Reducing transitory starch in cassava impairs photosynthesis and carbon export to storage roots

Abstract

Optimizing source-sink relations is a central strategy for improving crop yield. In potato, reducing transitory leaf starch synthesis can redirect carbon to storage organs without impairing growth. To test whether this strategy is transferable to cassava, we suppressed the major small subunit of ADP-GLUCOSE PYROPHOSPHORYLASE, MeAPS1a. Transgenic plants were nearly devoid of transitory leaf starch and showed impaired photosynthetic carbon fixation. Under greenhouse conditions, shoots acclimated to the reduced carbon supply and exhibited only moderate biomass losses, and the data indicated reduced carbon supply to developing storage roots. This was reflected by lower leaf sucrose levels, reduced storage root biomass and starch accumulation, and downregulation of carbohydrate-responsive genes, including the major AGPase large subunit MeAPL3. Under field conditions, the absence of transitory starch was even more detrimental, causing severe growth inhibition and drastic reductions in both shoot and root biomass. We also found that MeAPS1a-derived small interfering RNAs moved systemically from leaves to roots, with lower root siRNA abundance in older, larger plants than in younger, smaller plants. Together, our results demonstrate that transitory starch is indispensable for sustaining carbon flux to cassava storage roots, highlighting species-specific constraints on engineering source-sink relations.