The essential role of plastidial triose phosphate isomerase in the integration of seed reserve mobilization and seedling establishment.

Seed germination and seedling establishment are pivotal for the life cycle of seed plants. Storage reserve mobilization provides energy and carbon to support seedling development. Seedling establishment involves root elongation and plastid transition from a heterotrophic to photoautotrophic state, such that the seedling can attain independence once seed reserves have been depleted. At the biochemical level, this transition is likely complicated as it requires a spatial and temporal shift from degradation to synthesis for many metabolic pathways. The triose phosphate isomerase (TPI) catalyzes the reversible conversion of dihydroxyacetone phosphate (DHAP) to glyceraldehyde-3-phosphate (GAP) and is involved in many pathways including glycolysis, Calvin cycle, and glycerol metabolism. Plants contain both cytosolic and plastid forms of TPI and neither have been extensively characterized, presumably because TPI catalyzes a reversible reaction (i.e. substrate/product are in equilibrium) and is therefore unlikely to be of regulatory importance. In the recent study1, we discovered a knock-down mutant for pdTPI that reveals this enzyme plays a crucial, metabolic role during the heterotroph/autotroph transition phase, affecting both chloroplast development and seedling establishment. Inability of a functional cytosolic TPI to compensate for reduced pdTPI expression demonstrates plastid and cytosolic pools of DHAP and GAP are not in equilibrium and may reveal a novel plastid translocator.