TOWARD MORE PRODUCTIVE, EFFICIENT, AND COMPETITIVE NITROGEN-FIXING SYMBIOTIC BACTERIA

The prospects of developing strains of legume nodule bacteria that pro vide higher productivity of leguminous plants are described. The gener ic, biochemical, physiological, regulatory, and economic constraints t hat govern the ability of private and public efforts to construct bett er inoculants for legume nodulation are discussed. Success in construc ting better inoculants requires a two-pronged approach. First, strains need to be improved in order to compete successfully with indigenous strains for root nodulation of legumes. Several loci have been identif ied to date that affect competitiveness for strain nodule occupancy. U sually mutations in these loci affect the ability of a strain to form nodules rapidly and efficiently. Other loci, such as those that confer antibiotic production, can be added to strains to enhance nodulation competitiveness when co-inoculated with antibiotic-sensitive strains. Second, the inoculum strains must be improved with respect to symbioti c nitrogen fixation. Efforts to enhance the symbiotic productivity of legume nodule bacteria either by mutation pr genetic engineering are a lso described. The best characterized example of these is the hydrogen ase system. Due to nitrogenase-dependent catalysis of proton reduction , diazotrophs evolve large amounts of H-2. An approach to maximize the efficiency of symbiotic N-2 fixation, and therefore of legume product ivity, is to construct strains of Rhizobium with the ability to oxidiz e this otherwise wasted H-2. The electrons produced by H-2 oxidation a re funneled through energy-conserving electron transport chains; Our k nowledge of the genetics and biochemistry of H-2 oxidation in Bradyrhi zobium japonicum and Rhizobium leguminosarum has developed rapidly in recent years. At least 20 genes are needed for these bacteria to manuf acture and efficiently express a nickel-containing H-2-uptake hydrogen ase. These genes include those encoding regulatory elements, posttrans lational processing enzymes, nickel-sensing and nickel-metabolism prot eins, and electron transport components for integrating the electrons from H-2 oxidation into the respiratory chain. Some of the components for oxidizing H-2 in the symbiotic N-2 fixing bacteria are distinct fr om the analogous components in (nonsymbiotic) H-2 oxidizing bacteria.