Nitrite transport to the chloroplast is not a well documented process in spite of being a central step in the nitrate assimilation pathway. The lack of molecular evidence, as well as the easy diffusion of nitrite through biological membranes, have made this physiological process difficult to understand in plant nutrition. The aim of this review is to illustrate that nitrite transport to the chloroplast is a regulated step, intimately related to the efficiency of nitrate utilization. In Chlamydomonas reinhardtii, the Nar1;1 gene has been shown to have this role in nitrate assimilation. NAR1;1 corresponds to a plastidic membrane transporter protein related to the bacterial formate/nitrite transporters. At least four Nar1 genes might exist in Chlamydomonas. The existence of orthologous Nar1 genes in plants is discussed.
The expression of nitrite uptake activity and the induction of transcripts from several nitrate assimilation genes (Nii1, Nrt2;1, Nrt2;3, and Nar1) have been analysed in Chlamydomonas reinhardtii Dang. strains bearing several sets of genes encoding nitrate transport systems. Different nitrate concentrations resulted in a differential induction pattern of nitrite uptake activity depending on which particular nitrate transport system was present in the cells, and that was directly related to their relative efficiency for nitrate transport. The presence of the high-affinity nitrate transport system I (NRT2;1, NAR2) made cells able to sense the very low concentrations of nitrate present in culture medium with no added nitrate and to express optimally Nii1, Nrt2;1, Nrt2;3, and Nar1 genes involved in nitrate/nitrite assimilation. In addition, strains lacking nitrate reductase activity overexpressed these gene transcripts as a result of continuous signalling by nitrate, but only those bearing an active system I. This study supports the hypothesis that signalling of nitrate assimilation genes occurs intracellularly in a process dependent on the nitrate uptake activity. The bispecific nitrate/nitrite transport system I with the highest affinity for nitrate is the most efficient one for signalling the expression of nitrate/nitrite assimilation genes in C. reinhardtii.
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