Role of gibberellins and of the RGA and GAI genes in controlling nitrate assimilation in Arabidopsis thaliana

Screening of an Arabidopsis cDNA library for complementation of a yeast mutant affected in the regulation of nitrogen metabolism led to the isolation of the RGA and GAI cDNAs that were also known to be involved in plant response to gibberellins. This raised the question of RGA and GAI being also involved in controlling nitrogen metabolism in plants. To address this issue we studied whether loss of function (rga2, gai-t6) or gain of function mutations (gai-1) in RGA or GAI or in both (rga24 gai-t6) genes had any impact on nitrate assimilation in Arabidopsis plants grown in the greenhouse or in vitro. In addition, as the sole known plant function of RGA and GAI was their implication in gibberellin signal transduction, we analysed the effects of gibberellin treatment on nitrate assimilation of wild-type plants or of the gibberellin-deficient ga1-3 mutant. In most cases, no difference in expression of the NIA1, NIA2 (encoding nitrate reductase, NR EC 1.6.6.1), NII (encoding nitrite reductase, NiR EC 1.7.7.1), GS1 and GS2 genes (encoding cytosolic and chloroplastic glutamine synthetases EC 6.3.1.2) could be detected in the different genetic backgrounds or after GA treatment of wild-type plants or GA-deficient plants. This absence of effect is also in general supported by similar NR and NiR activities and nitrate or nitrogen content in the different plants. In conclusion, our studies show that RGA, GAI and gibberellins do not act as major factors controlling nitrate assimilation in Arabidopsis at the vegetative stage although they may have, in some instances, an effect on nitrate assimilation genes.