The Arabidopsis sugar-insensitive mutants sis4 and sis5 are defective in abscisic acid synthesis and response

Although soluble sugar levels affect many aspects of plant development and physiology, little is known about the mechanisms by which plants respond to sugar. Here we report the isolation of 13 sugar-insensitive (sis) mutants of Arabidopsis that, unlike wild-type plants, are able to form expanded cot yledons and true leaves when germinated on media containing high concentrat ions of glucose or sucrose. The sis4 and sis5 mutants are allelic to the AB A-biosynthesis mutant aba2 and the ABA-insensitive mutant abi4, respectivel y. In addition to being insensitive to glucose and sucrose, the sis4/aba2 a nd sis5/abi4 mutants also display decreased sensitivity to the inhibitory e ffects of mannose on early seedling development. Mutations in the ABI5 gene , but not mutations in the ABI1, ABI2 or ABI3 genes, also lead to weak gluc ose- and mannose-insensitive phenotypes. Wild-type and mutant plants show s imilar responses to the effects of exogenous sugar on chlorophyll and antho cyanin accumulation, indicating that the mutants are not defective in all s ugar responses. These results indicate that defects in ABA metabolism and s ome, but not all, defects in ABA response can also alter response to exogen ous sugar.