Chapter 3 58 0 10 20 30 0 20 40 60 80 100 Day Internode 1 length (mm) WL+FR MM WL MM WL+FR M82 WL M82 0 10 20 30 0 2 4 6 8 10 Day Diameter (mm) WL+FR MM WL MM WL+FR M82 WL M82 (a) (b) Figure 3.1. Tomato first internode length and diameter changes over time. (a) First internode length of tomato cultivars Moneymaker (MM) and M82 over 28 days of 7D supplemental far-red treatment. (b) Diameter of first internode of MM and M82 over 28 days of far-red treatment. WL = white light, WL+FR = white light supplemented with far-red. As discussed in the previous chapter, plant stems are composed of various cell types. Each cell type has a unique structure and macromolecular composition that allows it to carry out specific functions. For instance, the cambium consists of undifferentiated meristem cells that produce new cells for specialized functions. The response of different cell types to environmental signals varies, and specific pathways are activated depending on the cell type (Miller Coyle, 2004). This knowledge is crucial in understanding the role of plant stems in growth and development, as well as in the response to external stimuli (Kutschera, 2001; Kozuka et al., 2010; Mack and Davis, 2015). Stem growth is composed of various cell activities that lead to stem elongation and stem expansion. These pathways are modulated by multiple hormones, as shown in Figure 3.2, which demonstrates the interplay of different hormones in stem growth. In the earlier stage of primary growth, the shoot apical meristem is the major regulator of stem elongation. Therefore, genes such as ARGOs (Auxin-Regulated Gene involved in Organ Size) and EBP1 (Erbb-3 Binding Protein 1) play key roles in maintaining meristematic competence regulated by sucrose, auxin, and cytokinin (Mizukami and Fischer, 2000; Hu et al., 2003; Horváth et al., 2006). In the lateral growth of the stem, when the lateral meristem is involved, more complex signaling pathways are required. Auxin still dominates stem growth in secondary growth, but it is essential to have strigolactone signaling to mediate auxin function in secondary growth, and jasmonate positively regulates vascular cambium (Perrot-Rechenmann, 2010; Sehr et al., 2010; Agusti et al., 2011; Chandler and Werr, 2015; Ma and Li, 2019). In the cambium activities, WOX and CLAVATA are intensively studied in Arabidopsis (Hirakawa et al., 2010; Wang et al., 2018). In our model, tomato, clear secondary growth patterns were reported (Venning, 1949; Thompson and Heimsch, 1964). Auxin,
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