GUN4-mediated tetrapyrrole metabolites regulates starch biosynthesis during early seed development in rice

Seed formation requires the supply of sucrose derived from leaves and the functioning of starch biosynthetic enzymes during seed development, of which are related to the tetrapyrrole metabolism. Here, we found that, aside from leaves, the Genome Uncoupled 4 ( OsGUN4 ) was also predominantly expressed at 7–14 days after flowering (DAF) in developing seeds, whereas its down-regulation produced aberrant grain appearances and altered storage substances in the epi-genetic mutant of gun4 epi seeds, which was attributed to abnormal performance of metabolites and starch biosynthetic enzymes. Moreover, the OsGUN4 mutation would greatly affect the sucrose accumulation in leaves and the sucrose transportation in phloem, which was related to the weak photosynthesis resulted from reduced chlorophylls. Conversely, in developing grains, the OsGUN4 mutation led to the accumulation of sucrose and the tetrapyrrole metabolites, i.e., protoporphyrin IX (PPIX) and heme, but inhibited synthesis of storage starch. Additionally, feeding of the exogenous heme also greatly suppressed the expression of starch biosynthetic genes at 7 to 14 DAF in developing grains, suggesting its similar effects to the OsGUN4 mutation during starch biosynthesis. Collectively, these findings demonstrated that OsGUN4 was a potential regulator to mediate starch biosynthesis via tetrapyrrole metabolites early during seed development.