Q.21 Which one of the following statements about photoproteins in plants is
INCORRECT?
(A) Phytochromes are activated by red light
(B) Phytochromes are inactivated by far–red light
(C) Cryptochromes are sensitive to blue light
(D) Phototropins are insensitive to blue light
Option Analysis
Phytochromes exist in two interconvertible forms: Pr (inactive, absorbs red light ~660 nm) and Pfr (active, absorbs far-red light ~730 nm). Red light converts Pr to Pfr, activating phytochromes for responses like seed germination and shade avoidance. Far-red light reverses this by converting Pfr back to Pr, inactivating them.
Cryptochromes (cry1, cry2) are flavoproteins that absorb blue light (320-500 nm), regulating photomorphogenesis, hypocotyl inhibition, and flowering.
Phototropins (phot1, phot2) are serine/threonine kinases with LOV domains binding FMN chromophore; they absorb blue light to trigger phototropism, stomatal opening, and chloroplast relocation. Option (D) wrongly claims insensitivity to blue light.
Introduction to Photoproteins in Plants
Photoproteins in plants, such as phytochromes, cryptochromes, and phototropins, act as light-sensing photoreceptors critical for growth, development, and environmental adaptation. These molecules enable precise responses to light quality, quantity, and direction, forming the basis of sensory photobiology in CSIR NET Life Sciences. Understanding their activation spectra helps differentiate correct from incorrect statements in exam questions.
Phytochromes: Red and Far-Red Light Sensors
Phytochromes (phyA-phyE in Arabidopsis) toggle between Pr (red-absorbing, inactive) and Pfr (far-red-absorbing, active) forms. Red light (~660 nm) activates phytochromes by Pr-to-Pfr conversion, promoting germination and inhibiting shade avoidance. Far-red light (~730 nm) inactivates them via Pfr-to-Pr reversion, explaining phenomena like low-energy germination escape.
Cryptochromes: Blue Light Regulators
Cryptochromes (cry1, cry2) bind flavin and pterin chromophores, absorbing blue/UV-A light (320-500 nm). They mediate hypocotyl inhibition, anthocyanin accumulation, and photoperiodic flowering while suppressing senescence. Blue light stabilizes cryptochromes, triggering downstream signaling via HY5 and PIFs.
Phototropins: Blue Light Kinases
Phototropins (phot1, phot2) feature LOV domains that autophosphorylate upon blue light absorption (~450 nm), driving directional growth. They control phototropism, chloroplast movement, stomatal opening, and leaf expansion under low light. Unlike the incorrect claim of insensitivity, phototropins are quintessential blue light receptors.
