Balancing supply and demand: the spatial regulation of carbon metabolism in grass and cereal leaves.

Leaf primary metabolism responds to changes in both supply of inputs and demand for products. Metabolic control in leaves changes both spatially and temporally. Using leaves of C(3) temperate Gramineae, the spatial control of carbohydrate metabolism has been studied using a range of approaches. Single-cell sampling and subsequent analysis of metabolites, proteins and transcripts has indicated significant differences between epidermal, mesophyll and parenchymatous bundle sheath cells. These differences correlate with differentiated function as heterotrophic, autotrophic and transport pathway components of the leaf. The review emphasizes the key role of sucrose and discusses its catabolism to hexoses and its anabolism to fructans as mechanisms for the preservation of sucrose gradients within the leaf.