A SNARE-Aquaporin complex in stomatal hydraulics

气孔是通过植物叶片的不透水角质层介导气体交换的气孔。当光合作用消耗叶片内的二氧化碳时，叶片为二氧化碳进入打开；当大气湿度较低时，叶片关闭以减少水蒸气的蒸腾作用，防止叶片干燥。气孔是水资源供应和作物生产危机的中心，这种危机正在开始显现，而且随着全球需求，特别是农业需求超过淡水供应，这种危机只会升级。因此，气孔是提高作物生产性能和效率的重要目标。大多数植物的气孔通过光合作用追踪对二氧化碳的直接需求，对叶片内的二氧化碳做出反应，在光照下打开，在黑暗中关闭。然而，与光合作用相比，气孔响应较慢。日光的波动，例如云层经过头顶，会降低光合作用和水利用效率（WUE=光合作用中固定的碳量/蒸发量），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的速度，在光合作用和水分利用效率方面取得了实质性进展。我们现在需要了解如何利用气孔固有的过程来实现这些增益。气孔运动是由包围气孔的保卫细胞膜上的溶质和水分运输驱动的。保卫细胞携带离子通道蛋白促进溶质通量，水通道蛋白介导水通量，它们在气孔运动过程中依靠膜泡的运输来调节细胞表面积。因此，这三个过程的协调对气孔响应至关重要。从我们之前的工作中，我们知道通过K+通道的主导离子通量通过通道亚群和所谓的SNARE蛋白之间的结合与膜运输耦合，SNARE蛋白促进囊泡运输，并且在陆地植物中是保守的。这些相互作用确保了溶质通量和膜交通在保护细胞内“锁步”运行。有一些证据表明，通过水通道蛋白的水通量是平行协调的，但到目前为止，我们还缺乏对这种协调如何产生的理解。质膜水通道蛋白存在于所有被子植物中。在模式植物拟南芥中，有3个pip参与保护细胞的水通量，其中一个是PIP2；1,占主导地位。我们最近发现了三种pip和SYP121蛋白之间的选择性相互作用，SYP121蛋白是质膜上两个主要SNAREs之一。这些相互作用依赖于SYP121的胞质n端区域，该区域具有序列分化性，但在功能上可与其他SNAREs互换，并被广泛认为在所有真核生物中调节SNARE活性和囊泡运输。最令人兴奋的是，我们发现一个嵌合的SYP121包含了一个不相互作用的SNARE的相同区域，当在保护细胞中表达时，它会减慢气孔的打开和关闭，并在植物经历波动的日光时抑制水分利用效率和生长。我们的发现是SYP121-PIP在气孔运动中结合的第一个直接证据，他们指出SNARE亚结构域在体内负责这种作用。SYP121还结合保卫细胞的K+通道，以K+通量协调囊泡交通。因此，SYP121-PIP结合表明在气孔调节中存在SNARE联系；它提出了PIP与K+通道结合的协调性问题；它挑战了关于水通道蛋白水力学中囊泡交通影响水利用效率和植物生物量增加的既定教条。我们现在建议解决SYP121与保护细胞pip的结合和功能，并确定其对植物的影响。这项研究是为了了解生命的基本规律。尽管如此，了解这种SNARE联系还是为未来生物工程加速气孔运动和提高作物效率带来了潜在的目标。

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.