Chapter 2 46 and how are these developmental changes regulated at the molecular level? What is the uniqueness of pith cell compared to the collenchyma and parenchyma? What is the similarity between pith and epidermis in cell elongation activities? We are going to further explore this questions in following chapters. The comparison of our results to the well-charaterized Arabidopsis petiole SAS is complicated by the cell types present. The cellular morphology of Arabidopsis petiole and tomato stem are different in that petiole does not have pith cells, and the only structure of Arabidopsis containing pith cells is the inflorescence stem. Pith cells are spongy parenchyma cells, which provide support and nutrient storage for the stem and root (Abercrombie et al., 1968; Gallego-Giraldo et al., 2016; Yang et al., 2016). According to literature, the extensibility of outer layers is less than that of pith. The low water potential in the xylem has the effect of diminishing the transmission of tensile forces by compressed cells in the stem cortex to the epidermis, which is believed to be the critical cell layer for controlling extension growth. As the xylem water potential decreases, cells in the cortex undergo a reduction in turgor pressure (Brown et al., 1995a), and epidermis acts as a signal receptor to promote and restrict growth of the entire shoot (Savaldi-Goldstein and Chory, 2008). In this chapter, we show that the pith was very responsive to FR enrichment: more pith cell layers were generated and the pith cells elongated in response to FR enrichment. The epidermis cells elongated as well but less significantly compared to pith, while the other cell types in the internode 1 did not show FR-responsive elongation. Therefore, how did these cell types receive the SAS signals and mediate the SAS elongation response is worth investigation. Therefore, we continued to look into transcriptomic changes to understand the regulation of FRresponsive internode elongation in tomato. Another thing about pith is that this tissue is not present in all dicot and monocot species. Pith conducts progressive programmed cell death in many species including tomato (Esau, 1953; Fujimoto et al., 2018). This raises many questions for us about the role of pith in in FR-responsive internode elongation. If pith elongation consistently occurs during SAS elongation, what distinguishes it from elongation in other cell types? Do species with pith share this feature? Do species without pith elongate through different mechanisms? Some of these questions are explored further in the later chapters of this thesis.
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