Nramp5 Is a Major Transporter Responsible for Manganese and Cadmium Uptake in Rice

TLDR

Paddy rice can accumulate high levels of manganese without toxicity, yet the mechanisms driving Mn uptake remain unclear. The study demonstrates that the rice Nramp5 protein mediates manganese uptake and its accumulation in the plant. Nramp5 is constitutively expressed in roots, localizes to the plasma membrane, and is polarized to the distal side of both exodermis and endodermis cells. Loss of Nramp5 drastically reduces growth and grain yield under low manganese, lowers Mn and Cd concentrations in roots and shoots, abolishes Mn and Cd uptake, and these effects can be partially rescued by high manganese but not by iron, confirming Nramp5 as a major Mn and Cd transporter.

Abstract

Paddy rice (Oryza sativa) is able to accumulate high concentrations of Mn without showing toxicity; however, the molecular mechanisms underlying Mn uptake are unknown. Here, we report that a member of the Nramp (for the Natural Resistance-Associated Macrophage Protein) family, Nramp5, is involved in Mn uptake and subsequently the accumulation of high concentrations of Mn in rice. Nramp5 was constitutively expressed in the roots and encodes a plasma membrane–localized protein. Nramp5 was polarly localized at the distal side of both exodermis and endodermis cells. Knockout of Nramp5 resulted in a significant reduction in growth and grain yield, especially when grown at low Mn concentrations. This growth reduction could be partially rescued by supplying high concentrations of Mn but not by the addition of Fe. Mineral analysis showed that the concentration of Mn and Cd in both the roots and shoots was lower in the knockout line than in wild-type rice. A short-term uptake experiment revealed that the knockout line lost the ability to take up Mn and Cd. Taken together, Nramp5 is a major transporter of Mn and Cd and is responsible for the transport of Mn and Cd from the external solution to root cells.