The regulation of the nitrate assimilation and amino acid biosynthetic studied in transformed tobacco plants with reduced Fd-GOGAT activity

Abstract

Tobacco (Nicotiana tabacum) plants expressing a partial ferredoxin-dependent glutamine a-oxoglutarate aminotransferase (Fd-GOGAT) cDNA in the antisense orientation under the control of the 35S promoter, were used to study the metabolism of amino acids, 2-oxoglutarate and ammonium following the transition from CO2 enrichment (where photorespiration is inhibited) to air (where photorespiration is a major process of ammonium production in leaves). In the Fd-GOGAT antisense plants, the photorespiration-dependent increases in foliar ammonium, glutamine, a-KG and total amino acids were proportional to the decrease in Fd-GOGAT activity. The transgenic plants were used to examine the roles of ammonium, glutamine (Gln) and a-KG in the regulation of nitrate reductase (NR: EC 1.6.6.1) gene expression. In air, the simultaneous accumulation of Gln and a-KG was accompanied by an increase in the steady state level of NR transcripts. We conclude that the negative effect of Gln on NR transcript abundance is offset by high a-KG. It is therefore likely that the relative amounts of a-KG and Gln are more important in controlling NR gene transcription than the concentration of either metabolite alone. In addition we found that several pathways of amino acid biosynthesis were modified when NH4+ and Gln accumulate in leaves and that this accumulation was not due to increased proteolytic activity. The accumulation of a-KG, Gln and NH4+ in the leaves of the transformed plants with low Fd-GOGAT activity may initiate pathways of signal transduction leading to the simultaneous modulation of several pathways of nitrate assimilation and amino acid biosynthetsis.