Transitory Starch Metabolism in Guard Cells: Unique Features for a Unique Function.

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. Starch is a nonstructural polysaccharide synthesized inside plastids of plants and algae. It consists of two types of a-1,4-linked glucan polymers—amylose and amylopectin—that differ in chain length and frequency of a-1,6-branches. These polymers adopt complex secondary and tertiary structures that organize into insoluble, semicrystalline granules to store energy in a dense, osmotically inert form (Pfister and Zeeman, 2016). Starch is a vital substance for plants, both for shortand long-term storage of carbohydrates. In heterotrophic organs, such as potato tubers, cassava roots, cereal seed endosperm, and the stems of woody perennials, starch is synthesized in specialized amylopasts from imported sucrose (Suc) and stored over the seasons, or even for many years. Remobilization of this long-term storage starch takes place during seed germination, tuber sprouting, or regrowth, when photosynthesis has either not yet resumed or is insufficient to meet the demand for energy and carbon skeletons (Lloyd and Kossmann, 2015). In photosynthetic tissues, starch is synthesized in the chloroplasts of mesophyll cells during the day and remobilized at night to provide carbon and energy for maintenance and growth (Stitt and Zeeman, 2012). These short-term reserves, known as transitory starch, are formed directly from intermediates of the Calvin-Benson cycle in the light and when broken down at night provide substrates for respiration in the leaf and synthesis of Suc that can be exported to growing sink organs. Within the leaf epidermis, transitory starch is also present in the chloroplasts of the guard cells that surround the stomatal pore.