General discussion 6 217 6.2.3 SAUR regulation is not limited only to auxin SAUR genes, in addition to being influenced by auxin, are regulated by brassinosteroids (BR) and gibberellins (GA), suggesting their involvement in diverse hormone-mediated aspects of plant growth (Ren and Gray, 2015). BR, a growth-promoting hormone, shares extensive crosstalk with auxin, converging at the transcriptional level to regulate common target genes, including various SAUR genes (Fàbregas and Caño-Delgado, 2014; Wang et al., 2014). BES1 and BZR1 transcription factors in the BR signaling pathway have been identified as potential regulators of SAUR gene expression. Chromatin immunoprecipitation studies confirm the binding of BZR1 and BES1 to SAUR promoters, implicating SAURs as downstream effectors in BR-mediated expansion growth (He et al., 2005; Yin et al., 2005; Gory Vert et al., 2011). Similarly, GA, another growth-promoting hormone, impacts SAUR gene expression, with DELLAs, central regulators in the GA pathway, influencing SAUR promoters (Claeys et al., 2014; Xu et al., 2014). GA-induced degradation of DELLAs leads to enhanced ARF, PIF, and BZR1/BES1 transcription factor binding to SAUR promoters. Notably, SAUR36 is upregulated by RGA-LIKE2 (RGL2), a major DELLA protein in seed germination regulation, with further research needed to elucidate its specific role in other processes (Feng et al., 2008; De Lucas et al., 2008; Bai et al., 2012). The interaction between BR and auxin signaling is complex and involves overlapping gene activation, synergistic effects, and interdependent interactions. Microarray analyses have shown that BR and auxin activate similar sets of genes, and genetic and physiological studies have demonstrated their collaboration in various developmental processes, including hypocotyl elongation and vascular bundle patterning (Ibañes et al., 2009; Depuydt and Hardtke, 2011; Nemhauser and Torii, 2016). Several molecular mechanisms have been proposed to mediate BR-auxin interactions. BZR1, a key regulator in the BR pathway, controls genes involved in auxin synthesis, transport, and signaling (Sun et al., 2010). Auxin, in turn, activates BR biosynthetic genes and increases BR levels. The protein BIN2 phosphorylates an auxin-response factor (ARF2), and both BZR2 and ARF5 bind to the same promoter involved in BR and auxin responses (Vert et al., 2008; Walcher and Nemhauser, 2012). Additionally, the actin cytoskeleton, regulated by both BR and auxin, plays a role in integrating their responses and feedback regulating auxin transport and BR signaling (Lanza et al., 2012). However, further evaluation is needed to fully understand the physiological implications of these mechanisms in BR-auxin interactions. In chapter 3, we found SAUR upregulation and in chapter 4, we explored the role of hormones, specifically auxin (IAA), gibberellins (GA), and brassinosteroids (BR), in regulating stem elongation during SAS in tomato plants. In our RNAseq analysis,
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