Asymmetric auxin distribution is instrumental for the differential growth that causes organ bending on tropic stimuli and curvatures during plant development. Local differences in auxin concentrations are achieved mainly by polarized cellular distribution of PIN auxin transporters, but whether other mechanisms involving auxin homeostasis are also relevant for the formation of auxin gradients is not clear. Here we show that auxin methylation is required for asymmetric auxin distribution across the hypocotyl, particularly during its response to gravity. We found that loss-of-function mutants in Arabidopsis IAA CARBOXYL METHYLTRANSFERASE1 (IAMT1) prematurely unfold the apical hook, and that their hypocotyls are impaired in gravitropic reorientation. This defect is linked to an auxin-dependent increase in PIN gene expression, leading to an increased polar auxin transport and lack of asymmetric distribution of PIN3 in the iamt1 mutant. Gravitropic reorientation in the iamt1 mutant could be restored with either endodermis-specific expression of IAMT1 or partial inhibition of polar auxin transport, which also results in normal PIN gene expression levels. We propose that IAA methylation is necessary in gravity-sensing cells to restrict polar auxin transport within the range of auxin levels that allow for differential responses.

生长素的不对称分布有助于植物生长期间差异生长，该差异生长导致器官在热带刺激下弯曲并弯曲。生长素浓度的局部差异主要是通过PIN生长素转运蛋白的极化细胞分布来实现的，但是尚不清楚涉及生长素稳态的其他机制是否也与生长素梯度的形成有关。在这里，我们表明生长素甲基化对于整个下胚轴的不对称生长素分布是必需的，特别是在其对重力的响应过程中。我们发现拟南芥IAA羧甲基甲基转移酶1（IAMT1）过早地展开了根尖钩，并且它们的下胚轴在重力重新定向过程中受到了损害。此缺陷与PIN基因表达中的生长素依赖性增加有关，从而导致极性生长素转运增加，并且在iamt1突变体中缺乏PIN3的不对称分布。IAMT1的内胚层特异性表达或极性生长素转运的部分抑制可恢复iamt1突变体的重力定向，这也可导致PIN基因表达水平正常。我们建议在重力感应细胞中IAA甲基化是必要的，以将极性植物生长素运输限制在允许差异反应的植物生长素水平范围内。