Chapter 1 20 legumes has been studied in several species. In soybean, for example, low R:FR condition has been shown to induce the expression of several auxin biosynthesis and transport genes, including GmYUCCA1, and GmPIN1a, thought to be involved in promoting stem elongation (Wang et al., 2017; Zhang et al., 2022). 1.2.4.2 Role of Gibberellins in SAS Gibberellins (GAs) play a significant role in regulating the shade avoidance response in plants. Under low R:FR conditions, the accumulation of phytochrome-interacting factors (PIFs), modulate the expression of genes involved in GA biosynthesis and signaling pathways, and promote GA levels. Elevated GA levels results in a dynamic distribution of GA in organs, and further stimulate stem and petiole elongation (Djakovic-Petrovic et al., 2007; Casal, 2013; Küpers et al., 2018; Lyu et al., 2021). GA destabilizes DELLA proteins, which are suppressors of PIF action, thus providing a direct SAS modulation mechanism (Djakovic-Petrovic et al., 2007; Feng et al., 2008; De Lucas et al., 2008). Moreover, GAs are also involved in the regulation of other shade-induced responses, such as hyponasty, leaf expansion and reduced branching (Kurepin et al., 2007; UeokaNakanishi et al., 2011; Küpers et al., 2023). 1.2.4.3 Role of Strigolactones in SAS Another aspect of the shade avoidance hormonal network is the role of strigolactones, a class of plant hormones that have been shown to regulate shoot branching and plant architecture in response to environmental cues (Agusti et al., 2011; Siddiqi and Husen, 2017). In Arabidopsis, the transcription factors FAR-RED ELONGATED HYPOCOTYLS3 (FHY3) and FAR-RED IMPAIRED RESPONSE1 (FAR1), integral to phytochrome A-mediated light signaling, act with strigolactone pathway repressors, SUPPRESSORS OF MAX2 1-LIKE (SMXL6/7/8). This alliance directly inhibits SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 9, 15 (SPL9 and SPL15), suppressing their activation of BRANCHED 1 (BRC1), a pivotal branching repressor. FHY3 and FAR1 not only modulate SMXL6/7/8 but also directly up-regulate their expression, accentuating the central role of strigolactone in orchestrating branching. Under simulated shade conditions, reduced FHY3 accumulation results in elevated BRC1 expression, limiting branching. This integrated model underscores how light and strigolactone intricately regulate branching through concerted actions on the BRC1 promoter (Xie et al., 2020). Recent studies have suggested that strigolactones may also play a role in the shade avoidance response in legumes by regulating the balance between shoot and root growth (Siddiqi and Husen, 2017). In
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