This Update focuses on the starch that accumulates in the guard cells that control stomatal pore size and thus the exchange of water vapor, CO2 and O2 between the leaf and the atmosphere. Transitory starch in these cells plays a key role in determining the velocity of stomatal opening in the light. This significantly differs from the transitory starch in the mesophyll leaves, which acts primarily as a carbohydrate reserve to sustain plant metabolism during the night. We discuss how the unique function of transitory starch in guard cells is reflected in the timing of its deposition and mobilization, along with differences from mesophyll cells in the pathways and regulation of starch metabolism.
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