General discussion 6 215 induction of many shared target genes and reinforce each other’s activity at those targets (Oh et al., 2012; Oh et al., 2014). The interactions between these transcription factors explain why PIFs and BZR stimulate auxin sensitivity. However, each member of the BAP module also has its specific targets, indicating a complex growth-promoting network of interacting transcription factors stimulated by auxin, gibberellin, and brassinosteroid signaling, while being repressed by active phy and cry photoreceptors (Fu and Harberd, 2003; Ishida et al., 2010; Holalu and Finlayson, 2017; Galvāo et al., 2019). This interplay is important to keep in mind as a reference when interpreting the tomato data in the following sections. 6.2.2 SAURs are responsive during FR-triggered elongation of internode 1 and pith In Chapter 3, focused on RNA-seq analysis of cell elongation, the most enriched pathway emerged as the auxin response Gene Ontology (GO) category, specifically comprising SMALL AUXIN UP RNA (SAUR)s. This finding underscores the significance of SAURs in SAS. Auxin regulates hypocotyl growth in Arabidopsis (Gray et al., 1998; Zhao et al., 2001), and many SAURs are highly expressed in elongating hypocotyls (Franklin et al., 2011; Chae et al., 2012; Chalivendra et al., 2013; Stamm et al., 2013), and it has long been hypothesized that SAURs may be involved in auxin-regulated cell expansion (Stortenbeker and Bemer, 2019). Experimental results supporting a positive role for SAURs in cell expansion were obtained from several recent studies in Arabidopsis. Overexpression of SAUR36, SAUR41, or stabilized fusion proteins of SAUR19 or SAUR63 promotes hypocotyl elongation by enhancing cell expansion. Conversely, seedlings expressing artificial microRNAs targeting SAUR19 or SAUR63 exhibit slightly shorter hypocotyls with smaller epidermal cells, indicating a positive regulatory role for these SAURs in cell expansion (Knauss et al., 2003; Park et al., 2007; Narsai et al., 2011; Qiu et al., 2013). SAURs are key elements of the auxin growth mechanism to facilitate cell expansion that was established for more than 50 years (Spartz et al., 2014). Originating in the 1970s, the acid growth theory was initially introduced to elucidate auxin-mediated cell expansion (Rayle and Cleland, 1970; Rayle and Cleland, 1992; Hager, 2003). The auxin-mediated cell elongation response to various signals depends on the enhanced expression of SAUR19-24 (Ishida et al., 2010). This response involves the stimulation of H+-ATPase proton pumps, leading to rapid apoplast acidification and acid growth. In shade-exposed Arabidopsis petioles, apoplast acidification happens within minutes.
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