Involvement of Ca2+ signalling in the sugar-inducible expression of genes coding for sporamin and β-amylase of sweet potato

Genes coding for sporamin and β-amylase of sweet potato are inducible not only by high levels of metabolizable sugars, such as sucrose, but also by a low concentration of polygalacturonic acid (PGA). Calmodulin inhibitors and EGTA inhibited both the PGA-inducible and the sucrose-inducible accumulation of mRNAs for sporamin and β-amylase in sweet potato. Calmodulin inhibitors, EGTA and La³⁺, also inhibited the sucrose-inducible expression, in leaves of transgenic tobacco, of a fusion gene, β-Amy:GUS, which consists of the promoter of the β-amylase gene and the coding sequence for β-glucuronidase. The sucrose-inducible expression of the β-Amy:GUS fusion gene was also inhibited by two inhibitors of Ca²⁺ channels, diltiazem and nicardipine. These results suggest that the sugar-inducible expression of genes for sporamin and β-amylase involves, at least in part, Ca²⁺-mediated signalling, and that the cytosolic free Ca²⁺ may mediate cross-talk between signals related to carbohydrate metabolism and other stimuli. Treatment of coelenterazine-loaded leaf discs of tobacco expressing a Ca²⁺-binding photoprotein, aequorin, with 0.2 M sucrose for 24 h significantly reduced the level of luminescence that could be induced by cold shock, as compared to cold shock-induced luminescence in coelenterazine-loaded leaf discs treated with water. Repression of cold shock-induced luminescence was due to the conversion of holoaequorin to apoaequorin during the treatment with sucrose. Treatment of coelenterazine-loaded leaf discs with a 0.2 M solution of glucose or fructose, but not of mannitol or sorbitol, also reduced the cold shock-induced luminescence. It is suggested that non-synchronous increases in cytosolic level of free Ca²⁺ occur in leaf discs during treatment with high levels of metabolizable sugars.