A nitrite transporter associated with nitrite uptake by higher plant chloroplasts

Chloroplasts take up cytosolic nitrite during nitrate assimilation. In this study we identified a nitrite transporter located in the chloroplasts of higher plants. The transporter, CsNitr1- L, a member of the proton- dependent oligopeptide transporter ( POT) family, was detected during light- induced chloroplast development in de- etiolating cucumber seedlings. We detected a CsNitr1- L - green fluorescent protein ( GFP) fusion protein in the chloroplasts of leaf cells and found that an immunoreactive 51 kDa protein was present in the isolated inner envelope membrane of chloroplasts. CsNitr1- L has an isoform, CsNitr1- S, with an identical 484 amino acid core sequence; however, in CsNitr1- S the 120 amino acid N- terminal extension is missing. Saccharomyces cerevisiae cells expressing CsNitr1- S absorbed nitrite from an acidic medium at a slower rate than mock- transformed control cells, and accumulated nitrite to only one- sixth the concentration of the control cells, suggesting that CsNitr1- S enhances the efflux of nitrite from the cell. Insertion of T- DNA in a single CsNitr1- L homolog ( At1g68570) in Arabidopsis resulted in nitrite accumulation in leaves to more than five times the concentration found in the wild type. These results show that it is possible that both CsNitr1- L and CsNitr1- S encode efflux- type nitrite transporters, but with different subcellular localizations. CsNitr1- L may possibly load cytosolic nitrite into chloroplast stroma in the chloroplast envelope during nitrate assimilation. The presence of genes homologous to CsNitr1- L in the genomes of Arabidopsis and rice indicates that facilitated nitrite transport is of general physiological importance in plant nutrition.