Determining the role of Tie-dyed1 in starch metabolism: epistasis analysis with a maize ADP-glucose pyrophosphorylase mutant lacking leaf starch.

In regions of their leaves, tdy1-R mutants hyperaccumulate starch. We propose 2 alternative hypotheses to account for the data, that Tdy1 functions in starch catabolism or that Tdy1 promotes sucrose export from leaves. To determine whether Tdy1 might function in starch breakdown, we exposed plants to extended darkness. We found that the tdy1-R mutant leaves retain large amounts of starch on prolonged dark treatment, consistent with a defect in starch catabolism. To further test this hypothesis, we identified a mutant allele of the leaf expressed small subunit of ADP-glucose pyrophosphorylase (agps-m1), an enzyme required for starch synthesis. We determined that the agps-m1 mutant allele is a molecular null and that plants homozygous for the mutation lack transitory leaf starch. Epistasis analysis of tdy1-R; agps-m1 double mutants demonstrates that Tdy1 function is independent of starch metabolism. These data suggest that Tdy1 may function in sucrose export from leaves.