DIFFERENTIAL EXPRESSION OF THE ARABIDOPSIS NIA1 AND NIA2 GENES - CYTOKININ-INDUCED NITRATE REDUCTASE-ACTIVITY IS CORRELATED WITH INCREASED NIA1 TRANSCRIPTION AND MESSENGER-RNA LEVELS

Nitrate reductase (NR) activity increased up to 14-fold in response to treatment of Arabidopsis thaliana seedlings with the cytokinin benzyl adenine. NR induction was observed in seedlings germinated directly on cytokinin-containing medium, seedlings transferred to cytokinin mediu m, and seedlings grown in soil in which cytokinin was applied directly to the leaves. About the same level of induction was seen in both wil d-type and Nia2-deletion mutants, indicating that increased NR activit y is related to the expression of the minor NR gene, Nia1. The steady- state Nia1 mRNA level was increased severalfold in both wild-type and mutant seedlings after benzyladenine treatment. Transcript levels of t he Nia2 gene, which is responsible for 90% of the NR activity in devel oping wild-type seedlings, did not show any changes upon cytokinin tre atment. Nuclear run-on assays demonstrated that Nia1 gene transcriptio n increased dramatically after cytokinin treatment.