Chapter 1 14 converted to Pfr (the far-red light-absorbing form) by the red light. Pfr is the active form of phytochrome, and can be converted back to Pr by far-red light. Consequently, red light-induced positive phototropism mediated by phytochromes are typically reversible by far-red light (Reed et al., 1993; Kiss et al., 2003). Among all the phytochromes, phyB is essential for mediating SAS (Taiz et al., 2010); phyA can be activated by far-red light and therefore can antagonize phyB-meditated responses in light-grown seedlings (Sheerin and Hiltbrunner, 2017). Figure 1.3. Light absorption graph of pigments in plants, adapted from literature (Glenn, 2022; Domenici et al., 2014). The molecular changes accompanying this photoconversion have a significant impact on signal transduction pathways, leading to adjustments in developmental responses based on the prevailing light environment (Bischoff et al., 2001). In the presence of sunlight, phytochrome becomes active and translocates from the cytosol into the nucleus. Within the nucleus, it interacts with PHYTOCHROME INTERACTING FACTORS (PIFs) and facilitates their inactivation and degradation. PIFs are growth-promoting factors involved in various physiological processes (Shin et al., 1997; Ruberti et al., 2012; Yang and Li, 2017). The family of basic helix-loop-helix (bHLH) transcription factors, which PIFs belong to, serves as the central hub in a signaling cascade that promotes cell elongation. These bHLH factors play a crucial role in coordinating and regulating the processes involved in cell elongation (Oh et al., 2014). In response to low R:FR light conditions, the active pool of phytochrome decreases, leading to the
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