We identified a mutant of Arabidopsis thaliana ectotype Col-O in which significantly reduced levels of expression of the gene for beta-amylase (AT beta-Amy) were detected in leaves in response to high concentrations of sucrose, glucose or fructose. Genetic studies, including a cross with transgenic plants that harbored the AT beta-Amy:GUS transgene with the promoter of AT beta-Amy, indicated that this phenotype was caused by a recessive mutation, Iba1, that affected expression of AT beta-Amy in trans. We also found a reduced level of sugar-induced expression of AT beta-Amy in the Landsberg erecta (Ler) ecotype compared with other ecotypes. This phenotype seemed to be due to a recessive trait, provisionally designated Iba2, that was linked to neither erecta nor Iba1. The Iba2 mutation also affected expression of AT beta-Amy:GUS transgene. Accumulation of starch and sugars after treatment of leaves with sucrose was not affected in the Iba1 mutant and Ler plants. However, both Iba1 mutant and Ler plants accumulated low levels of anthocyanin in response to sucrose, results that suggested the existence of some genetic linkage between regulation of the expression of AT beta-Amy and regulation of the accumulation of anthocyanin. Although the Iba1 and Iba2 mutations did not affect sugar-inducible gene expression in general, the expression of sugar-regulated genes other than the gene for beta-amylase was differentially affected in the Iba1 mutant and Ler plants. These results suggest that the sugar-regulated expression of many genes in plants might be mediated by multiple signal-transduction pathways.