Chapter 5 192 diameter appears to be unresponsive or even reduced in FR in our set up, leading to a fragile long stem (Figure 5.2 c), which is a common problem in SAS that plants become very vulnerable to lodging. Meanwhile, Arabidopsis also shows limitations in secondary growth (Turnerag1 and Somervilleb, 1997; Oh et al., 2003; Paul-Victor and Rowe, 2011). This could be a plausible explanation for the weak FR-response of stem elongation in Arabidopsis. After investigating Arabidopsis, we then sought to unravel whether the FR-induced stem elongation and associated cellular changes would be conserved in other dicot species with stem growth habit. Overall, we sampled S. lycopersicum, C. annuum, P. sativum, G.max, S. melongena, B. nigra, and O. basilicum. Most of the species we looked at were responsive to FR in their internode elongation (Figures 5.4-5.9). In contrast, we also found that P. sativum and O. basilicum were only mildly or not responsive to FR, and neither showed secondary growth despite the development of lateral meristem (Ko et al., 2004; Skubisz et al., 2007). Is there a relationship between radial growth, elongation growth and pith cell expansion and elongation? We still don’t have a clear answer. Herbaceous (non-woody) plants mostly undergo primary growth, with very little secondary growth, However, some nonwoody plants, such as potato tubers, carrot taproot, and sweet potato tuberous root, do undergo remarkable secondary growth (Hardwick and Elliott, 2016; Groover, 2023). Therefore, the occurrence of secondary growth in plants varies depending on the species, and it is not a universal feature of all plants. Recent studies have explored the genetic space of radial plant growth in Arabidopsis thaliana (Greb, 2019), but the question remains of how conserved it is among dicot species. Pith cell elongation is one of the clearest responses we found in our shade avoiding species whose stems elongated. While specific information regarding the conservation of pith cell elongation in Arabidopsis and other plants is not readily available, it is known that the expansion of stem tissues, including pith cells, is influenced by various factors such as hormonal signaling and genetic regulation (Nagata et al., 2004; Ye et al., 2021). 5.3.2 The responsiveness of the three TFs to FR is unique to the Solanaceae family In addition to investigating the conservation of plant architectural and cellular traits in response to FR, we also wanted to see if some of the TFs we found as novel FR-responsive genes that are regulated in the tomato internode (Chapter 3) were conserved across FRresponsive dicots. Understanding the extent of the conservation of these TFs’ expression patterns can shed light on evolution of the regulatory networks of stem elongation and potentially help us understand the cell activities in SAS.
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