Identification and expression analyses of two genes encoding putative low-affinity nitrate transporters from Nicotiana plumbaginifolia

Higher plants have both high- and low-affinity nitrate uptake systems (HATS and LATS respectively). Here we report the isolation and characterization of two genes, NpNRT1.1 and NpNRT1.2, from Nicotiana plumbaginifolia whose s tructural features suggest that they both belong to the NRT1 gene family; w hich is involved in the LATS. Amino acid sequence alignment showed that the N. plumbaginifolia proteins have greater similarity to their corresponding tomato homologues than to each other. Genomic Southern blot analysis indic ates that there are probably more than two members of this family in N. plu mbaginifolia. Northern blot analysis shows that NpNRT1.2 expression is rest ricted strictly to roots, whereas NpNRT1.1, in addition to roots, is expres sed at a basal level in all other plant organs. Likewise, differential expr ession in response to external treatments with various N sources was observ ed for these two genes: NpNRT1.1 can be considered as a constitutively expr essed gene whereas NpNRT1.2 expression is dependent strictly on high nitrat e concentrations. Finally, over-expression of a gene involved in the HATS d oes not lead to any modification of LATS gene expression.