We isolated two allelic mutants of Arabidopsis thaliana that showed an early senescence phenotype under light/dark and continuous dark photo regimes, and designated the mutant hypersenescence1 (hys1). The hys1 mutants decreased chlorophyll and protein content, lowered the efficiency of photosystem II, and accumulated several senescence upregulated gene transcripts earlier than the wild-type plants. In addition to these senescence features, the hys1 seedlings responded more intensely to exogenously applied sugars than did wild-type seedlings in sugar-induced growth inhibition and sugar-mediated transcript accumulation, both of which are known to be regulated by the sugar sensor hexokinase. The hys1 mutant also had abnormal trichomes. Map-based cloning of the HYS1 gene identified a novel gene that encodes a protein with a potential nuclear localization signal in the amino-terminal region, and five putative transmembrane domains in the carboxyl-terminal region. Furthermore, we found that the constitutive expressor of pathogenesis-related genes 5 (cpr5) mutant, which shows spontaneous pathogen-defence responses and abnormal trichomes, has a point mutation in the HYS1 gene, suggesting that these independently isolated mutants are allelic to each other. Although no definite conclusion can be drawn from these results, we suggest that altered sensitivity to sugars and/or enhanced efficiency of sugar signalling in the hys1/cpr5 mutant may have important roles in the initiation processes of leaf senescence and pathogen-defence responses in Arabidopsis.