Hormone interplay in the regulation far-red-responsive stem elongation in tomato 4 145 4.3.2 IAA can induce partial elongation response Building on previous knowledge of SAS regulation with hormones and supported by the GO enrichment of RNA sequencing data (Chapter 3) and the behaviour of auxinrelated genes (Figure 4.2a), we hypotheses that IAA has a major function in SAS. We, therefore, first continued with examining the effect of IAA on internode elongation. Although the primary source of auxin production is understood to be in young leaves and meristems, the exact origin and distribution of auxin during the shade avoidance response remains elusive (Chandler, 2009). The hypothesis that the first and second true leaves and the meristem are prime candidates due to their elevated position in the plant was supported by our results, as IAA treatment on the first true leaf and first internode both caused similar increases in hypocotyl and first internode length (Figure 4.5, S4.3). In local internode IAA treatment, all tested concentrations induced the same elongation response (Figure 4.6), leaving the optimal application concentration uncertain. At the microscopic level, IAA treatment resulted in increased cell elongation but did not significantly result in a high level of total elongation of the internode to recapitulate FR (Figure 4.7). However, we observed no change in the overall IAA concentration of first internode or first true leaf at 1h, 2h and 6h treatments with supplemental FR (Figure 4.12). Notably, the inhibitors PEO-IAA and BBo also did not significant affects supplemental FR-induced elongation (Figure 4.10, 4.11, S4.7), which indicates either that the application was not effective (did not penetrate to the necessary tissues, or did not target the pathway in tomato), or that IAA regulated pathway are regulated by the feedback loop of other hormones in the tomato FR-induced internode elongation. Overall, we were not able to find a clear indication that IAA level is increased in FR response building upon current experiment. Interestingly, the auxin transport inhibitor NPA decreased the stem diameter in comparison to the mock treatment (Figure 4.8). Disturbance of IAA transport made plants tend to bend over due to fragility (Figure 4.8a). The results imply that auxin plays a role in regulating stem growth to ensure proper support. The NPA effect is likely causing an auxin imbalance in the first internode. Above this region, higher NPA concentrations hindered development and caused leaf curling, accompanied by a surge in hypocotyl length (Figure 4.8 S4.6). However, due to the complexity of IAA transport, we cannot further infer the distribution dynamic of IAA concentration. Collectively, these findings indicate that auxin alone does not replicate the plant growth observed during the shade avoidance response. Hence, we followed up by investigating if the FR-induced internode elongation could be explained by hormone balance between
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