A SNARE-Aquaporin complex in stomatal hydraulics

气孔是介导气体交换穿过植物叶片的不可渗透的角质层的孔。当光合作用耗尽叶片内的CO2时，它们会打开以供CO2进入，当大气湿度较低时，它们会关闭以减少水蒸气的蒸腾作用并防止叶片干燥。气孔是水供应和作物生产危机的核心，这场危机正在开始显现，随着全球需求，特别是农业需求，超过淡水供应，危机只会升级。因此，气孔是提高作物性能和效率的重要目标。大多数植物的气孔通过光合作用跟踪对CO2的即时需求，在叶片内响应CO2，在光照下打开，在黑暗中关闭。然而，气孔的反应与光合作用相比是缓慢的。日光的波动，例如当云层从头顶经过时，会降低光合作用并降低水分利用效率（WUE=光合作用中固定的碳量/蒸腾的水量），主要是因为气孔通常滞后于光的变化。合成生物工程通过加快气孔响应速度，在光合作用和WUE方面显示出实质性的收益。我们现在需要了解这些增益是如何利用气孔本身的过程来实现的。气孔运动是由溶质和水分穿过气孔周围保卫细胞的膜来驱动的。保卫细胞窝藏离子通道蛋白，以促进溶质通量和水通道蛋白介导的水通量，他们依赖于交通的膜泡，以调整细胞表面积在气孔运动。因此，这三个过程的协调是至关重要的气孔反应。从我们以前的工作中，我们知道，通过K+通道的主要离子通量耦合到膜交通的通道和所谓的SNARE蛋白，促进囊泡交通和陆地植物之间的子集之间的结合。这些相互作用确保溶质通量和膜流量在保卫细胞内以“锁步”操作。有一些证据表明，水通量通过水通道蛋白的平行协调，但到目前为止，我们还没有了解这种协调可能会出现。质膜（PIP）水通道蛋白在所有被子植物中发现。三种PIP有助于模式植物拟南芥保卫细胞中的水通量，尽管其中一种PIP 2;1占主导地位。我们最近发现了所有三种PIP和SYP 121蛋白之间的选择性相互作用，SYP 121蛋白是质膜上两种主要SNARE之一。这些相互作用依赖于SYP 121的胞质N-末端区域，该区域是序列发散的，但在功能上可与其他SNARE互换，并且被广泛认为在所有真核生物中调节SNARE活性和囊泡交通。最令人兴奋的是，我们发现，嵌合SYP 121纳入非相互作用的SNARE相同区域时，在保卫细胞表达减缓气孔打开和关闭，并抑制WUE和生长时，植物经历波动的日光。我们的研究结果是第一个直接证据SYP 121-PIP结合气孔运动，他们指出SNARE亚域负责这一行动在体内。SYP 121还结合保卫细胞K+通道，协调囊泡交通与K+通量。因此，SYP 121-PIP结合表明气孔调节中的SNARE关系;它回避了PIP和K+通道结合的协调问题;并且它挑战了关于囊泡交通在水通道蛋白水力学中影响WUE和植物生物量增益的作用的既定教条。我们现在建议解决SYP 121与保卫细胞PIP的结合和功能，并建立对植物的后果。这项研究是为了了解生命的基本规则。尽管如此，理解这种SNARE关系仍有望成为未来生物工程的潜在目标，以加速气孔运动并提高作物效率。

Stomata are pores that mediate gaseous exchange across the impermeable cuticle of plant leaves. They open for CO2 entry when photosynthesis depletes CO2 inside the leaf, and they close to reduce the transpiration of water vapour and prevent leaf drying when atmospheric humidity is low. Stomata are at the centre of a crisis in water availability and crop production that is beginning to unfold and can only escalate as the global demand, especially in agriculture, outstrips fresh water supplies. Thus stomata are an important target in efforts to enhance crop performance and efficiencies.Stomata of most plants track the immediate demand for CO2 by photosynthesis, responding to CO2 within the leaf, opening in the light and closing in the dark. However, stomatal responses are slow by comparison with that of photosynthesis. Fluctuations in daylight, for example as clouds pass overhead, degrade photosynthesis and reduce water use efficiency (WUE=amount of carbon fixed in photosynthesis/amount of water transpired), principally because stomata generally lag behind changes in light. Synthetic bioengineering has shown substantial gains in photosynthesis and WUE by accelerating the speed of stomatal response. We need now to understand how such gains might be achieved using the processes native to the stomata.Stomatal movement is driven by solute and water transport across the membrane of the guard cells that surround the stomatal pore. Guard cells harbour ion channel proteins to facilitate solute flux and aquaporins to mediate water flux, and they rely on a traffic of membrane vesicles to adjust cell surface area during stomatal movements. Thus, coordination of these three processes is essential for stomatal responses. From our previous work, we know that the dominant ion flux through K+ channels is coupled to membrane traffic by binding between subsets of channels and so-called SNARE proteins that facilitate vesicle traffic and are conserved across land plants. These interactions ensure solute flux and membrane traffic operate in 'lock-step' within guard cells. There is some evidence for a parallel coordination of water flux through aquaporins, but until now we have lacked an understanding of how this coordination might arise.Plasma membrane (PIP) aquaporins are found across all angiosperms. Three PIPs contribute to water flux in guard cells of the model plant Arabidopsis although one, PIP2;1, dominates. We recently uncovered a selective interaction between all three PIPs and the SYP121 protein, one of two principal SNAREs at the plasma membrane. These interactions depend on a cytosolic N-terminal region of SYP121 that is sequence-divergent, but functionally interchangable with other SNAREs and is widely recognised to regulate SNARE activity and vesicle traffic in all eukaryotes. Most exciting, we find that a chimeric SYP121 incorporating the same region of a non-interacting SNARE slows stomatal opening and closing when expressed in guard cells and suppresses WUE and growth when plants experience fluctuating daylight.Our findings are the first direct evidence for SYP121-PIP binding in stomatal movements, and they point to the SNARE subdomain responsible for this action in vivo. SYP121 also binds guard cell K+ channels, coordinating vesicle traffic with K+ flux. Thus, SYP121-PIP binding suggests a SNARE nexus in stomatal regulation; it begs questions about the coordination of PIP and K+ channel binding; and it challenges established dogma about the roles of vesicle traffic in aquaporin hydraulics that impact on WUE and plant biomass gain.We propose now to resolve the binding and function of SYP121 with the guard cell PIPs and to establish the consequences for the plant. This research is to understand the fundamental rules of life. Understanding this SNARE nexus nonetheless carries the promise of a potential target for future bioengineering to accelerate stomatal movements and enhance crop efficiencies.