15 NSFBIO SAUR regulation of stomatal aperture

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Guard cells control photosynthesis and transpiration by regulating opening and closing of stomatal pores. We know much about how guard cells shrink in response to the drought stress hormone ABA, leading to stomatal closure. In contrast, we have sparse knowledge of the biochemical events by which guard cells swell to cause stomatal opening in response to light and diurnal rhythms. We have found that Arabidopsis SAUR (Small Auxin Up RNA) proteins can promote stomatal opening. This proposal will be to unravel under what conditions and by what mechanisms they do so.SAUR genes are a large class in all land plants. Several Arabidopsis SAUR proteins localize to the plasma membrane and promote cell expansion. These proteins are normally short-lived, but can be stabilized in the plasma membrane. We know that closely related SAUR proteins promote growth by inhibiting membrane-associated protein phosphatases that otherwise dephosphorylate plasma membrane H+-ATPases at a key regulatory site and increase H+-ATPase proton pumping. The same SAURs also promote stomatal opening when expressed in guard cells and a subset of these are preferentially expressed in guard cells under conditions in which stomata are open, but are repressed by drought or ABA. By contrast, ABA activates other SAUR genes in guard cells which may have opposing effects on stomatal function. Thus, what we do not know is whether these opposing actions are unique, whether they overlap, or whether SAUR-mediated signalling affects other transport and homeostatic processes in guard cells. Multiple transporters in both the plasma membrane and the tonoplast influence guard cell shrinking and swelling, and their activities often depend either directly or indirectly on the activation status of the others. Thus, to understand stomatal dynamics, it is essential to gain a global, systems view of guard cell transport and homeostasis. A major component of this project will exploit computational models that simulate these couplings using parameters based on experimental measurements. Starting points for current modelling will draw on past predictions that changes in H+-ATPase activity will have significant effects on stomatal aperture. For example, increased H+-ATPase activity in the model leads to slower stomatal closing at dusk and also to changed cytoplasmic concentrations of several other ions. Such knowledge will aid in experimental design and will also form a basis for subsequent validation.The collaborating researchers have complementary expertise in guard cell physiology (Blatt), computational modeling (Blatt), biochemistry (Gray), genetics (Reed, Nagpal), and auxin response (Gray, Nagpal, Reed). The collaboration will pool resources and expertise developed in the three labs to make possible the multipronged approach.