Chapter 1 16 In Arabidopsis, shade-induced elongation is regulated by a complex interplay of hormonal signaling pathways. Elongation of the petiole in Arabidopsis involves multiple hormone signaling pathways, including gibberellins, auxins, brassinosteroids, and ethylene. These hormones collectively coordinate cell division, cell elongation, and cell wall modifications to promote elongation in response to shading (Djakovic-Petrovic et al., 2007; MüllerMoulé et al., 2016). Similarly, in tomato, stems elongate in SAS (Figure 1.4) and this process potentially involves key regulators such as gibberellins, auxins, and ethylene, which are have been implicated to play a role (Courbier et al., 2021). Low red/far-red ratios (R:FR) stimulate stem growth-related gene expression and concurrently reduce the expression of genes associated with flavonoid synthesis, isoprenoid metabolism, and photosynthesis. This results in decreased levels of flavonoids and isoprenoid derivatives, alongside a reduction in stem jasmonate levels and photosynthetic capacity. While this model partially explains shade-avoidance responses, further understanding of the complex links between shade and auxin networks is needed. (Cagnola et al., 2012; Iglesias et al., 2018; Schrager-Lavelle et al., 2019; Courbier et al., 2021). However, the intricacies of shade-induced elongation in tomato, particularly the specific molecular mechanisms and signaling components, remain areas that require further investigation. Other dicot plants, such as soybean (Glycine max) and sunflower (Helianthus annuus) also display significant elongation responses in SAS response to shade cues (Green-Tracewicz et al., 2011; Page et al., 2011; Tang and Liesche, 2017; Lyu et al., 2021). The underlying hormonal and molecular mechanisms regulating elongation in these plants are similar to those observed in tomato and Arabidopsis and involve gibberellin and auxin. Figure 1.4. Shade avoidance response leads to a significant elongation response in tomato. Comparison of 21-day-old Moneymaker plants grown in white light (WL) vs far-red light treatment (WL+FR) for 14 days.
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