Nitrate reductase and growth of Arabidopsis thaliana in solution culture

Abstract

As a principal limiting nutrient, the metabolism of nitrogen is central to understanding plant growth and performance. Nitrate reductase (NR) is the first, and thus a critical, enzyme in the overall pathway for N incorporation. As such, and because it is highly regulated at the transcriptional, translational and post-translational levels, NR has received a large amount of experimental attention. On the other hand, there have been a number of reports that plant growth can be maintained with much less, or much more, NR activity than is normally present, and without the normal controls. The actual relationship between NR activity and growth is, however, poorly understood. The objective of this study was to examine the integration of growth, protein accumulation and patterns of nitrate reductase (NR) activity in wild-type Arabidopsis thaliana L. plants and a mutant lacking NR. Our results show that WT plants, during their exponential growth phase, had as much as 15-times more NR activity than needed to support the recorded growth. NR-deficient mutants, on the other hand, achieved equivalent exponential growth rates, lagging WT by about two weeks, but had less than 20% of the NR needed to support that growth. Both WT and mutants grew equally well when nitrite was supplied instead of nitrate, so long as the nitrite was continuously present, suggesting involvement of N-conversions in the growth medium. When N was supplied only in short pulses, nitrite toxicity was indicated, but the mutants still survived and grew on nitrate. We conclude that the absolute requirements for NR in plants may be highly flexible. The mechanism by which NR-deficient plants incorporate nitrate and support growth remains unclear, however.