Chapter 6 216 According to this theory, auxin induces proton release in susceptible cells, lowering apoplastic pH, increasing membrane potential, and activating cell wall-loosening proteins like EXPANSINs (Sasidharan et al., 2008; Sasidharan et al., 2010; Arsuffi and Braybrook, 2018), also upregulated in our data (Figure 4.22-23). Intriguingly, certain SAURs have been suggested to exert a negative regulatory influence on cell expansion. For example, transgenic seedlings overexpressing SAUR32 exhibit reduced hypocotyl elongation in conditions of darkness or red light exposure (Park et al., 2007). Conversely, a potential negative regulatory role of SAUR36 in inhibiting leaf growth is hinted by saur36 T-DNA loss-of-function plants displaying larger leaves with expanded epidermal cells (Hou et al., 2013). However, SAUR36 overexpression results in seedlings displaying an extended hypocotyl phenotype (Stamm et al., 2013). This indicates that SAURs can have both promoting or inhibiting effects on cell expansion, and this might be also indicated in our data from RNAseq (Figure 4.2a). This interplay among SAURs, with their potential dual roles, adds layers of complexity to our understanding of their functions. These nuanced regulatory mechanisms underscore the need for comprehensive investigations to unravel the diverse roles played by SAUR family members in tomato SAS. Roig-Villanova et al., 2007 proposed a connection between SAURs and SAS in Arabidopsis. The atypical bHLH proteins, PHYTOCHROME RAPIDLY REGULATED1 (PAR1) and PAR2, lacking a basic domain for DNA-binding, are upregulated by shade, inhibiting SAS. PAR1 interacts with PIF4 to suppress its DNA-binding activity, resulting in the inhibition of SAUR15 and SAUR67, potential direct targets of PIF4; this interaction, coupled with the transient induction of SAUR15 and SAUR67 by shade, suggests a feedback mechanism through PAR1 and PAR2 to mitigate shade-induced SAUR expression (Oh et al., 2014). Hornitschek et al., 2012 also supported the role of SAURs in shade-stimulated elongation growth, identifying SAUR19 subfamily genes and additional SAUR genes as potential direct targets of PIF5. The rapid induction of specific SAUR genes in elongating hypocotyls under far-red enrichment indicates their involvement in cell elongation, contributing to the increased growth observed in shade avoidance. In Chapter 3, we showcase the shade avoidance syndrome induction of SAUR in the stem of S. lycopersicum. Despite limited annotation in S. lycopersicum, in the expression analysis of IAA-related genes (Fig 4.2a), we performed ortholog search for SAURs, and revealed a significant upregulation of many SAURs.
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