Endogenous regulation of translocation and of
carbon partitioning, major factors for integrating plant function, depend on
diurnal regulation of starch synthesis and mobilization. Regulated diurnal
cycling of transitory starch provides a steady carbon supply to sink growth
and avoids potentially adverse 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 regulation in response to carbohydrate status. Despite
their key roles in plant metabolism, mechanisms for endogenous regulation of
starch synthesis and degradation are not well characterized. Time course
studies with labeled carbon reveal endogenous diurnal regulation of starch
metabolism, by which sucrose synthesis from starch and newly-fixed carbon are
mutually regulated in support of translocation at night, under low light, and
during periods of water 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 Arabidopsis 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-level carbon metabolism in
mutant plants with impaired ability to mobilize starch identified steps in
starch mobilization that support diurnal regulation of translocation.
Endogenously regulated exit of glucose across the chloroplast membrane appears
to regulate starch mobilization.
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