Research Highlight Keeping Nitrate in the Roots: An Unexpected Requirement for Cadmium Tolerance in Plants

Li et al. (2010) identified AtNRT1.8 as a membrane transporter involved in the control of long-distance transport of nitrate between roots and shoot. Both the regulation of AtNRT1.8 expression and the phenotype of an nrt1.8 mutant indicate that this transporter plays an important role in protecting the plant against cadmium toxicity, and possibly against a wide range of biotic and abiotic stresses. The nitrate ion (NO3 2) is a major nitrogen source for higher plants. Following uptake from the soil solution into the plant, nitrate can be either stored in the vacuoles or assimilated to supply the nitrogen atom for amino acid biosynthesis. In many her-baceous species, nitrate assimilation occurs predominantly in the leaves (Andrews, 1986), where a large part of the reducing power needed for this process directly originates from the photosystems. Hence, most of the nitrate taken up by epi-dermal or cortical cells at the periphery of the root undergoes radial transport to the most inner tissues in the stele, and is secreted into the xylem sap for transloca-tion to the shoot. Xylem vessels are made of tracheary elements, which are dead when mature, and thus constitute an extra-cellular space. As a consequence, nutrients that move between xylem vessels and the adjacent cells have to cross a plasma mem-brane, thus requiring specific intrinsic transporter proteins. The molecular physiology of nitrate transport has been the matter of numerous studies during the past decade, especially in the model plant Arabidopsis thaliana. Nitrate uptake from the external medium into epidermis and cortex involves specific transporters of the NRT1 and NRT2 families (AtNRT1.1/CHL1, AtNRT1.2, AtNRT2.1), which are now quite well characterize