Glutamine and α-ketoglutarate are metabolite signals involved in nitrate reductase gene transcription in untransformed and transformed tobacco plants deficient in ferredoxin-glutamine-α-ketoglutarate aminotransferase

Transformed tobacco (Nicotiana tabacum L.) plants with varying activities of the key enzyme of ammonia assimilation, ferredoxin-glutamine-alpha -ketoglutarate aminotransferase (Fd-GOGAT; EC 1.4.7.1), were used to examine the roles of ammonium, glutamine (Gln) and alpha -ketoglutarate (alpha -KG) in the regulation of nitrate reductase (NR; EC 1.6.6.1) transcript abundance. In wild-type leaf discs, NR mRNA abundance was increased following feeding with NO3-, sucrose and alpha -KG and decreased by feeding Gln. In air, leaves with decreased GOGAT accumulated Gln and alpha -KG simultaneously; this was accompanied by increased NR transcripts. The inhibition of NR transcription by Gln observed in leaf-disc experiments was therefore not observed in the low-Fd-GOGAT plants that accumulate Gln in vivo. The results suggest that the negative effect of Gln on NR transcript abundance was offset by high alpha -KG and that the relative amounts of alpha -KG and Gln are more important in controlling NR gene transcription than the concentration of either metabolite alone.