Chapter 4 146 IAA and other hormones, including gibberellins (GA) and brassinosteroids (BR), which are also implicated in SAS (Yang et al., 1996; Djakovic-Petrovic et al., 2007; Keuskamp et al., 2012; Colebrook et al., 2014). 4.3.3 GA and BR induce full elongation, while their inhibitors fully block FR-induced elongation-are they more important than IAA? Therefore, we examined the roles of gibberellins and brassinosteroids, also jointly with auxin in far-red light-induced shade avoidance response in Solanum lycopersicum. GA, BR and GA+BR treated plants in white light exhibited comparable internode elongation responses as tomato plants in supplemental far-red (Figures 4.13, 4.16, 4.17). Internode elongation appeared responsive to GA concentration changes (Figure 4.13). Interestingly, combining GA with IAA did not significantly impact hypocotyl or first internode length/ diameter (Figure 4.14). Experiments with GA and BR treatments demonstrated that these hormones have a more pronounced effect on internode elongation compared to IAA. This observation raises some possibilities. One potential explanation for the observed effects is that GA and BR may activate distinct downstream pathways or engage with pivotal regulators, fostering increased cell expansion and elongation, possibly linked to their divergent impacts on cell wall extensibility or the modulation of cell division processes. But this function is not be influenced by IAA Understanding the molecular mechanisms underlying the distinct elongation responses to GA, BR, and IAA would provide valuable insights into the nuanced roles each hormone plays in plant growth and development. Inhibition of GA and BR biosynthesis, paclobutrazol (PBZ) and brassinazole (BRZ) respectively, led to shortened first internodes in FR-enriched plants (Figure 4.11). The inhibition of GA in the presence of far-red light resulted in the cessation of elongation, emphasizing the crucial role of GA in promoting stem growth during shade avoidance. Both IAA+BR or IAA+GA can overcome the depletion of GA or BR with inhibitor suggest a potential interactions between IAA,BR and GA. We were not able to carry on an experiment with PBZ or BZ complemented with only IAA/BR/GA compared to IAA+BR, IAA+GA, which might help us to confirm if there is a functional redundancy in the absence of IAA. This highlights the potential compensatory role of GA and IAA in overcoming the effects of BR deficiency, further emphasizing the intricate balance between these hormones. In conclusion, our findings highlight the essential role of both GA and BR in FR-induced elongation. Interestingly, the ability of either GA or BR to overcome the inhibition of the
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