Altered patterns of sucrose synthase phosphorylation and localization precede callose induction and root tip death in anoxic maize seedling

Root extracts made from maize (Zea mays) seedlings submerged for 2 h showed an increased P-32-labeling of a 90-kD polypeptide in a Ca2+-dependent mann er. This protein was identified as sucrose synthase (SS) by immunoprecipita tion and mutant analysis. Metabolic labeling with P-32(i) indicated that th e aerobic levels of SS phosphorylation were maintained up to 2 h of anoxia. Ln contrast, during prolonged anoxia the protein was under-phosphorylated, and by 48 h most of the protein existed in the unphosphorylated form. In s eedlings submerged for 2 h or longer, a part of SS became associated with t he microsomal. fraction and this membrane localization of SS was confined o nly to the root tip. This redistribution of SS in the root tip preceded cal lose induction, an indicator of cell death. The sh1 mutants showed sustaine d SS phosphorylation and lacked the anoxia-induced relocation of SS, indica ting that it was the SH1 form of the enzyme that was redistributed during a noxia. The sh1 mutants also showed less callose deposition and greater tole rance to prolonged anoxia than their non-mutant siblings. EGTA accentuated anoxic effects on membrane localization of SS and callose accumulation, whe reas Ca2+ addition reversed the EGTA effects. These results indicate that t he membrane localization of SS is an important early event in the anoxic ro ot tip, probably associated with the differential anoxic tolerance of the t wo SS mutants. We propose that beside the transcriptional control of genes encoding SS, the reversible phosphorylation of SS provides a potent regulat ory mechanism of sugar metabolism in response to developmental and environm ental signals.