Hormone interplay in the regulation far-red-responsive stem elongation in tomato 4 123 We also investigated the effects of NPA treatment on the cellular morphology in the stem. The application of 50 μM of NPA resulted in a decrease in cell length in all cell types under both WL and WL+FR treatments (Figure 4.9). Conversely, at a higher concentration of 125 μM of NPA, we observed a slight increase in cell lengths. Furthermore, the thickness of the pith decreased in response to NPA application at both concentrations, regardless of the light treatment (Figure 4.9). Interestingly, at 125 μM of NPA, there was a significant decrease in the number of pith cells. This finding suggests that NPA or its effects on auxin localization may inhibit cell division. Overall, the effects of NPA were not what we had hypothesised. Instead of blocking the FR-responsive elongation, we observed strong internode 1 elongation even at WL. We considered alternative explanations, and one was that the NPA treatment in the internode would cause IAA to pool in the internode. In other words, instead of NPA inhibiting IAA from entering, NPA would lock it in. To try an alternative approach to circumvent the possible side effects of our NPA treatment, we tried also other IAA inhibitors. 4.2.6 Auxin antagonist PEO-IAA and biosynthesis inhibitor BBo did not affect stem elongation Next, we tested the effect from an application with PEO-IAA, an auxin antagonist (Hayashi et al., 2008). PEO-IAA interacts with TIR1/AFB (TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX) proteins similarly to -alkyl-IAA, and leads to an anti-auxin effect on auxin-responsive gene expression, cell division, and elongation pathways (Šenkyřík et al., 2023). In this experiment, we tested different concentrations of PEO-IAA applied to the first internodes of tomato plants in either WL or WL+FR conditions on one day before FR treatment (Figure 4.10). Above, we showed that IAA application triggered a mild stem elongation response. Unexpectedly, no significant differences were observed in stem elongation with any concentration of PEO-IAA. These outcomes suggest that PEO-IAA does not influence stem elongation in tomato, and this could be attributed to unsuccessful penetration. Alternatively, it may be indicative of a limited responsiveness of tomato plants to PEO-IAA.
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