Metabolism within the specialized guard cells of plants

Contents Summary 1018 I. Introduction 1018 II. Guard cell photosynthesis 1019 III. Guard cell central metabolism 1022 IV. Guard cell starch metabolism differs from that of mesophyll cells and plays a key role in stomatal movement 1025 V. Connectors between mesophyll and guard cells 1026 VI. Challenges and perspectives in understanding and modelling guard cell metabolism 1029 Acknowledgements 1030 References 1030 Summary Stomata are leaf epidermal structures consisting of two guard cells surrounding a pore. Changes in the aperture of this pore regulate plant water‐use efficiency, defined as gain of C by photosynthesis per leaf water transpired. Stomatal aperture is actively regulated by reversible changes in guard cell osmolyte content. Despite the fact that guard cells can photosynthesize on their own, the accumulation of mesophyll‐derived metabolites can seemingly act as signals which contribute to the regulation of stomatal movement. It has been shown that malate can act as a signalling molecule and a counter‐ion of potassium, a well‐established osmolyte that accumulates in the vacuole of guard cells during stomatal opening. By contrast, their efflux from guard cells is an important mechanism during stomatal closure. It has been hypothesized that the breakdown of starch, sucrose and lipids is an important mechanism during stomatal opening, which may be related to ATP production through glycolysis and mitochondrial metabolism, and/or accumulation of osmolytes such as sugars and malate. However, experimental evidence supporting this theory is lacking. Here we highlight the particularities of guard cell metabolism and discuss this in the context of the guard cells themselves and their interaction with the mesophyll cells.