植物根并非绝对向重力方向生长。如果根下方是高盐环境，则根会避盐生长，此现象称为“halotropism”。在根避盐过程中，面向高盐的表皮细胞PIN2内吞增强，从而改变生长素分布，但其分子机制还仍未知。前期工作中我们已鉴定编码NPH3-like蛋白的MAB4基因家族，可阻止PIN内吞。此外，高盐处理下MAB4丰度降低，且过表达MAB4阻碍避盐反应。这表明在避盐中MAB4丰度降低可能导致PIN内吞增强。另外，突变MAB4酪氨酸以模拟非磷酸化，发现高盐处理下，其仍可稳定定位于质膜。因此我们假设避盐中酪氨酸磷酸化降低MAB4丰度，导致PIN2内吞。进一步发现，阿米洛利可阻断避盐反应，其可在动物体中阻断感受盐的钠离子通道。可推测阿米洛利敏感的钠离子通道可能作用于生长素上游参与避盐反应。我们已设计一系列实验进一步证明避盐中盐感应和级联反应的分子机制。本项目研究成果将对植物中盐应答的分子机制提供新见解。

Plant roots grow downwardly in the direction of gravity, but this is not unconditional. If plant roots sense high concentration of salt coming up from below, they diminish gravity responses and grow away from high salinity environment. This response of root to salt is called “halotropism”. In halotropism, increased PIN2 internalization in the epidermis facing higher salt concentration is thought to change auxin distribution in response to salt. However, the molecular mechanism of salt sensing, salt signaling, and PIN2 internalization in halotropism is still unknown. We have previously identified MAB4 family genes, encoding NPH3-like proteins that interact with PIN proteins in the plasma membrane, as a blocker of PIN endocytic internalization. Recent our results showed that the abundance of MAB4 family proteins was decreased in response to high salt, whereas overexpression of MAB4 blocked halotropic response. These results indicate that salt-responded reduction of MAB4 family proteins causes PIN2 internalization in halotropism. In addition, unphospho-mimicking mutation of a single tyrosine residue of MAB4 provided protein stability even in the presence of high salt. Furthermore, our results of pharmacological analysis showed that amiloride, a blocker of sodium channels that function as a salt taste sensor in animal, specifically blocked halotorpims, but not gravitropism, indicating that amiloride-sensitive sodium channels are involved in halotropism. Here, we propose a hypothesis that amiloride-sensitive sodium channels sense salt and decreases the abundance of MAB4 family proteins via tyrosine phosphorylation-dependent degradation, leading to PIN2 internalization in halotropism. We have designed a series of experiments to further investigate the molecular mechanism of salt sensing and signaling cascade in halotropism. Our research projectwould provide novel insights into the molecular mechanism of salt response in plants.