Exploring conservation of cellular-level traits in shade avoidance syndrome among species 5 165 To address these challenges, certain seed plants evolved an additional stem cell population known as the vascular cambium. This remarkable adaptation gave rise to a hollow cylinder within the plant’s stems, facilitating radial growth of primary axes (Venning, 1949). This phenomenon, termed secondary growth, independently evolved multiple times in seed plants, suggesting that factors governing primary growth were repurposed to coordinate secondary growth (Melzer et al., 2008). Mechanical strength was not solely achieved through stem thickening via vascular cambium activity; it also involved specialized cell types with thickened cell walls found within the two tissues— the phloem and xylem—that the vascular cambium generates (Etchells et al., 2012). This growth flexibility serves as a crucial survival strategy in diverse stressful conditions. The detailed study and reporting of pith cell elongation (Chapter 2) in the context of the SAS is relatively unexplored territory. Pith, a tissue located in the center of dicotyledonous stems and composed of undifferentiated parenchyma cells (Zabel and Morrell, 2020). While pith is found in dicots, monocots also have pith, which extends in the center of monocot roots, composed of parenchyma (Roodt et al., 2019). The diversity of pith in vascular plants can provide insights into their evolution. Medullary bundles, which are vascular units in the pith, have evolved multiple times in vascular plants (da Cunha Neto et al., 2020). The development of the primary vascular system within Nyctaginaceae has been studied, and it was found that medullary bundles diversified within the family (da Cunha Neto et al., 2020). In the Early Devonian euphyllophyte Leptocentroxyla, a protoxylem pathway to the evolution of pith has been hypothesized (Tomescu and Mcqueen, 2022). The delayed and shortened protoxylem differentiation hypothesis explains the evolution of pith by delaying the onset of differentiation and lengthening cell growth duration in a central protoxylem strand, and shortening the interval of differentiation of those tracheids, leading to the evolution of pith (Tomescu and Mcqueen, 2022). Here, we are able to limit our investigation of pith inside of dicots, where diversity of pith patterns was also observed. By exploring the relationship between SAS and pith elongation, we hope to query the conservation of pith elongation within dicots’ adaptation strategies. 5.1.3 Target dicot species for studying evolution of FRresponses in the stem In this chapter, our aim is to see how conserved the internode FR-responses are within dicots with stem growth habit. We subjected seven plant species from four families (Figure 5.1) —Pisum sativum (Pea) and Glycine max (Soybean) from Fabaceae,
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