General introduction 19 1 1.2.4.1 Role of Auxins in SAS Auxins are a class of phytohormones that are essential for plant growth and development. Auxin is synthesized in the shoot apical meristem and young leaves, and transported to other parts of the plant through polar transport (Woodward and Bartel, 2005) Auxin biosynthesis is regulated by a family of genes known as YUCCAs (YUC), which encode flavin-containing monooxygenases that catalyze the rate-limiting step in auxin biosynthesis (Zhao, 2010). In Arabidopsis, there are 11 YUC genes, and their expression is regulated by light (Sato et al., 2015). YUCs have also been identified in other species such as tomato (Expósito-Rodríguez et al., 2011) and soybean (Wang et al., 2017). Auxin signaling is mediated by a family of transcription factors, known as AUXIN RESPONSE FACTORs (ARFs), which bind to auxin-responsive elements (AuxREs) in the promoters of target genes (Woodward and Bartel, 2005). Auxin is sensed by a family of F-box proteins known as TIR1/AFB that function as auxin receptors (Kepinski and Leyser, 2005). ARFs have the ability to bind tandem repeat AuxRE sequences either as homodimers, in conjunction with other ARFs, or in combination with repressive Aux/IAA proteins. In the absence of auxin, the function of ARFs is inhibited by Aux/IAA proteins, which form dimers with ARFs to prevent their activity (Woodward and Bartel, 2005; Li et al., 2016). Upon binding of auxin to its receptors, this complex can now interact with AUX/IAA proteins and facilitate their ubiquitination and subsequent degradation. This allows ARFs to homodimerize and regulate auxin-dependent gene expression. Light-dependent interactions of cryptochrome 1 (CRY1) and phytochrome B with AUX/ IAA proteins leads to the stabilization of AUX/IAAs, resulting in the inhibition of auxin signaling (Luo et al., 2018; Xu et al., 2018). When auxin levels increase, auxin binds to TIR1/AFB proteins, causing the degradation of AUX/IAA proteins and the activation of ARF-mediated gene expression (Stacey et al., 2016). As a consequence, hypocotyl elongation is stimulated. This elongation is also regulated by the ARF7 and ARF19 transcription factors, which are induced by low R:FR ratios (Nozue et al., 2015). ARF7 and ARF19 activate the expression of a number of downstream genes, including several encoding cell wall-modifying enzymes, which promote cell elongation (Okushima et al., 2007). It has been shown that auxin primarily acts in the hypocotyl epidermis to regulate low R:FR-induced hypocoyl elongation in Arabidopsis SAS (Keuskamp et al., 2010b; Procko et al., 2016). Extensive investigation into auxin signaling in SAS has been conducted primarily in Arabidopsis, prompting a broader examination to assess the conservation of auxin signaling across different contexts. The role of auxin in the shade avoidance response in
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