Isolation and Initial Characterization of Constitutive Nitrate Reductase-Deficient Mutants NR328 and NR345 of Soybean (Glycine max)

The value of biodiversity in legume symbiotic nitrogen fixation and nodulation for biofuel and food production

Much of modern agriculture is based on immense populations of genetically identical or near-identical varieties, called cultivars. However, advancement of knowledge, and thus experimental utility, is found through biodiversity, whether naturally-found or induced by the experimenter. Globally we are confronted by ever-growing food and energy challenges. Here we demonstrate how such biodiversity from the food legume crop soybean (Glycine max L. Merr) and the bioenergy legume tree Pongamia (Millettia) pinnata is a great value. Legume plants are diverse and are represented by over 18,000 species on this planet. Some, such as soybean, pea and medics are used as food and animal feed crops. Others serve as ornamental (e.g., wisteria), timber (e.g., acacia/wattle) or biofuel (e.g., Pongamia pinnata) resources. Most legumes develop root organs (nodules) after microsymbiont induction that serve as their habitat for biological nitrogen fixation. Through this, nitrogen fertiliser demand is reduced by the efficient symbiosis between soil Rhizobium-type bacteria and the appropriate legume partner. Mechanistic research into the genetics, biochemistry and physiology of legumes is thus strategically essential for future global agriculture. Here we demonstrate how molecular plant science analysis of the genetics of an established food crop (soybean) and an emerging biofuel P. pinnata feedstock contributes to their utility by sustainable production aided by symbiotic nitrogen fixation.

A variety of regulatory mechanisms are involved in the nitrogen-dependent modulation of the nodule organogenesis program in legume roots

Combined nitrogen (N) sources are known to strongly affect initiation, development and functioning of Nitrogen-Fixing-Nodules whose formation is triggered by lipochitin-oligosaccharide signals secreted in the rhizospere by the Rhizobium partner. The rapid effects of N supply on nodule initiation have been mainly described when N sources are present at the moment of Rhizobium inoculation or purified Nod Factors addition. We recently reported that high ammonium nitrate growth conditions might also strongly affect the nodulation competence of Lotus japonicus plants, prior to the Rhizobium inoculation. This is a long-term effect, which suggests a change of the general nutritional status as the signal controlling the reduced nodulation capacities. The mechanisms underlying these inhibitory pathways are apparently different and the identification of the molecular actors involved may provide new insights into the linkage between N environmental changes and root organogenesis programs.

Enhanced detection of polymorphic DNA by multiple arbitrary amplicon profiling of endonuclease-digested DNA: identification of markers tightly linked to the supernodulation locus in soybean

Multiple endonuclease digestion of template DNA or amplification products can increase significantly the detection of polymorphic DNA in fingerprints generated by multiple arbitrary amplicon profiling (MAAP). This coupling of endonuclease cleavage and amplification of arbitrary stretches of DNA, directed by short oligonucleotide primers, readily allowed distinction of closely related fungal and bacterial isolates and plant cultivars. MAAP analysis of cleaved template DNA enabled the identification of molecular markers linked to a developmental locus of soybean (Glycine max L. Merrill). Ethyl methane sulfonate (EMS)-induced supernodulating, near-isogenic lines altered in the nts locus, which controls nodule formation, could be distinguished from each other and from the parent cultivar by amplification of template pre-digested with 2-3 restriction enzymes. A total of 42 DNA polymorphisms were detected using only 19 octamer primers. In the absence of digestion, 25 primers failed to differentiate these soybean genotypes. Several polymorphic products co-segregated tightly with the nts locus in F2 families from crosses between the allelic mutants nts382 and nts1007 and the ancestral G. soja Sieb. & Succ. PI468.397. Our results suggest that EMS is capable of inducing extensive DNA alterations, probably around discrete mutational hot-spots. EMS-induced DNA polymorphisms may constitute sequence-tagged markers diagnostic of specific genomic regions.

Inheritance of supernodulation in soybean and estimation of the genetically effective cell number

Provided the nature of inheritance is known, the frequency of homozygous mutant plants in individual M2 families (derived from M1 seed) can be used to estimate the genetically effective cell number (GECN). Segregation ratios in M3 families derived from M2 wild-type plants indicated that the supernodulation characters nts382, nts1007 and nts183 are inherited as Mendelian recessives. The nature of inheritance was also known or confirmed to be recessive by crossing the wild type to these and several other mutants derived from the same population of M2 families. Subsequently, using the frequency of mutant plants in individual M2 families, the GECN for soybean was calculated to be approximately two.

A method for the separation and partial purification of the three forms of nitrate reductase present in wild-type soybean leaves

A rapid and simple purification method was used to separate and purify nitrate reductases (NR) from Williams soybean leaves. Blue Sepharose columns were sequentially eluted with 50 millimolar NADPH and 50 millimolar NADH, thus separating NAD(P)H:NR from NADH:NRs. Subsequent purification of the collected peaks on a fast protein liquid chromatography-Mono Q column enabled separation of two NADH:NRs. Sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed that the subunit relative molecular mass for all three NR forms (constitutive NAD(P)H:NR [pH 6.5], EC 1.6.6.2; constitutive NADH:NR [pH 6.5], EC not assigned; and inducible NADH:NR [pH 7.5], EC 1.6.6.1) was approximately 107 to 109 kilodaltons. All three NRs showed similar spectra with absorption maxima at 413 and 273 nanometers in the oxidized state, and with the characteristics of a cytochrome b type heme upon reduction with NADH (absorption maxima at 556, 527, and 424 nanometers). The technique developed provides an improved separation of the three NR forms from soybean leaves. The similarity of the NRs with regard to their cytochrome b(556) type heme content and in relative molecular mass indicated that other differences must exist to account for the different kinetic and physical properties previously reported.