Differential expression of the nitrite reductase gene family in tobacco as revealed by quantitative competitive RT-PCR.

Tobacco (Nicotiana tabacum L. cv. Xanthi XHFD8) possesses four nitrite reductase (NiR) genes: nii1, nii2, nii3, and nii4. Their differential expression in leaves and roots was investigated by quantitative competitive RT-PCR using gene-specific primer pairs. These results appear to contradict existing views on the expression of these NiR genes: (i) the mRNA of each of the four NiR genes was distinguishable both in leaves and roots; (ii) nitrate treatment increased nii1 and nii3 mRNA in leaves and roots by at least 4-fold (at least 5-fold in nii2 and nii4 mRNA); and (iii) the steady-state levels of nii1 and nii3 mRNA were almost the same in leaves (6-7 x 10(5) and about 3 x 10(6) copies microg(-1) of total RNA before and after nitrate treatment, respectively) and in roots (3-4 x 10(4) and 3-6 x 10(5) copies microg(-1) of total RNA before and after nitrate treatment, respectively). Very similar relationships were obtained for the steady-state levels of nii2 and nii4 mRNA in roots (2-4 x 10(5) and 8 x 10(6) copies microg(-1) of total RNA before and after nitrate treatment, respectively), and in leaves (5-9 x 10(4) and 4 x 10(5) copies microg(-1) of total RNA before and after nitrate treatment, respectively). These results demonstrate that nii1 and nii3 transcripts are a dominating, but not exclusive, NiR mRNA in leaves, and the same is true for nii2 and nii4 transcripts in roots.