General discussion 6 211 As the tissue ages in some plants, the pith in the middle of the stem may dry out and disintegrate, resulting in a hollow stem (Abercrombie et al., 1968; Kutschera and Köhiler, 1992; Brown et al., 1995b). The formation of the pith cavity, intricately linked to programmed cell death and governed by diverse physiological and molecular processes, is caused by stress conditions (Guo et al., 2019; Wang et al., 2022). Pith cavities in plant stems, such as those found in bamboo, are not unexpected, as they can form due to various factors, including disease, aging, and environmental conditions (Wei et al., 2017). Wheat stems typically exhibit solidity in the nodal region, with the formation of an internodal cavity occurring as pith cells undergo programmed cell death during internode elongation. The genes influencing stem pith thickness are likely associated with pith cell death and cell wall composition. For instance, modulating pith thickness can be achieved by either activating or inhibiting programmed cell death (which is also observed in sorghum) (Fujimoto et al., 2018) or altering cell wall composition, leading to increased stem cell wall thickness and lignin content (Kong et al., 2013; Liu et al., 2023b) (Kong et al., 2013).The significance of the pith and its programmed cell death extends beyond monocots, as evidenced by the presence of pith cavities also in dicots. In Figure 6.1, we observed a dicot Vicia faba forming a pith cavity early in the development of the third internode, while the older internodes remained filled with pith. This could offer a dicot model to study the role of pith cavity in phenotypic plasticity of the stem. 6.1.2 Pith cell elongation, a key contributor to stem elongation in multiple species within SAS We observed a range of pith responses exhibited by different plant species when it comes to shade avoidance. We first noticed a pronounced pith cell elongation and radial divisions in FR in S. lycopersicum cultivars Moneymaker and M82 (Chapter 2). In the 8 species we selected (Chapter 5), most of them demonstrated typical stem elongation and similar to tomato, pith cell elongation in response to reduced R:FR. In SAS, the persistent correlation between pith elongation and stem elongation underscores its conservation across the panel. While radial growth of pith was observed in several species, its significance varied, leading us to hypothesize that pith activities during rapid growth primarily emphasize longitudinal direction. The presence or absence of pith in plant stems is influenced by various factors related to the plant’s growth habit, evolution, and ecological niche. One key reason for its presence in herbaceous plants, such as the species in our panel, is its role in nutrient storage and transport. Herbaceous plants often require the capacity to store nutrients and water for
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