The role of rice phenolics efflux transporter in solubilizing apoplasmic iron.

Iron (Fe) is an essential micronutrient for plants whose deficiency presents a major worldwide agricultural problem. Moreover, Fe is not easily available in neutral to alkaline soils, rendering plants deficient in Fe despite its abundance. Plants secrete phenolics, such as protocatechuic acid (PCA) and caffeic acid (CA), to take up and utilize apoplasmic precipitated Fe, but despite the rapid progress in understanding cellular and subcellular Fe transport, the molecular mechanisms of phenolics synthesis and secretion are not clear. Recently, we isolated and characterized a phenolics efflux transporter in rice by characterizing a mutant in which the amount of PCA and CA in the xylem sap was dramatically reduced, which we hence named phenolics efflux zero 1 (pez1). PEZ1 is a plasma membrane protein that transports PCA when expressed in Xenopus laevis oocytes, and characterization of PEZ1 knockdown and overexpressing plants revealed that it plays an essential role in solubilizing precipitated apoplasmic Fe. The identification of PEZ1 will increase our understanding of apoplasmic Fe solubilization as well as promote research on phenolics efflux mechanisms in different organisms.