Possible involvement of heterotrimeric G-protein signaling in Al-induced secretion of organic acid anions in Arabidopsis and rye

Secretion of organic acid anions (OAs) from the roots in response to aluminum (Al) is a common mechanism for Al resistance in plants. Transporter genes involved in the Al-induced secretion of malate and citrate have been identified, but the signal cascades mediating the regulation of the secretion of OAs has not been understood. Here, we investigated the possible role of heterotrimeric G-proteins in the Al-induced secretion of OAs. Seedlings of rye and Arabidopsis thaliana were grown in an environment-controlled growth room. Organic acid anions in the exudates of roots were determined spectrophotometrically. Heterotrimeric G-proteins and GTPase activity were assayed using protein gel blot analyses with G alpha antibody and spectrofluorometer, respectively. GPA1 gene expression level was determined by semi-quantitative RT-PCR. Immunoblotting revealed a heterotrimeric G-protein in the plasma membrane fraction of rye and Arabidopsis roots. Exposure of the rye roots to Al resulted in activation of GTPase. Al-induced secretion was inhibited by pertussis toxin, a heterotrimeric G-protein antagonist, but was stimulated by cholera toxin, a heterotrimeric G-protein agonist. Furthermore, Al-induced OA secretion was blocked in G alpha- or G beta-deficient mutants of Arabidopsis. Al induced GPA1 expression in Arabidopsis roots. Heterotrimeric G-proteins-coordinated transduction of Al-stress signals may be associated with the secretion of OAs from roots.