Guard cells are now recognized as the premier cell model in plants. Much of this notoriety can be traced to research over the previous three decades into the signalling mechanisms behind abscisic acid (ABA), its ability to regulate ion transport across the plasma and vacuolar membranes of guard cells, and thereby to drive stomatal movements. Guard cells surround stomatal pores of plant leaves. They control the aperture of the pore and, so, CO2 entry into the leaf for photosynthesis and water loss via transpiration. Their sensitivity to environmental and endogenous stimuli makes them ideal for physiological studies, and their situation makes them well-suited for cellular and subcellular research. The ‘read-out’ of stomatal aperture is also central to the crisis in water availability and crop production that is expected to unfold over the next 20–30 years. Not surprisingly, there is growing interest in ‘reverse engineering’ stomatal guard cells to improve agricultural water use. It seems like an openand-shut case. But is it?

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