Light is the most important environmental signal, providing the ultimate source of biological energy.
Plants have the ability to respond to fluctuations in multiple parameters of light. They can monitor the presence, absence, wavelength, intensity, directionality and diurnal duration of the incident light signals, and can modulate their growth and development appropriately towards optimal radiant energy capture, survival and reproduction. ---Photomorphogenesis--- facilitated through several informational photoreceptors, made up of small families of cryptochromes and phototropins, which monitor the UV region of the spectrum and phytochromes, which monitor the far-red region
Red light stimulates seed germination, but this induction can be inhibited by subsequent exposure to far-red light.
The phytochrome phy molecule is a soluble dimeric chromoprotein with each polypeptide folding into 2 main structural domains ---an amino terminal photosensory domain that carries a single, covalently attached tetrapyrrole chromophore, phytochromobulin and a –terminal domain that mediates dimerization. The photosensory activity of the molecule results from its capacity to undergo light-induced, reversible switching between two conformers the biologically inactive Pr (for red light absorbing) form and the biologically active Pfr (for far red light absorbing) form. Pfr formation (signal perception) initiates an intracellular transduction process causing altered expression of selected genes that are responsible for directing the morphogenesis that is appropriate for the conditions, whereas reconversion for Pr can reverse the process
Small mutigene families encode phytochromes. The phy family in Arabidopsis consists of five members from phyA to phyE. Sometimes different members of the family monitor essentially the same light signals but have predominant regulatory roles in different physiological processes. Whereas different members of the family monitor different parameters of the light environment but control essentially the same physiological response.
Signaling
Phytochromes are light regulated protein kinases.
Most research has come form studies on the seedling de-etiolation process in Arabidopsis, which is regulated by phyA and phyB. There are both shared and separate early signaling pathways used by phyA and phyB, there are both positively and negatively acting components in each pathway and these pathways meet downstream and regulate photomorphogenesis and the circadian clock.
Phy molecules are induced to translocate from the cytoplasm to the nucleus following photoactivation, (therefore not have a second messenger pathway). Studies have shown that full-length phytochromes are initially present in the cytosol in their inactive Pr conformer but after photoconversion to the inactive Pfr conformer, the photoreceptor molecules are induced to translocate into the nucleus.
A phy-interacting transcription factor---PHYTOCHROME-INTERACTING FACTOR 3 (PIF3)—member of a basic helix-loop-helix superfamily of transcriptional regulators.
Both phyA and phyB bind to PIF3, but only after light induced conversion of the biologically active Pfr form. Arabidopsis seedlings that express reduced levels of PIF3 show strongly reduced phenotypic responsiveness to light signals perceived by phyA.
PIF3 localizes constitutively to the nucleus and binds to sequence specific fashion to a G-box DNA sequence that is present in various light regulated promoters. Moreover phyB can bind specifically and photoreversibly to PIF3 that is already bound to its cognate DNA-binding site. Studies have shown that PIF3 could recruit the photoreceptor in its active form to G-box-containing promoters.
PIF3 is functionally necessary for phyB-induced expression of two key photoresponsive genes CCA1 and LHY. The promoters of both of these genes contain G-box motifs to which PIF3 binds in a sequence specific manner, which is consistent with a direct role in regulating their gene expression.
CCA1 and LHY themselves encode MYB class transcription factor related proteins with functions in regulating the expression of CAB genes and or the circadian clock.
Proposed is that PIF3 might represent the central control point through which the phy system regulates both a main branch of photomorphogenesis and the circadian oscillator and this regulation might be done through a short transcriptional cascade using MYB-related transcription factors as intermediates.
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