Role of starch in carbon translocation and partitioning at the plant level

Endogenous regulation of translocation and of carbon partitioning, major fa ctors for integrating plant function, depend on diurnal regulation of starc h synthesis and mobilization. Regulated diurnal cycling of transitory starc h provides a steady carbon supply to sink growth and avoids potentially adv erse high sugar levels. Carbon availability from starch affects development and alters carbon availability with respect to nitrogen. Along with sugar sensing, the level and turnover of starch are involved in endogenous regula tion in response to carbohydrate status. Despite their key roles in plant m etabolism, mechanisms for endogenous regulation of starch synthesis and deg radation are not well characterized. Time course studies with labeled carbo n reveal endogenous diurnal regulation of starch metabolism, by which sucro se synthesis from starch and newly-fixed carbon are mutually regulated in s upport of translocation at night, under low light, and during periods of wa ter stress. Even under steady irradiance, which supports photosynthesis at midday levels, starch synthesis begins gradually and slows under an end-of- day circadian regulation that anticipates the dark period. Studies with Ara bidopsis mutants identified two requisite components of starch mobilization , endoamylase, and glucose transport across the chloroplast inner envelope. Time course studies of carbohydrate levels and labeling studies of plant-l evel carbon metabolism in mutant plants with impaired ability to mobilize s tarch identified steps in starch mobilization that support diurnal regulati on of translocation. Endogenously regulated exit of glucose across the chlo roplast membrane appears to regulate starch mobilization.