ARIA, an Arabidopsis Arm Repeat Protein Interacting with a Transcriptional Regulator of Abscisic AcidResponsive Gene Expression, Is a Novel Abscisic Acid Signaling Component

Arabidopsis (Arabidopsis thaliana) genome contains more than 90 armadillo (arm) repeat proteins. However, their functions are largely unknown. Here, we report that an Arabidopsis arm repeat protein is involved in abscisic acid (ABA) response. We carried out two-hybrid screens to identify signaling components that modulate ABA-responsive gene expression. Employing a transcription factor, ABF2, which controls the ABA-dependent gene expression via the G-box type ABA-responsive elements, we isolated an arm repeat protein. The ABF2-interacting protein, designated as ARIA (arm repeat protein interacting with ABF2), has another conserved sequence motif, BTB/POZ (broad complex, tramtrak, and bric-a-brac/poxvirus and zinc finger) domain, in the C-terminal region. The physiological relevance of ABF2-ARIA interaction was supported by their similar expression patterns and similar subcellular localization. Plants overexpressing ARIA are hypersensitive to ABA and high osmolarity during germination and insensitive to salt during subsequent seedling growth. By contrast, an ARIA knockout mutant exhibits ABA and glucose insensitivities. Changes in the expression levels of several ABF2-regulated genes were also observed in ARIA overexpression lines, indicating that ARIA modulates the transcriptional activity of ABF2. Together, our data indicate that ARIA is a positive regulator of ABA response.