Overexpression of barley nicotianamine synthase 1 confers tolerance in the sweet potato to iron deficiency in calcareous soil

Iron (Fe) is an essential micronutrient for all higher organisms. Fe is sparingly available in calcareous soils and Fe deficiency is a major agricultural problem worldwide. Nicotianamine (NA) is a metal chelator involved in metal translocation in plants. Sweet potato is an attractive crop that can grow in poor soil and thus is useful for planting in uncultivated soil. In addition, the sweet potato has recently been suggested as a source of bioethanol. Our aim is to increase NA concentration in sweet potato to ameliorate Fe deficiency. Sweet potato plants expressing the barley NA synthase 1 (HvNAS1) gene under the control of CaMV 35S promoter were produced by Agrobacterium-mediated transformation. The transgenic sweet potato exhibited tolerance to low Fe availability when grown in calcareous soil. The level of tolerance to low Fe availability was positively correlated with the HvNAS1 expression level. The NA concentration of the transgenic sweet potato leaves was up to 7.9-fold greater than that of the non-transgenic (NT) plant leaves. Furthermore, the Fe and zinc concentrations were 3- and 2.9-fold greater, respectively, in transgenic sweet potato than in NT plant leaves. Our results suggest that increasing the NA concentration of sweet potato by overexpression of HvNAS1 could significantly improve agricultural productivity and energy source.