Functional Analysis of a MATE Gene OsFRDL2 Revealed its Involvement in Al-Induced Secretion of Citrate, but a Lower Contribution to Al Tolerance in Rice

The multidrug and toxic compound extrusion (MATE) transporters represent a large transporter family in plants, but the role of most genes in this family has not been examined. We functionally characterized a MATE family member, OsFRDL2, in rice (Oryza sativa). OsFRDL2 showed an efflux transport activity for citrate when it was expressed in both Xenopus oocytes and cultured tobacco cells. OsFRDL2 was mainly expressed in the roots and its expression was not induced by iron (Fe) deficiency, but it was rapidly up-regulated by aluminum (Al). Furthermore, the expression of OsFRDL2 was regulated by ART1, a C2H2-type zinc-finger transcription factor for Al tolerance. OsFRDL2 protein was localized at unidentified vesicles in the cytosol, but not co-localized with either mitochondria or peroxisomes when expressed in both onion epidermal cells and cultured tobacco cells. Knockout of OsFRDL2 decreased Al-induced secretion of citrate from the roots, but did not affect the internal citrate concentration. The Al-induced inhibition of root elongation was similar between the OsFRDL2 knockout line and its wild-type rice. Knockout of OsFRDL2 did not affect the translocation of Fe from the roots to the shoots. A double mutant between osfrdl2 and osfrdl4 or osfrdl1 did not further decrease the Al-induced citrate secretion and Fe translocation compared with the single mutant. Collectively, our results indicate that although OsFRDL2 is involved in the Al-induced secretion of citrate, its contribution to high Al tolerance is relatively small in rice.