ABA Hypersensitive Germination2-1 Causes the Activation of Both Abscisic Acid and Salicylic Acid Responses in Arabidopsis

ABA and salicylic acid (SA) are believed to act antagonistically. We previously reported that an ABA-hypersensitive mutant ahg2-1, which had reduced expression of poly(A)-specific ribonuclease (PARN), exhibited pleiotropic phenotypes including unique enhanced ABA- and SA-sensitive phenotypes. In this study, we characterized the increased SA-sensitive phenotype of this mutant in detail and addressed its relationship with ABA-related and dwarf phenotypes. We found that the ahg2-1 mutant had a high endogenous SA level and an elevated resistance to bacterial pathogens. Double mutant analyses showed that Arabidopsis plants defective in the SA signaling pathway (npr1 and pad4 mutants and nahG transgenic plants) could suppress neither the ABA hypersensitivity nor the dwarf phenotypes. These results indicate that ABA-related, SA-related and dwarf phenotypes of the ahg2-1 mutant are independent of each other. To obtain more insight into the molecular basis of the effect of ahg2-1, microarray analyses were conducted not only for ahg2-1 but also for ahg2sid2 or ahg2abi1 so as to reduce the secondary effects of SA or ABA. The resulting data indicate that ahg2-1 has a unique gene expression profile, consistent with the novel phenotype of this mutant. Detailed comparison of the expression profiles of up- or down-regulated genes implied that ahg2-1 somehow affects mitochondrial function. Our data suggest that a partial loss of PARN activity affects ABA, SA and mitochondrial function independently, and that the regulation of mRNA levels is deeply implicated in diverse cellular functions.