Chapter 4 134 4.2.13 Interplay of IAA, GA and BR Given the significant elongation response observed with the application of GA, BR and IAA individually, we wanted to evaluate the effects of the combined treatment of GA+IAA+BR on plant growth (Figure 4.19). Specifically, internode 1 was brushed three times over a week with either 50 or 100 μM of the treatment. The results showed that the combined treatment induced similar elongation levels on internode 1 compared to the FR treatment. This is an interesting contrast to the GA only and GA+BR combined treatments (Figure 4.18), where internode elongation exceeded that of FR-induction. This possibly indicates hormones have addictive effect, since auxin potentially has a restricting role in the elongation and GA with high concentration in our experiment was able to induce elongation. Additionally, a noticeable and significant elongation was observed in the hypocotyl, which could potentially be attributed to the effect of BR because this phenomenon was not observed with IAA or GA treatment alone. Combined treatment involving GA, IAA, and BR significantly promotes cell elongation, surpassing the elongation induced by FR treatment alone (Figure 4.20). In our endeavour to comprehensively explore the effects of GA, IAA, and BR, we sought to investigate whether the suppression of supplemental FR-induced elongation by applying PBZ or BZ could be rescued by the other two hormones. Building upon insights from our previous results and assumptions, we formulated a hypothesis to determine if the combination of two hormones could rescue the blockage caused by the absence of one hormone. We had previously shown that GA treatment in WL leads to internode elongation greater than FR (Figure 4.15), and that inhibiting BR signaling by BZ abolishes the FR-induced internode elongation (Figure 4.11). We investigated if GA with IAA would overcome the BZ-induced inhibition, and also vice versa. We conducted an experiment using the soil penetration method. Four days before supplemental FR, we stopped watering and added 40 ml of the inhibitors, PBZ or BZ, to the soil two days prior to the experiment. During the experiment, we applied either IAA+BR with PBZ, or IAA+GA with BZ under supplemental FR light conditions (Figure 4.21). Here, we observed that GA+IAA treatment in supplemental FR, while inhibiting BR with BZ, led to internode elongation even stronger than supplemental FR alone. This indicates that GA+IAA treatment can rescue the internode elongation in BR inhibition, suggesting that brassinosteroid is not needed for these two hormones to promote elongation. Conversely, BR+IAA treatment in supplemental FR combined GA inhibition via PBZ led to internode elongation similar to the effect of supplemental FR. This indicates that BR-induced internode elongation is not dependent on GA availability, or that BR+IAA treatment can rescue the internode elongation during GA inhibition.
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