Higher plants survive terrestrial water deficiency and fluctuation by arresting cellular activities (dehydration) and resuscitating processes (rehydration). However, how plants monitor water availability during rehydration is unknown. Although increases in hypo-osmolarity-induced cytosolic Ca²⁺ concentration (HOSCA) have long been postulated to be the mechanism for sensing hypo-osmolarity in rehydration^1,2, the molecular basis remains unknown. Because osmolarity triggers membrane tension and the osmosensing specificity of osmosensing channels can only be determined in vivo^3,4,5, these channels have been classified as a subtype of mechanosensors. Here we identify bona fide cell surface hypo-osmosensors in Arabidopsis and find that pollen Ca²⁺ spiking is controlled directly by water through these hypo-osmosensors—that is, Ca²⁺ spiking is the second messenger for water status. We developed a functional expression screen in Escherichia coli for hypo-osmosensitive channels and identified OSCA2.1, a member of the hyperosmolarity-gated calcium-permeable channel (OSCA) family of proteins⁶. We screened single and high-order OSCA mutants, and observed that the osca2.1/osca2.2 double-knockout mutant was impaired in pollen germination and HOSCA. OSCA2.1 and OSCA2.2 function as hypo-osmosensitive Ca²⁺-permeable channels in planta and in HEK293 cells. Decreasing osmolarity of the medium enhanced pollen Ca²⁺ oscillations, which were mediated by OSCA2.1 and OSCA2.2 and required for germination. OSCA2.1 and OSCA2.2 convert extracellular water status into Ca²⁺ spiking in pollen and may serve as essential hypo-osmosensors for tracking rehydration in plants.

高等植物通过阻止细胞活动（脱水）和复苏过程（补水）来应对陆地水分缺乏和波动。然而，植物在补水过程中如何监测水的可用性尚不清楚。尽管低渗透压诱导的胞质 Ca ²⁺ 浓度增加 (HOSCA) 长期以来一直被认为是在补液 ^1,2 中感知低渗透压的机制，但其分子基础仍然未知。由于渗透压触发膜张力，并且渗透传感通道的渗透传感特异性只能在体内确定 ^3,4,5 ，因此这些通道被归类为机械传感器的亚型。在这里，我们鉴定了拟南芥中真正的细胞表面低渗透压传感器，并发现花粉 Ca ²⁺ 尖峰是通过这些低渗透压传感器直接受水控制的，也就是说，Ca ²⁺ 尖峰是水状况的第二信使。我们在大肠杆菌中开发了低渗透敏感通道的功能表达筛选，并鉴定了 OSCA2.1，它是高渗透压门控钙渗透通道 (OSCA) 蛋白家族的成员 ⁶ 。我们筛选了单敲除和高阶 OSCA 突变体，并观察到 ​​osca2.1/osca2.2 双敲除突变体在花粉萌发和 HOSCA 方面受到损害。 OSCA2.1 和 OSCA2.2 在植物和 HEK293 细胞中充当低渗透敏感 Ca ²⁺ 通透通道。培养基渗透压的降低增强了花粉 Ca ²⁺ 振荡，这是由 OSCA2.1 和 OSCA2.2 介导的，是发芽所必需的。 OSCA2.1 和 OSCA2.2 将细胞外水状态转化为花粉中的 Ca ²⁺ 加标，并可作为跟踪植物补液的重要低渗透压传感器。