General introduction 15 1 accumulation of specific phytochrome-interacting factors (PIFs) such as PIF4, PIF5, and PIF7. These PIFs activate growth-promoting genes containing E-box and G-box motifs. This activation stimulates various processes involved in the biosynthesis and transport of important plant hormones like auxin, gibberellins, brassinosteroids, cytokinins, and ethylene (Ueoka-Nakanishi et al., 2011; Casal, 2012; Ruberti et al., 2012; Pantazopoulou et al., 2017; Küpers et al., 2018). Specifically, auxin modulates cell wall remodeling and cell elongation via regulation of expansins and xyloglucan endotransglucosylase/ hydrolases (XTHs) (Fry, 2004; Sasidharan et al., 2010; Sasidharan et al., 2014; de Wit et al., 2015). 1.2.3 Phenotypic response in shade avoidance 1.2.3.1 Hyponasty Hyponasty is the upward bending of leaves or petioles. Hyponastic growth allows plants to reorient their leaves towards available light sources, thereby enhancing light interception and photosynthetic efficiency (Pierik and De Wit, 2014; Pantazopoulou et al., 2017). In recent years, extensive research has shed light on the molecular and physiological mechanisms underlying hyponasty in Arabidopsis in the context of shade avoidance. Studies have revealed the central role of phytohormones, particularly auxins, in regulating hyponasty. Auxins act as key signaling molecules: they are synthesized in the leaf blade in response to FR and transported into the petiole where they induce a hyponastic response specifically localized to the leaf perceiving the FR signal (Michaud et al., 2017). Auxins promote cell elongation and influence the directionality of leaf and petiole bending in response to shade-induced stress (Yang and Li, 2017). The dynamic changes in auxin distribution and transport, mediated by the auxin efflux carriers and signaling pathways, orchestrate the precise adjustments in cell expansion and tissue growth required for hyponasty. The involvement of other phytohormones such as gibberellins and brassinosteroids has been recognized in modulating hyponastic responses. The interplay between these enable plants to fine-tune their growth patterns and optimize resource allocation in response to shade conditions (Bou-Torrent et al., 2014; Pantazopoulou et al., 2017; Yang and Li, 2017; Küpers et al., 2018; Küpers et al., 2023). 1.2.3.2 Elongation When plants are subjected to shading conditions, they exhibit a remarkable ability to elongate their stems, petioles, and hypocotyls to reach and capture more light with their leaves.
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