Mutation of OsGUN4 Uncoupled the Sugar-dependent Signals to Regulate Starch Biosynthesis in Rice

Abstract Background Starch biosynthesis requires plastid-to-nucleus signals to ordinate the flow of carbon, which is partly mediated by tetrapyrrole intermediates. We previously revealed that mutation of the Genomes Uncoupled 4 (OsGUN4) would affect tetrapyrrole intermediates, but the underlying mechanisms for regulatory roles of OsGUN4 on starch biosynthesis remains largely unknown. Results In this study, we revealed that the OsGUN4 mutation not only retarded the carbon flow from sucrose to starch but also disabled the sensitive response to exogenous feeding sucrose. Moreover, extra addition of norflurazon (NF) would aggravate insensitivity to the sucrose-dependent induction in gun4epi, especially exhibiting collapse declines of the starch biosynthetic enzymes’ activities. However, genes encoding starch synthetic enzymes performed discordance with the activities of starch biosynthesis associated enzymes upon on the scarce expression of OsGUN4 in gun4epi. These results indicated that OsGUN4 played regulatory roles on biosynthetic genes and enzyme activity in starch biosynthesis. Furthermore, we also concluded that 1O2 derived from GUN4/protoporphyrin IX (proto) might be responsible for the sugar-dependent signals to regulate starch biosynthesis, due to positive correlations between the singlet oxygen (1O2) and many starch biosynthetic genes that were subjected to the control of three reported transcriptional factors (TFs) during starch biosynthesis. Eventually, the OsGUN4 mutation would also relieve the repression of glucose on Snf1-related protein kinases (SnRK1) but seem to negatively mediate the functioning of ADP-Glc pyrophosphorylase (AGPase). Conclusion In summary, we demonstrated that OsGUN4 serve as a broker to activate 1O2-mediated signals in the sugar signaling cascade, possibly functioning upstream through TFs, and that OsGUN4 played roles in the SnRK1A-mediated signals, partly through the accumulations of sugars, e.g. glucose or sucrose.