Hormone interplay in the regulation far-red-responsive stem elongation in tomato 4 127 Notably, when we tested different concentrations of bioactive gibberellin 3 (GA3), we observed a dose-dependent trend in the elongation of the first internode (Figures 4.13, S4.7). This experiment was performed by brushing GA3 three times (on days 1, 3 and 5 of the light treatment) onto the first internode of tomatoes placed in either WL or WL+FR, and the parameters were measured at 7 days. We found that tomato plants growing in WL responded to a rising GA3 concentration. Namely, the stem length and the first internode length were gradually getting bigger. At 100 μM GA3, the internode 1 elongation reached the level of FR-induced elongation. In supplemental FR, no significant differences were found in stem- and first internode length except for 100 μM GA3. In addition, no significant differences were found in hypocotyl length, hypocotyl diameter and first internode diameter in both the WL and FR light treatment. Therefore, we conclude that GA3 causes an significant elongation response only with 100 μM in FR and WL. 4.2.9 Adding IAA to the GA treatment had minimal effect on elongation We wanted to test if the GA effect was additive with the previously observed mild effect from IAA, and if these effects could also be additive with FR-induced elongation. Hence, we treated plants with both GA and IAA, either 50 or 100 μM of each. All the plants grown under white light (WL) and treated with 100 μM GA3, or with 50 or 100 μM GA3+IAA had longer internode 1 lengths compared to the control group (Figure 4.14). Furthermore, the first internode of plants treated with 50 or 100 μM GA3 and IAA displayed a significantly larger diameter in comparison to the control group. However, the treatment with 100 μM GA3 alone did not lead to a notable difference in the diameter of the first internode. Conversely, no noteworthy disparities were observed in the stem and first internode lengths between plants treated with 100 μM GA3 and those treated with 50 or 100 μM GA3 and IAA. Likewise, there were no significant variations detected in hypocotyl length and diameter. These outcomes collectively indicate that the addition of IAA, when compared to the exclusive GA3 treatment, does not induce any supplementary response (Figure 4.14). However, the 100 μM GA3 and IAA treatment induced an additive internode elongation effect on top of the FR effect. Treating with GA alone also affected cellular morphology (Figure 4.15). GA alone resulted in a reduction in the number of pith cell layers, suggesting an inhibition of cell division by GA. However, this effect was overcome when GA was combined with IAA. Remarkably, under FR conditions, the combination of GA and IAA not only overcame the cell division inhibition but also induced even more cell division (Figure
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