Dr. Mohit Gupta
Questions asked in Section - C of CSIR-NET December 2014.
Q103. Light is crucial for plant growth and development. Following are certain statements related to photoreceptors in model plant Arabidopsis thaliana.
A. Among the five phytochrome genes, representing a gene family, PHYB plays a predominant role in red- light perception.
B. Cryptochromes are involved in the regulation of flowering time and hypocotyl length.
C. phyA photoreceptor is predominantly involved in far-red light perception.
D. The LOV domain of phytochrome C (PHYC) is important domain for signal transmission.
Which one of the following combinations of above statements is correct?
1. A, B and C 2. A,C and D 3. B,C and D 4. A, B and D
Ans (1) This question is part of Photosensory Biology and this is an advance area of research with many facets still emerging. Infact in Delhi University south campus, Plant Molecular Bio. Dept., a lot of research is being conducted in lab of Dr. Khurana.
But the perplexing part is that this topic is either not taught in most universities in India or teachers just skip it superficially....because to have clarity in this topic, one has to take the pain of going through many research papers as well as advance books........which is rarely seen now days.
Infact students are not even aware of names of different photoreceptors and their locations in the cell....
Light is an important signal which regulates plant development.....and we call it Photomorphogenesis. The receprots for light are called Photoreceptors e.g
Phytochrome........present in cytoplasm
Cryptochrome........present in nucleus
Phototropin...........present in plasma membrane
Zeaxanthin (a pigment which serves as receptor, recently discovered)........present in plastids
Protochlorophyllide-a..........present in plastids
Discussing the entire topic here is not possible.......in classroom programme it takes us multiple lectures involving many hours to justify this area....and no question shall skip our teaching.
Phytochrome is a red and far-red light photoreceptor.......present in two interconvertible photochromatic forms i.e. Pr and Pfr, Pr absorbs red light and gets converted to Pfr form which is said to be physiologically active form of phytocphrome and Pfr absorbs far-red light and converts to Pr......Phytochrome is present in cytoplasm............it is made up of two components:
1) A light absorbing part called Chromophore (synthesized in plastid)
2) Polypeptide chain called Apoprotein (synthesized in cytosol)
Phytochrome infact is a light activated dimeric protein kinase........they get autophosphorylated by light and it is an enzyme i.e. serine/threonine kinase. In cytosol it exists in inactive Pr form............upon absorption of red light chromophore part undergoes conformational change..........which induces a change in apoprotein also.....thus converting Pr form to Pfr form...........In the Pfr form, the NLS, nuclear localizing signals are exposed thus Pfr shuttles to nucleus..........where it phosphorylates regulatory proteins including transcription factors.........thus modulating gene expression.
In Arabidopsis 5 different genes for phytochrome have been discovered i.e. PHY-A, PHY-B, PHY-C, PHY-D & PHY-E....and these have been grouped under two different classes i.e. Type-1 & Type-2 phytochromes.
Type-1 includes Light sensitive form (i.e. Pfr is unstable) e.g. phytochrome encoded by PHY-A.
Type-2 includes Light stable form (i.e. Pfr is stable) e.g. phytochromes encoded by PHY-B, C, D, E.
Genetic studies have revealed that Phytochrome-A & Phytochrome-B have contrasting roles in plant development...........Phytochrome-A is the photoreceptor involved in far red light whereas Phytochrome-B mediates responses to red light or white light..............i.e. choice A & C are correct.
Cryptochrome were first discovered in Arabidopdis.......but later found in various organisms e.g. cyanobacteria, algae, Drosophilla, mice and humans........Cryptochrome is a blue light photoreceptor located in nucleus and encoded by cry-1 & cry-2 genes.
Cryptochromes mediate several blue light responses e.g.
Inhibition of hypocotyl (stem) elongation
Anthocyanin (are pigments responsible for colour of flowers) prodduction
Regulation of circadian clocks
promotion of cotyledon expansion
Choice 'C' says Cryptochromes are involved in the regulation of flowering time and hypocotyl length.....so it is a correct choice.
Studies have indicated that protein degradation and phosphorylation are important steps in signal transduction of cryptochrome.
They also mediate blue light responses........and are responsible for
Phototropic movement of plant towards blue light.
Chroloplast movements in a cell.
They are localized on plasma membrane and are encoded by phot-1 and phot-2 genes.
C- terminus half of phototropin is a serine/threonine kinase whereas
N-terminus half contains LOV domains i.e. light, oxygen and voltage domains. LOV domain binds FMN and undergoes blue light dependent autophosphorylation..............the downstream signalling beyond receptor is not clear as of now.
Choice 'D' says The LOV domain of phytochrome C (PHYC) is important domain for signal transmission..........is a wrong statement as LOV domaind are present in phototropins and not phytochromes.
So choice A,B & C are correct options and choice D is a wrong one, so answer is
1) A,B and C.