Abstract
Since the early days of Darwin, monocot coleoptiles have been used to investigate indole-3-acetic acid (IAA) production, polar transport, and tropisms. This tropic response, as explained by the Cholodny-Went Hypothesis, is the result of differential growth on the two sides of the elongating shoot, which can be explained by the asymmetrical IAA distribution. Here, using maize coleoptiles, we first showed that the asymmetric IAA flow after gravi-stimulus directly affects the TIR/AFBs-mediated auxin signaling pathway, which results in tropic curvature. ZmPIN1(s) immunohistochemical analyses revealed that ZmPIN1(s) was present in almost all cells, but a basal cellular localization at lower regions. Application of the IAA transport inhibitors NPA and BFA at the very tip region almost completely inhibited IAA movement from the tip. These inhibitors also severely suppressed gravitropic bending. PEO-IAA (α-(phenylethyl-2-one)-IAA), an auxin antagonist that binds to TIR1/AFBs, suppressed not only the expression of an auxin responsible ZmSAUR2 gene, but also gravitropic curvature. Expression of ZmSAUR2 was up-regulated on the lower side and down-regulated on the upper side of the elongation zone, corresponding to the asymmetric IAA distribution. These results indicate that the asymmetric downward streams of IAA control the differential growth rate of the cells by attenuating TIR1/AFBs-mediated auxin response genes, including ZmSAUR2, and therefore result in tropic curvature.
