Chapter 6 210 characteristics of its cells, contributes to its functions in nutrient storage, transport, and support within the plant, but the molecular mechanisms underlying these pith functions remain largely uncharacterized. Recent research has unveiled new roles for pith. Firstly, the intercalation of the xylem with the pith to separate xylem strands from each other decreases the risk of cavitation (Bouda et al., 2022), making complex pith patterns an evolutionary advantage in arid environments. Secondly, pith drives primary thickening growth (the diameter and strength of the stem structure) and culm size in bamboo underground shoots (Wei et al., 2017). The spiral growth of bamboo internodes, a manifestation of shoot wall cell expansion, exerts anisopleural force on the internal pith tissue, ultimately leading to the asymmetrical formation of the pith, this is an example of the unique growth niche of pith (Guo et al., 2019; Long et al., 2023). In this thesis, we saw radial pith divisions in FR-treated tomato stems, hand in hand with radial thickening of the elongating internode (Figures 2.6, 2.16), potentially linking pith with radial growth also in tomato. The pathway of pith growth remains elusive, raising questions about its origin. It is commonly assumed that pith cells originate from the rib meristem, located at the top of the shoot and in close proximity to the shoot apical meristem (SAM). In this thesis, local treatment of SAM with far-red light induced a similar elongation effect as the overall farred treatment (Figure 3.2). This observation suggests that the rib meristem, particularly in the early stages of growth, could serve as a trigger for pith generation. In Chapter 2, we found that pith elongated and thickened radially in response to FR in tomato stems, and in Chapter 3 we generated pith-specific transcriptome data. When comparing the transcriptomes of pith and the whole internode in our data, we saw pithspecific pattern of multiple transcription factor (Sasidharan et al., 2010)(Table S.3.5 red color) encoding genes and EXPANSINs. This same observation can be made from Arabidopsis pith transcriptome (Shi et al., 2020). This suggests a noteworthy pattern that does not effectively prove pith function but does has an evolutionary significance of pithrelated processes conserved across species. An early study on testing the presence of cAMP in plants actually utilized excised tobacco pith tissue on auxin-containing media (Lundeen et al., 1973). In this setup, auxin induced both cell enlargement and DNA replication phases of the cell cycle, along with cytokinesis (Lundeen et al., 1973). Similarly, our Gene Ontology (GO) analysis (Figure 3.7) revealed a substantial upregulation in DNA replication and cell activities upon FR treatment in pith, offering molecular support to the cell divisions and elongation observed in Figure 2.16.
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