Phytochrome belongs to red/far-red light family of photoreceptors. It exists in two spectral forms named red light absorbing form (Pr), said to be the inactive form and far-red light absorbing form (Pfr) which is the active form. This photoreceptor is structurally divided into two modules- the amino acid (N-) terminal photosensory module and caboxylic acid (C-) terminal His kinase-like catalytic output module. Five different types exist in Arabidopsis (PhyA-E). Roots and hypocotyls elongation in Arabidopsis is regulated by photoreceptors one class of which is phytochrome. The role of phytochrome B (PhyB) in red light responses has been established through studies using PhyB mutant and truncated versions. N-terminal module of PhyB containing 651 amino acids was shown to be biologically active in regulating photomorphogenesis. Meanwhile, the C-terminal module was long assumed to be involved in downstream signal transduction. Recently, this module was suggested to play a role in integrating red and blue light signaling to circadian clock. Here, the study shows that the C-terminal module of PhyB is needed for root growth and strongly modulates the root to hypocotyl ratio at 22°C. At an elevated temperature (34°C), this ratio was altered suggesting a role of this module in temperature signaling during plant growth.
Key words: Phytochrome, red/far-red, Roots, hypocotyls.
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