRNA contributes to floral induction by acting as the long-distance signal between leaves and shoot apical meristem (SAM) in shoot apical meristem, FT protein forms complex with another transcription factor FD. They both initiate flowering by activating floral meristem identity genes.
14.8.    Vernalization Pathway
Vernalization is a process in which flowering is promoted by low-temperature treatment applied on a fully hydrated seed. The effective temperature range for vernalization is usually between about 1°C and 7°C. Vernalization is successful when followed by a treatment of long days. It involves suppression of the FLC gene which is a floral repressor. Three genes participate in this pathway namely Vernalization insensitive 3 (VIN3), VRN-1 VRN-2 VIN-3 genes encodes a chromatin remodelling plant homeodomain (PHD) finger protein. It is essential for FLC regulation. VIN-3 gets activated by long cold periods and establishes the vernalization response by promoting histone de-acetylation at the FLC locus and effectively represses FLC expression. VRN1 encodes a DNA binding protein while VRN2 encodes nuclear protein. Thus VIN-3 is induced in response to vernalization and establishes the initial silencing of FLC. Consequently, VRN1 and VRN 2 are required to maintain FLC in a silenced state. FLC represses meristem identity genes and blocking FLC results in expression of them and initiate flowering. 
It is also proposed that vernalization results inform the action of active gibberellic acid the promote flowering by the GA pathway.
14.9.    Gibberellic Acid pathway
Flowering in plants is regulated by some proteins which block expression floral transition genes. This regulation is supported by the activity of DELLA repressor proteins like RGA, GAI, etc. as observed in Arabidopsis. Gibberellin promotes degradation of DELLA repressors via 26s proteasomal pathway thereby increasing the transcriptional activity of the floral meristem identity gene LFY. Gibberellin promotes early flowering and for flowering under non-inductive short days. Exogenous application of GA stimulates augmentation in SOC1 gene expression also contributes to products of Agamous like (AGL) gene transcripts. Genes like SOC1 and AGL turns on the activity of other floral homeotic genes like APETELA1 (API), APETELA3 (AP3), PISTILLATA (PI) for floral organ development.
14.10.    Autonomous Pathway
Some plants, with the help of internal signals, control floral development. Genes associated with the pathway are expressed in the meristem, as guided by the production of a fixed number of leaves. It contributes to flowering by regulating expression of Flowering Locus C (FLC) gene. In vernalization, FLC gene expression is silenced by epigenetic modelling. With help of some proteins, but in this case, a series of activities takes place that involves RNA-mediated chromatin silencing of FLC RNA transcripts. FY mRNA shows catalytic activity and induces a decline in negative charge on histone protein so that gene fragment wrapped around that histone become less available for transcription.
14.11.    Ageing pathway:- 
In addition to the above mentioned flowering pathways, flowering time is influenced by chronological factors. It is governed by active on of micro RNAs (miRNA). miRNA-172 and miRNA-156 are key factors that play a distinct role in developmental phase transitions. miRNA-156 is highly abundant in the juvenile stage and decreases during subsequent adult age, whereas miRNA-172 has an opposite expression pattern i.e., it is abundant during adult phase but present in trace amount during juvenile phase. Overexpression of miRNA-156 negatively regulates genes like SQUAMOSA Promoter Like (SPL) and delays transition from juvenile to adult. But with the increase in miRNA-172 expression, it encodes transcription factor that positively regulates SPL and flowering locus T (FT) gene along with repression of flowering time negative regulators acting dour strenuous of FT gene.

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