A molecular perspective on starch metabolism in woody tissues

The elucidation of the molecular mechanisms of starch synthesis and mobilization in perennial woody tissues is of the utmost scientific and agricultural importance. Starch is the main carbohydrate reserve in plants and is fundamental in human nutrition and several industrial processes. In leaves, starch accumulated during the day is degraded throughout the night and the resulting sugars, glucose and maltose, are exported to the cytosol by the specialized transmembrane translocators pGT and MEX, respectively. Nevertheless, the degradation of the starch granule is a complex process not completely elucidated. While the mechanisms of starch mobilization during germination in the dead endosperm of cereal seeds are well described, the molecular and biochemical mechanisms involved in starch storage in the heterotrophic tissues of woody plants and its utilization in spring and winter are still puzzling. It is known that some biochemical steps of starch synthesis are conserved in heterotrophic tissues and in the leaves, but some aspects are particular to sink organs. From an agronomic standpoint, the knowledge on starch storage and mobilization in woody tissues is pivotal to understand (and to optimize) some common practices in the field that modify source-sink relationships, such as pruning and defoliation. Soluble sugars resulting from starch are also pivotal to cold adaptation, and in several fruits, such as banana and kiwifruit, starch may provide soluble sugars during ripening. In this review, we explore the recent advances on the molecular mechanisms and regulations involved in starch synthesis and mobilization, with a focus on perennial woody tissues.