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Growth and development of plant occur under great influence of light, that quality and quantity changes are received by the specific photoreceptors. Various photoreceptors perceive the changing light condition and transform them into a molecular signal that results in the appropriate response. The photoreceptors of red/far-red light are phytochromes the dimeric proteins covalently linked with phytochromobilin that acts as a chromophore. The decoding process starts with the perception of red light (666 nm), which occurs through photoisomerization of a chromophore leading to structural changes in apoprotein. This form of the phytochrome is called Pfr and is considered the biologically active form. This change is reversible, with far-red light (730 nm) illumination restoring Pr form. The phytochromes are synthesized in their inactive Pr form and are localized in the cytoplasm. Upon light excitation they are activated and translocated into the nucleus, where they interact with different proteins and modulate gene expression. PhyB enters the nucleus in response to red light, but phyA is efficiently transported into nucleus in response to far-red light and in response to very law levels of light over a broad range of colours. Nuclear accumulation of phyA is dependent on two proteins FHY1 and FHL which preferentially interact with the light activated form of phyA. Phytochromes, after translocation into the nucleus, interact with nuclear proteins. To date, more than 20 phytochrome-interacting proteins have been reported. Several recent studies have shown that multiple related bHLH (basic helix-loop-helix) class transcription factors play key roles in phytochrome signal transduction. All the bHLH proteins involved in light signaling belong to a single evolutionarily related subclass. These bHLH transcription factors are known as PIF (phytochrome interacting factor) or PIL (phytochrome interacting factor-like). Some PIF/PILs preferentially interact with phyB whereas others interact with equal affinity with both phyB and phyA. Analysis of pif/pil mutants have led to the suggestion that they mainly act as negative rather than positive regulators. Because phytochromes can phosphorylate PIF1/PIL5, PIF3, PIF4, PIF5 and HFR1, it is suggesting that phosphorylated proteins may then by ubiquitinylated by an E3 ubiquitin ligase, leading to degradation by the 26S proteasome. Recent studies demonstrated that several PIF/PIL proteins may interact with DELLA proteins the key repressors of gibberellic acid signaling. The DELLA proteins physically interact with several members of the PIF/PIL family in such a way that the interaction inhibits the ability of the PIF/PILs to bind to, and regulate their target genes. Members of the PIF/PIL transcription factors subclass are involved in phytochrome-regulated processes such as seed germination, seedling de-etiolation, and response to shade signals. Light, specifically red light, is a crucial external factor that induces seed germination. On the other hand, the plant hormones, gibberellins (GA) and abscisic acid (ABA) are internal cues that play important but antagonistic roles in seed germination. Recent research has identified PIF1/PIL5 as a key negative regulator in phytochrome-mediated seed germination. In the dark, PIF1/PIL5 represses germination through reducing GA responsiveness and regulating GA and ABA levels. Light-activated phytochromes directly interact with PIF1/PIL5 and promote its degradation, negating PIF1/ PIL5 repressive effects. To a postgerminative seedling, light is a decisive environmental factor that determines its developmental program. In the dark, a seedling undergoes scotomorphogenesis, however, under light, it adopts genetic program of photomorphogenesis. Light signals from phytochromes and cryptochromes converge on a group of conserved proteins termed COP/DET/FUS, which are central repressors of photomorphogenesis. In darkness, they work in concert to target a number of photomor- phogenesis-promoting transcription factors, such as HY5, HYH and LAF1, for degradation. In the dark, PIF/PIL proteins are active and regulate gene expression to promote scotomorphogenic growth. Under light, activated phytochromes in their Pfr form, interact with PIF/PILs and result in the phosphorylation and subsequent degradation of PIF/PILs by the proteasome, resulting in photomorphogenesis. Besides, PIF/PIL and HY5 proteins are signaling integrators that link light signals to the signaling of phytohormones. Phytochromes sense changes in light quality due to shading by competing vegetation, using the ratio of red to far-red light (R/FR). PIF4 and PIF5 have recently been shown to be positive regulators of shade avoidance responses, participating in the regulation of some key players in these responses, such as ATHB2, ATHB4 and PIL1, the proteins positively regulating shade avoidance, and HFR1, a transcription factor with a negative role in shade avoidance. Responses to low R/FR ratio are primarily mediated by phyB. In daylight, phyB exists predominantly in the Pfr form, and following import into the nucleus, Pfr B binds PIF4 and PIF5 proteins, resulting in their degradation via the 25S proteasome. In vegetational shade, a reduction in R/FR ratio results in conversion of phyB to the inactive Pr form. The reduction in phyB Pfr would therefore result in increased abundance of nuclear PIF4 and PIF5 proteins.

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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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