Chapter 6 214 Shade avoidance is a well-documented plant response to low light conditions, where plants adjust their growth patterns to compete for sunlight. In shade conditions, plants often exhibit traits like elongated stems and increased apical dominance, focusing on reaching light sources. While shade avoidance primarily affects primary growth (the lengthening of stems and roots), our data in S. lycopersicum in Chapter 2 showed it can also influence secondary growth – either directly, or indirectly by altering the allocation of resources. However, the extent to which shade avoidance directly impacts secondary growth can vary among plant species. In Chapter 5 we observed diameter increase also in Arabidopsis inflorescence stems in response to FR, but no radial growth in response to FR for any other species in our panel. While there is evidence that shade avoidance can influence resource allocation and growth patterns, more research is needed to understand the precise mechanisms and consequences of this relationship on secondary growth across different plant species and environments. 6.2 HORMONAL REGULATION OF TOMATO SAS IN A BROADER CONTEXT 6.2.1 Interplay of hormone signals in Arabidopsis SAS The perception and signaling of low R:FR in Arabidopsis involve complex interactions between photoreceptors, hormones, and transcription factors. In response to low R:FR, auxin concentrations increase rapidly, but this increase is often transient and lost on the second day of treatment (Bou-Torrent et al., 2014; de Wit et al., 2015; Pucciariello et al., 2018). A subsequent increase in auxin sensitivity is required to maintain auxin signaling for a longer duration in low R:FR. Arabidopsis senses low red:far red (R:FR) light conditions in the leaf lamina, which triggers PIF-regulated gene expression and auxin biosynthesis. The auxin signal then travels to the petiole/hypocotyl, where brassinosteroids (BRs) are involved in the elongation responses (Leivar and Monte, 2014; Galvāo et al., 2019; Zhang et al., 2020). Additionally, other hormones may further stimulate growth beyond the first day, such as the increased synthesis of gibberellic acid (GA) through enhanced GA20-OXIDASE (Hisamatsu et al., 2005; Bou-Torrent et al., 2014; Gommers et al., 2017). Increasing GA concentrations promote the degradation of growth-repressive DELLA proteins, which are nuclear-localized repressors that inhibit the activity of many transcription factors, including the BR-responsive growth promoter BZR1 (BRASSINAZOLE-RESISTANT 1) and BES1 (BRI1-EMS-SUPPRESSOR 1), but also ARFs (AUXIN RESPONSE FACTOR) and PIFs. These transcription factors, together with their DELLA repressor, constitute the BZR-ARF-PIF/DELLA (BAP/D) module (Oh et al., 2014). BZR1, ARF6, and PIF4 stimulate cell growth through the
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