Tissue-Specific Localization of an Abscisic Acid Biosynthetic Enzyme, AAO3, in Arabidopsis

Arabidopsis aldehyde oxidase 3 (AAO3) is an enzyme involved in abscisic acid (ABA) biosynthesis in response to drought stress. Since the enzyme catalyzes the last step of the pathway, ABA production sites may be determined by the presence of AAO3. Here, AAO3 localization was investigated using AAO3 promoter:AAO3-GFP transgenic plants and by an immunohistochemical technique. AAO3-GFP protein exhibited an activity to produce ABA from abscisic aldehyde, and the transgene restored the wilty phenotype of the aao3 mutant. GFP-fluorescence was detected in the root tips, vascular bundles of roots, hypocotyls and inflorescence stems, and along the leaf veins. Intense immunofluorescence signals were localized in phloem companion cells and xylem parenchyma cells. Faint but significant GFP- and immuno-fluorescence signals were observed in the leaf guard cells. In situ hybridization with antisense AAO3 mRNA showed AAO3 mRNA expression in the guard cells of dehydrated leaves. These results indicate that the ABA synthesized in vascular systems is transported to various target tissues and cells, and also that the guard cells themselves are able to synthesize ABA.