To better understand the role of ABI1 in abiotic stress response and ABA signaling, we characterized the phenotype of a null allele for the ABI1 gene selected from SALK T-DNA insertion collection. The absence of the ABI1 protein was manifested by a hypersensitivity to ABA and glucose in germination assays and also in root growth. The ABA-induced stomatal closure was also more pronounced in abi1 compared with WT, which resulted in substantial drought avoidance in the mutant line. We demonstrated that ABI1 interferes with guard cell signaling upstream of ROS production and cytosolic calcium elevation. Furthermore, to identify candidate genes associated with abiotic stress responses, we performed a microarray screen of abi1 and WT Col-0 plants. Microarray analysis has shown a modified transcriptional profile in abi1 upon ozone and drought stress. Using 2 ANOVA testing we obtain 154 and 193 genes that showed significantly different responses between WT and abi1 mutant to drought and ozone stress, respectively. In both treatments, some genes showed a stronger transcriptional response in abi1 than in WT (‘gain of regulation’), whereas other genes displayed a weaker response in abi1 than in WT (‘loss of regulation’) where regulation refers to either up- or down-regulation in response to the stress in the abi1 mutant. Furthermore to verify that the microarray approach to functional discovery is effective we demonstrated an ABI1-dependent regulatory mechanism controlling both ABA and ethylene biosynthesis. In summary, performed analysis revealed a complex ABI1 signaling network in ozone and drought stress conditions, suggesting a relationship between ABI1, stress signaling, and metabolic management. Thus, the results provide relevant information to integrate ABI1 with processes related to plant stress tolerance.

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