Plasma membrane-localized transporter for aluminum in rice

TLDR

Aluminum is abundant yet highly toxic, and how it enters cells remains unknown. The study aimed to identify a rice transporter, Nrat1, that specifically mediates trivalent Al uptake. Nrat1, when expressed in yeast, transports only trivalent Al and is localized to the plasma membrane of root‑tip cells, excluding epidermal cells. Loss of Nrat1 reduces Al uptake, increases cell‑wall binding and sensitivity, while its expression is Al‑induced via ART1 and is essential for vacuolar sequestration of Al.

Abstract

Aluminum (Al) is the most abundant metal in the Earth's crust, but its trivalent ionic form is highly toxic to all organisms at low concentrations. How Al enters cells has not been elucidated in any organisms. Herein, we report a transporter, Nrat1 (Nramp aluminum transporter 1), specific for trivalent Al ion in rice. Nrat1 belongs to the Nramp (natural resistance-associated macrophage protein) family, but shares a low similarity with other Nramp members. When expressed in yeast, Nrat1 transports trivalent Al ion, but not other divalent ions, such as manganese, iron, and cadmium, or the Al–citrate complex. Nrat1 is localized at the plasma membranes of all cells of root tips except epidermal cells. Knockout of Nrat1 resulted in decreased Al uptake, increased Al binding to cell wall, and enhanced Al sensitivity, but did not affect the tolerance to other metals. Expression of Nrat1 is up-regulated by Al in the roots and regulated by a C2H2 zinc finger transcription factor (ART1). We therefore concluded that Nrat1 is a plasma membrane-localized transporter for trivalent Al, which is required for a prior step of final Al detoxification through sequestration of Al into vacuoles.