Modulation of carbon and nitrogen metabolism, and of nitrate reductase, in untransformed and transformed Nicotiana plumbaginifolia during CO2 enrichment of plants grown in pots and in hydroponic culture
Abstract
Transformed plants of Nicotiana plumbaginifolia Viv. constitutively expressing nitrate reductase (35S-NR) or b-glucuronidase (35S-GUS) and untransformed controls were grown for two weeks in a CO 2enriched atmosphere. Whereas CO 2 enrichment (1000 ll á l A1) resulted in an increase in the carbon (C) to nitrogen (N) ratio of both the tobacco lines grown in pots with vermiculite, the C/N ratio was only slightly modi®ed when plants were grown in hydroponic culture in high CO 2 compared to those grown in air. Constitutive nitrate reductase (NR) expression per se did not change the C/N ratio of the shoots or roots. Biomass accumulation was similar in both types of plant when hydroponic or pot-grown material, grown in air or high CO 2 , were compared. Shoot dry matter accumulation was primarily related to the presence of stored carbohydrate (starch and sucrose) in the leaves. In the potgrown tobacco, growth at elevated CO 2 levels caused a concomitant decrease in the N content of the leaves involving losses in NO À 3 and amino acid levels. In contrast, the N content and composition were similar in all plants grown in hydroponic culture. The 35S-NR plants grown in air had higher foliar maximum extractable NR activities and increased glutamine levels (on a chlorophyll or protein basis) than the untransformed controls. These increases were maintained following CO 2 enrichment when the plants were grown in hydroponic culture, suggesting that an increased ¯ux through nitrogen assimilation was possible in the 35S-NR plants. Under CO 2 enrichment the NR activation state in the leaves was similar in all plants. When the 35S-NR plants were grown in pots, however, foliar NR activity and glutamine content fell in the 35S-NR transformants to levels similar to those of the untransformed controls. The dierences in NR activity between untransformed and 35S-NR leaves were much less pronounced in the hydroponic than in the pot-grown material but the dierence in total extractable NR activity was more marked following CO 2 enrichment. Foliar NR message levels were decreased by CO 2 enrichment in all growth conditions but this was much more pronounced in potgrown material than in that grown hydroponically. Since b-glucuronidase (GUS) activity and message levels in 35S-GUS plants grown under the same conditions of CO 2 enrichment (to test the eects of CO 2 enrichment on the activity of the 35S promoter) were found to be constant, we conclude that NR message turnover was speci®cally accelerated in the 35S-NR plants as well as in the untransformed controls as a result of CO 2 enrichment. The molecular and metabolic signals involved in increased NR message and protein turnover are not known but possible eectors include NO À 3 , glutamine and asparagine. We conclude that plants grown in hydroponic culture have greater access to N than those grown in pots. Regardless of the culture method, CO 2 enrichment has a direct eect on NR mRNA stability.