Cellular anatomy of tomato stems in response to far-red light 2 47 2.3.3 Shade avoidance affects both primary and secondary growth, but the relationship between these two processes remains unknown. The correlation of secondary growth, measured via interfascicular cambium thickening, with internode 1 elongation in SAS is another new insight. Plant growth is divided into primary growth (increases the length or height of the plant) and secondary growth (increases the diameter of the plant). Secondary growth is formed by lateral meristems; vascular cambium and cork cambium (Etchells & Turner, 2009). Vascular cambium is a closed ring of lateral meristematic tissue in the stem. Vascular cambium is composed of cells which proliferate and then based on location, differentiate to xylem on the inside or to phloem on the outside of the cambium (Etchells & Turner, 2009). Proliferation and differentiation of the vascular cambium lead to formation of secondary phloem and xylem (Mazur et al., 2014). However, in our data, we were not able to distinguish a significant difference in the xylem proportion of the vascular bundle. Interfascicular cambium is defined as cambium that fuses to form a continuous ring in the stem (Ragni and Greb, 2018; Etchells et al., 2019). In our study, we observed an increase in interfascicular cambium thickness under the supplemental FR treatment, indicating its active proliferation; however, we did not observe additional xylem development, suggesting that differentiation into xylem is inhibited at this stage. This inhibition of cambium differentiation activity has been reported by recent research in Arabidopsis (Ghosh et al., 2022). The authors proposed a model in which dark conditions lead to the accumulation of PIFs (PHYTOCHROME INTERACTING FACTORs), resulting in the inhibition of vascular differentiation and secondary cell wall deposition through the activation of CLE44 (CLAVATA3/ESR-RELATED 44). Conversely, the presence of light rapidly deactivates PIFs, consequently reducing CLE44 transcription and promoting xylem differentiation (Ghosh et al., 2022). Botterweg-Paredes et al. (2020) showed that tomato, dill, and carrot exhibit significant hypocotyl elongation when exposed to white light supplemented with FR. This response is accompanied by changes in vascular organization and the development of larger tracheary elements in tomato and carrot plants. However, not all shade-responsive species, such as dill, display an increase in tracheary elements (Botterweg-Paredes et al., 2020). There are currently no reports of how stem elongation (primary growth) and interfascicular cambium activity (secondary growth) relate to each other. Literature on how secondary growth links with pith is also very scarce, and as we observed a radial thickening coming from increased pith thickness, we are interested to learn how pith is associated with secondary growth in SAS.
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