Transcriptional control of the inducible nitrate reductase isoform from soybeans

Identification of cDNA clones corresponding to two inducible nitrate reductase genes in soybean: analysis in wild-type and nr1 mutant

Among higher plants, soybean is unique in that biochemically it has been characterized as having two constitutive nitrate reductase (cNR) isoforms and one substrate-inducible nitrate reductase (iNR) isoform in leaves. All three NR isoforms are expressed in cv. Williams 82 while the nr1 mutant expresses only the iNR isoform. The genetic and molecular mechanisms for regulation of these isoforms have not been elucidated. We describe here the isolation, by reverse transcription-polymerase chain reaction (RT-PCR), of two cDNA clones encoding soybean NR. They were designated as iNR1 and iNR2, respectively, since both were inducible by nitrate. The iNR1 and iNR2 cDNAs cover total encoding regions of 2661 and 2673 nucleotides, respectively. The iNR1 clone shows a 12 bp deletion at the 5' end, relative to iNR2. They show overall similarity of 89% at the nucleotide level, and 87% at the amino acid level. Like all plant NRs cloned so far, deduced amino acid sequences between iNR1 and iNR2 show greatest variation at the N-terminal region while no difference was observed at the C-terminus. Soybean iNR mRNAs were found to be different from those of maize and tobacco in response to tungsten inhibitor treatment, since the inhibitor decreased the steady-state levels of mRNA for soybean iNR and for NiR. Using the same 5' regions of both cDNAs as the probes, Southern blot analysis of genomic DNA revealed differences in organization between iNR1 and iNR2. The genomic DNA from wild-type Williams 82 soybean was shown to have three Eco RI fragments while the nr1 mutant lacked an 8 kb fragment when probed with iNR1 cDNA. Likewise, the nr1 mutant lacked three Hae III restriction fragments when probed with iNR1 cDNA. When probed with iNR2, both wildtype and nr1 mutant showed one identical Eco RI band and two identical Hae III bands. In northern blots, the steady-state level of iNR1 mRNA was similar for the nr1 mutant and the wild-type parent after 20 to 48 h induction by nitrate. Based on the Eco RI and Hae III restriction enzyme digestion patterns observed in Southern blot analysis of soybean DNA, it is concluded that in soybean iNR1 is encoded by a small multiple gene family and iNR2 is a single gene.

Regulation of the inducible soybean nitrate reductase isoform in mutants lacking constitutive isoform(s)

In soybean, three nitrate reductase isoforms have been identified based on metabolic regulation, substrate specificity, and kinetic parameters. Two isoforms have been termed constitutive, as their activities are present without the addition exogenous nitrate to soybean seedlings. The third activity is termed inducible, as its activity is present only when soybean plants have been supplied with nitrate. The purpose of this study was to examine the regulation of the inducible nitrate reductase isoform in soybean mutants lacking one or both of the constitutive isoforms. Based on evidence obtained through measurements of enzyme activity, Western blotting, and RNA determinations, the absence of one or both of the constitutive nitrate reductase isoforms has no effect on the metabolite regulation of the inducible nitrate reductase isoform.

A chimeric Lhcb::Nia gene: an inducible counter selection system for mutants in the phytochrome signal transduction pathway

One approach towards understanding the transduction pathways of phytochromes is the selection of mutants impaired in various steps. We report here the construction of an inducible counter-selection system for such mutants employing the enzyme nitrate reductase. This enzyme can convert the benign substrate analogue chlorate to the toxic product chlorite, resulting in severe growth inhibition. An Arabidopsis thaliana nitrate reductase gene (Nial*2) was placed under the regulation of an Arabidopsis thaliana light-harvesting chlorophyll a/b protein (Lhcb1*3) promoter that is phytochrome-responsive. The chimeric Lhcb::Nia gene was transformed into A. thaliana. Homozygous transformant lines were selected and grown in the absence of nitrate and the presence of L-glutamine, conditions that substantially inhibited the expression of the endogenous nitrate reductase genes. In darkness seedlings of the transformed lines were resistant to chlorate; however, when seedlings were grown with intermittent red light, increased sensitivity to chlorate was observed. This sensitivity was correlated with an increase in both Nia1*2 RNA levels and nitrate reductase activity. The resistant seedlings were clearly distinguishable from the sensitive ones based on hypocotyl length, with no overlap in this parameter between the two populations. Thus, this system should allow for the selection of mutants that are impaired in phytochrome regulation of the transcription of Lhcb genes.

Regulation of the inducible nitrate reductase isoform from soybeans

The activity of the pH 7.5 NADH-linked nitrate reductase isoform from soybeans is termed inducible. Activity can be observed only in seedlings which have been supplied nitrate. Steady-state levels of mRNA for this isoform also show an absolute requirement for nitrate. Nitrate reductase specific mRNA can be observed within 2 h after nitrate treatment. Levels peaked 48 h after nitrate treatment, while the addition of glutamine to nitrate diminished amounts of nitrate reductase specific mRNA. Using nuclear runoff transcription assays, we have shown that one level of control of nitrate reductase synthesis is transcriptional.