Post-doctoral researcher Academia Sinica New Taipei City, Taipei, Taiwan (Republic of China)
Body of Abstract: Zinc is an essential nutrient for plants but can cause toxicity in excess. Therefore, zinc accumulation is tightly regulated in plants. The metal chelator, nicotianamine (NA), is indispensable in heavy metal homeostasis in vascular plants. It chelates divalent metal cations and is involved in zinc intracellular as well as long-distance transport activities. The Zn/Cd hyperaccumulator Arabidopsis halleri, a close relative of Arabidopsis thaliana, can hypeaccumulate and hypertolerate heavy metals. It secretes large amounts of NA into the rhizosphere. Moreover, secreted NA forms a stable complex with zinc thus preventing rapid Zn uptake through reducing Zn bioavailability. Nevertheless, the NA transporter responsible for NA secretion into the rhizosphere has not yet been reported. In this study, we identified and characterized EFFLUX TRANSPORTER OF NA1 (ENA1) as a plasma membrane-localized NA efflux transporter in Arabidopsis. The expression of ENA1 is higher in A. halleri than in A. thaliana and induced by excess Zn only in A. halleri but not in A. thaliana. Increase of ENA1 expression by overexpressing AhENA1 led to elevated NA secretion from roots into the rhizosphere, but did not affect internal NA accumulation or long-distance root-to-shoot NA transport. With 67Zn uptake experiment, we found that increased NA secretion alleviated Zn toxicity in plants by reducing the uptake of excess Zn. Our discovery of ENA1 uncovers the molecular mechanism underlying the root NA secretion.