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

Abstract

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.

Identification of an abscisic acid transporter by functional screening using the receptor complex as a sensor

“…It has been suggested that ABA synthesized in vascular tissues is transported to guard cells (16,17). Therefore, we investigated whether AITs are involved in this process.…”

“…5F). The expression pattern of AIT1 in vascular tissues resembles the localization of ABA biosynthetic enzymes such as AAO3, ABA2, and NCED3 (16,17). Accordingly, we propose a model in which AIT1 functions to maintain the ABA pool size at the site of biosynthesis (vascular parenchyma cells), and this might be required for efficient translocation of ABA to guard cells (Fig.…”

“…Immunohistochemical studies using antibodies raised against Arabidopsis ABA biosynthetic enzymes AAO3, AtABA2, and AtANCED3 suggested that ABA is actively synthesized in vascular tissues (parenchyma cells) in response to water stress (16,17). This, in turn, indicates that ABA synthesized in vascular tissues must be translocated to guard cells to induce stomatal closure.…”

The multifaceted role of ABA in disease resistance

“…ABA in the form of its inactive glucose ester conjugate is highly mobile throughout the plant and is released into its active form when it has reached the target tissue. Localization studies of ABA biosynthetic enzymes indicate that the vascular bundles are the active sites of ABA synthesis in turgid, non-stressed plants [58,59]. Therefore, all vascular plant tissues have the capacity to synthesize ABA.…”

Tocopherols Play a Crucial Role in Low-Temperature Adaptation and Phloem Loading in Arabidopsis

“…That this occurs in both vte2 and vte1 strongly suggests that tocopherols play a crucial, previously unrecognized role in low-temperature adaptation, specifically in phloem loading. Several studies have suggested that vascular tissues, including vascular parenchyma, are metabolically distinct, sensitive to changing environmental conditions, and hence critical sites for stress responses (Orozco-Cardenas et al, 2001;Hibberd and Quick, 2002;Fryer et al, 2003;Koiwai et al, 2004;Narvaez-Vasquez and Ryan, 2004). Our data are consistent with this thesis and suggest that tocopherols have important function(s) in regulating the responses of these specific cell types to environmental stress, such as low temperatures.…”