| The Arabidopsis hyponastic leaves 1 (hyl1) Ds insertion mutant exhibits reduced sensitivity to auxin and cytokinin and hypersensitivity to ABA. Null mutant plants are small and late-flowering, have hyponastic leaves, and show reduced gravitropism and apical dominance. Plants overexpressing the HYL1 gene are large, show exaggerated apical dominance, and bleach during development. The HYL1 gene encodes a 419 as protein with 2 dsRNA binding motifs in the N-terminal half, 6 near-perfect repeats of a 28-aa repeat in the C-terminal half and a central nuclear localization sequence. The HYL1 protein binds dsRNA, but not dsDNA, ssDNA, or ssRNA and a GUS-HYL1 fusion protein localizes in the nucleus. Both N- and C-terminal halves of the protein are essential for function and overexpression of either in a wildtype plant gives a dominant negative phenotype with respect to ABA hypersensitivity. The N terminal half of the protein is phosphorylated by two kinases of approximately 42 and 48 kd, one of which is activated by ABA. Microarray analysis showed a >2.5-fold increase in transcript levels in mutant compared to wildtype seedlings for 35 of ∼1000 stress-related genes tested, while only 3 showed a comparable decrease in transcript levels. Most of the genes with elevated transcript levels encode proteins which are induced by stresses, including members of the PR1 and GST gene families, ACC synthase, glucanase, as well as stress signaling proteins, such as ANP1 and AtMAPK3. Similar comparative microarray analysis of plants overexpressing the HYL1 protein and wildtype plants showed lower transcript levels for 56 genes, many of which encoded photosynthetic proteins, and increased transcript levels for 8 genes. These observations are consistent with a negative regulatory role of HYL1 in gene expression. Two experimental results offer preliminary evidence that HYL1 affects the stability of proteins regulated by the ubiquitinproteasome pathway. First, we observe the accumulation of the AB15 transcription factor at a much lower ABA concentration in hyl1 mutant plants than in wildtype plants. This observation offers an explanation for the hyl1 mutant's ABA hypersensitivy because ABA-triggered accumulation of ABI5 leads to early post-germination growth arrest. Second, we observe that application of the auxin transport inhibitor NPA results in the strong induction of an auxin promoter-driven GUS gene in root tips of wildtype plants, but not those of hyl1 plants. Thus the hyl1 mutant's auxin phenotypes may reflect a requirement for the HYL1 protein in the localization or metabolism of the auxin efflux carrier. |
