Final report covering period 10/1/90 to 10/1/95 : optimizing the Rhizobium - host symbiosis for increased bean production

The development of high-nitrogen-fixing bean-rhizobia pairs could have a tremendous impact on nutrition in many LDC"s in Africa and Latin America where beans are an important source of protein and calories and nitrogen fertilizers are often unavailable or unaffordable. The major barrier to the use of highly effective inoculant strains for beans is the highly competitive indigenous R. etli populations, which often exclude the inoculant strains from the nodules, but fail to fix sufficient nitrogen to satisfy the host needs. The long-term goal of this project is to increase bean production by establishing an effective bean-Rhizobium symbiosis through genetic manipulation of the plant and microsymbiont. In order to overcome the competitiveness problem, we will manipulate the host and symbiont genotypes to maximize modulation by the inoculant strains. Our first objective is to breed beans for the ability to select a highly effective strain from a mixture in the soil. To accomplish this, we developed a rapid screen in which visible symptoms correlate with nodule occupancy, we then collected widely diverse bean germplasm, and we screened the germplasm for the ability to select the high nitrogen fixing strain, KIM5, into the nodules. Of the 2,000 accessions screened, we found 92 accessions that are significantly selective for KIM5. This trait was passed on, indicating that it is heritable. In addition, we are constructing strains of R. etli that are more competitive in modulation by selecting for superior root colonizers. We found that strains that are superior in root colonization are also superior in modulation competitiveness. This past year we determined that addition of methionine prevented the mutants from dominating in the rhizosphere and nodules, suggesting that the parent strain is impaired in methionine uptake, utilization, or synthesis. We also made progress toward understanding the genetic basis of this trait, with preliminary evidence suggesting that we have cloned the gene responsible. (Author abstract)