Two nitrite reductase isoforms are present in tomato cotyledons and are regulated differently by UV-A or UV-B light and during plant development

The regulation by UV-A or UV-B light of the nuclear gene(s) encoding the pl astidic enzyme nitrite reductase (NIR; EC 1.7.7.1) was examined in the coty ledons of tomato (Lycopersicon esculentum L.). Two NiR isoforms designated NiR1 and NiR2 with apparent molecular masses of 63 kDa and 62 kDa, respecti vely, were detected by immunoblot analysis in total soluble protein extract s derived from tomato seedling cotyledons. Genomic Southern blot analysis i ndicated the presence of two NiR genes per haploid tomato genome. In etiola ted tomato cotyledons, the total NIR protein pool was almost exclusively co nstituted by NiR1. In contrast, NiR2 was the predominant NIR isoform in the cotyledons of tomato seedlings grown in white light. Illumination of etiol ated tomato cotyledons with UV-A or UV-B light resulted in an increase in b oth the total NIR transcript level and the NiR2 protein abundance. Blue lig ht stimulated the NiR2 protein pool above the level obtained with red light of equal photon fluence rate. These results show that NiR2 protein express ion is light-inducible and that the light-stimulation of NiR2 protein accum ulation involves the action of both phytochrome and a specific blue-light p hotoreceptor. The NiR1 protein level remained virtually unaffected by the l ight treatments. The change in the relative proportion of the NiR isoforms during greening of etiolated tomato cotyledons is, therefore, due to the di fferent light-responsiveness of the genes corresponding to NiR1 or NiR2. Th e physiological significance of the presence of NIR isoforms that are regul ated differently by light in tomato cotyledons is discussed.