Chapter 4 110 Brassinosteroids, another important plant hormone, is essential for normal plant growth (Li and Chory, 1999). Studies in Arabidopsis seedlings have demonstrated that brassinosteroid and auxin activity are required for shade avoidance initiated by reduced blue light (Keuskamp et al., 2012). Besides, it has been demonstrated that BRs play a role in plant growth and development by interacting with auxin and other phytohormones (Kim et al., 2006). Brassinazole (BZ) has been validated as a specific inhibitor of BR biosynthesis, a number of which have been shown to inhibit cytochrome P450s (Asami et al., 2000). Arabidopsis plants treated with BZ exhibit dwarf phenotypes reminiscent of BR biosynthesis mutants (Asami et al., 2000). It is noted that shade-induced stem elongation relies on BRs in Arabidopsis, as BR biosynthesis mutants and BZ-treated Arabidopsis plants fail to elongate in shade condition (Keuskamp et al., 2011)(Luccioni et al., 2002). However, numerous questions regarding how BRs and their interactions with other phytohormones modulate shade avoidance still remain to be answered. Plant responses to shade involve also other hormones (Figure 4.1). Ethylene plays a role in stem elongation and petiole growth under low light in tobacco (Pierik et al., 2004). In leaves, auxin accumulation stimulates cytokinin oxidase, inhibiting leaf growth (Yang and Li, 2017). Shade-induced abscisic acid (ABA) inhibits branching, and ABA biosynthesis mutants exhibit increased branching under low R:FR (Finlayson et al., 2010; Su et al., 2011). Strigolactone (SL), known for controlling lateral shoot growth, influences branching suppression in shade. max2 mutants, affected in SL signaling, show longer hypocotyls under various light conditions (Shen et al., 2012; GonzálezGrandío et al., 2013). Karrikins, enhancing light sensitivity, could be key in mitigating shade-induced responses (Waters and Smith, 2013; Meng et al., 2017). Recent studies in Arabidopsis have provided an insight into the intricate interplay among all these hormones. However, many details remain unclear, and the regulation of SAS beyond Arabidopsis has still a lot to explore.
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