背景回顾：To survive fluctuating water availability on land, terrestrial plants must be able to sense water stresses, such as drought and flooding. The plant hormone abscisic acid (ABA) and plant-specific SNF1-related protein kinase 2 (SnRK2) play key roles in plant osmostress responses. 

前期基础：We recently reported that, in the moss Physcomitrium patens, ABA and osmostress-dependent SnRK2 activation requires phosphorylation by an upstream RAF-like kinase (ARK). This RAF/SnRK2 module is an evolutionarily conserved mechanism of osmostress signaling in land plants. Surprisingly, ARK is also an ortholog of Arabidopsis CONSTITUTIVE RESPONSE 1 (CTR1), which negatively regulates the ethylene-mediated submergence response of P. patens, indicating a nexus for cross-talk between the two signaling pathways that regulate responses to water availability. 

提出问题：However, the mechanism through which the ARK/SnRK2 module is activated in response to water stress remains to be elucidated. 

主要发现：Here, we show that a group of ethylene-receptor-related sensor histidine kinases (ETR-HKs) is essential for ABA and osmostress responses in P. patens. 

结果1-ETR-HK与ARK互作：The intracellular kinase domain of an ETR-HK from P. patens physically interacts with ARK at the endoplasmic reticulum in planta.

结果2-hk突变体：Moreover, HK disruptants lack ABA-dependent autophosphorylation of the critical serine residue in the activation loop of ARK, leading to loss of SnRK2 activation in response to ABA and osmostress. 

结论：Collectively with the notion that ETR-HKs participate in submergence responses, our present data suggest that the HK/ARK module functions as an integration unit for environmental water availability to elicit optimized water stress responses in the moss P. patens.

 摘 要 

为了适应陆地上不断波动变化的可用水环境，陆地植物必须要能够感知水分胁迫，比如干旱和洪涝。植物激素脱落酸ABA和植物特异性的SNF1相关蛋白激酶SnRK2在植物的渗透响应中发挥重要作用。作者最近的研究显示，在苔藓植物小立碗藓中，ABA和渗透依赖性的SnRK2激活需要被上游RAF类激酶ARK磷酸化。RAF/SnRK2模块是陆地植物中演化上比较保守的渗透信号转导机制。有趣的是，ARK还是拟南芥CTR1蛋白的同源物，其在小立碗藓中负调控乙烯介导的淹水响应，说明ABA和乙烯信号在调控植物对可用水的响应方面存在交联关系。但是，ARK/SnRK2模块通过何种机制被激活以响应水分胁迫还不清楚。本文中，作者发现一类乙烯受体相关的感应器组氨酸激酶ETR-HKs对于小立碗藓中的ABA和渗透胁迫响应是必需的。小立碗藓ETR-HK蛋白的胞内激酶结构域与ARK蛋白在内质网上发生物理互作。此外，HK缺失突变会导致ARK激活环上关键丝氨酸残基缺少ABA依赖性的自磷酸化，从而导致SnRK2在响应ABA和渗透胁迫时不被激活。综和ETR-HKs参与淹水响应的研究结果，作者在本文中的研究揭示了HK/ARK模块作为环境可用水的集成单元，保证苔藓植物小立碗藓最佳的水分胁迫响应。

 通讯作者 

** Yoichi Sakata **