Cellular anatomy of tomato stems in response to far-red light 2 45 we conducted a microscopy-based examination of internode 1 elongation at the cellular level. Our findings indicate that, during the far-red (FR)-responsive stem elongation in tomatoes, the most pronounced cellular changes involve the elongation of pith cells and the generation of pith layers. Interestingly, our cross-sectional analysis revealed no alterations in the pith cell area, indicating that pith cells expanded exclusively in the longitudinal direction. We characterised in detail the FR-induced changes on juvenile tomato plants. 2.3.1 Cultivar-specific variations in leaf area changes under low R:FR Overall, our characterization of tomato aligns with the generally expected SAS phenotype. Previous reports on tomato SAS are limited, but a preceeding study on tomato cultivar Moneymaker in older growth stages (5-6 weeks) also demonstrated the FR-responsive stem elongation and leaf thickness changes (Courbier et al., 2021). Our phenotyping data on stem length and leaf thickness confirmed these findings. Secondly, a previous report (Bush et al., 2015) showed leaf area increases and leaf edge changes in FR. We observed a FR-responsive leaf area increase only in M82 and not Moneymaker. Therefore, leaf area response in SAS might be a cultivar-specific feature. More cultivars are needed to studied for a better conclusion of how the leaf areas change in SAS. 2.3.2 Cell elongation in SAS occurs predominantly in the pith in tomato SAS has been characterized in great detail in Arabidopsis, with elongation phenotypes for hypocotyl and petiole that show resemblance to the response observed in tomato. Investigations into SAS have revealed a key phenomenon in Arabidopsis: XTH (XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE) cell wallmodifying proteins play a role in relaxing the cell wall, facilitating cell expansion in response to shading (Sasidharan et al., 2010; Sasidharan et al., 2014). This process plays a pivotal role in facilitating petiole elongation under FR light (Sasidharan et al., 2010). In addition, epidermis cells of hypocotyl (18d) (Procko et al., 2016) and petiole (28d) (Pantazopoulou et al., 2017) in Arabidopsis were found to elongate in FR light treatment. However, the Arabidopsis and tomato have different tissue morphologies and growth habits. Hence, direct comparison between species is difficult. This leads us to ask the question: how do these different cell types play a role in stem elongation,
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