Cell and Developmental Biology Section, Division of Biological Sciences, University of California San Diego, La Jolla, CA 92093.
Institute of Technology, University of Tartu, Tartu 50411, Estonia.
Proc Natl Acad Sci U S A. 2021 Nov 23;118(47). doi: 10.1073/pnas.2107280118.
Stomatal pores close rapidly in response to low-air-humidity-induced leaf-to-air vapor pressure difference (VPD) increases, thereby reducing excessive water loss. The hydroactive signal-transduction mechanisms mediating high VPD-induced stomatal closure remain largely unknown. The kinetics of stomatal high-VPD responses were investigated by using time-resolved gas-exchange analyses of higher-order mutants in guard-cell signal-transduction branches. We show that the slow-type anion channel SLAC1 plays a relatively more substantial role than the rapid-type anion channel ALMT12/QUAC1 in stomatal VPD signaling. VPD-induced stomatal closure is not affected in / double mutants that completely disrupt stomatal CO signaling, indicating that VPD signaling is independent of the early CO signal-transduction pathway. Calcium imaging shows that osmotic stress causes cytoplasmic Ca transients in guard cells. Nevertheless, //// Ca-permeable channel quintuple, /-channel double, /-channel double, -channel single, /-channel double, /-channel double, kinase quintuple, //// quintuple, and / double mutants showed wild-type-like stomatal VPD responses. A B3-family Raf-like mitogen-activated protein (MAP)-kinase kinase kinase, M3Kδ5/RAF6, activates the OST1/SnRK2.6 kinase in plant cells. Interestingly, B3 Raf-kinase and /// (///) quadruple mutants, but not a 14-gene mutant including osmotic stress-linked B4-family Raf-kinases, exhibited slowed high-VPD responses, suggesting that B3-family Raf-kinases play an important role in stomatal VPD signaling. Moreover, high VPD-induced stomatal closure was impaired in receptor-like pseudokinase GUARD CELL HYDROGEN PEROXIDE-RESISTANT1 (GHR1) mutant alleles. Notably, the classical transient "wrong-way" VPD response was absent in mutant alleles. These findings reveal genes and signaling mechanisms in the elusive high VPD-induced stomatal closing response pathway.
气孔会迅速关闭，以应对由低空气湿度引起的叶气蒸气压差（VPD）增加，从而减少过多的水分流失。介导高 VPD 诱导气孔关闭的水力信号转导机制在很大程度上仍不清楚。通过对保卫细胞信号转导分支的高级突变体进行时间分辨气体交换分析，研究了气孔对高 VPD 的快速响应动力学。我们表明，慢型阴离子通道 SLAC1 在气孔 VPD 信号转导中比快速型阴离子通道 ALMT12/QUAC1 发挥更重要的作用。在完全破坏气孔 CO 信号的 // 双突变体中，VPD 诱导的气孔关闭不受影响，表明 VPD 信号转导独立于早期 CO 信号转导途径。钙成像显示，渗透胁迫会导致保卫细胞质内钙瞬变。然而，//// 钙通透通道五重突变体、//-通道双突变体、//-通道双突变体、-通道单突变体、//-通道双突变体、//-通道双突变体、激酶五重突变体、//// 五重突变体和 // 双突变体表现出与野生型相似的气孔 VPD 反应。B3 家族 Raf 样丝裂原激活蛋白激酶（MAPK）激酶激酶 M3Kδ5/RAF6 在植物细胞中激活 OST1/SnRK2.6 激酶。有趣的是，B3 Raf 激酶和///（///）四重突变体，但不是包括与渗透胁迫相关的 B4 家族 Raf 激酶在内的 14 个基因的突变体，表现出较慢的高 VPD 响应，表明 B3 家族 Raf 激酶在气孔 VPD 信号转导中发挥重要作用。此外，在受体样拟激酶 GUARD CELL HYDROGEN PEROXIDE-RESISTANT1（GHR1）突变体等位基因中，高 VPD 诱导的气孔关闭受损。值得注意的是，在 突变体等位基因中，经典的瞬态“错误方式”VPD 响应缺失。这些发现揭示了在难以捉摸的高 VPD 诱导气孔关闭反应途径中的基因和信号转导机制。
