Triplett E W, Sadowsky M J
Department of Agronomy, University of Wisconsin, Madison 53706.
Annu Rev Microbiol. 1992;46:399-428. doi: 10.1146/annurev.mi.46.100192.002151.
An economically important problem in microbial ecology concerns the efficacy of rhizobial inoculants for the formation of nitrogen-fixing root nodules on legume crop plants such as soybean, alfalfa, and clover. Some strains of rhizobia can increase symbiotic nitrogen fixation under controlled conditions. However, attempts to improve nitrogen fixation under agricultural conditions with such strains often fail, usually as a result of the presence of indigenous rhizobia limiting nodulation by the inoculum strains. This problem is referred to as the Rhizobium competition problem, and molecular genetics is being used to address the problem from two perspectives. First, the host specificity of rhizobia is being characterized with the long term goal of developing strains that can nodulate a very strain-specific host-legume genotype. Second, the genetic basis of competitiveness in several strains is being examined. Genetic determinants of nodulation competitiveness have been isolated and mechanisms for their stable integration into the genome of superior nitrogen-fixing strains have been developed. Several phenotypes have been identified as playing an important role in nodulation competitiveness including antibiosis, motility, speed of nodulation, cell-surface characteristics, and nodulation efficiency. Several solutions to this problem are likely to result from these strategies and will be useful for certain legumes in specific locations.
微生物生态学中一个具有重要经济意义的问题涉及根瘤菌接种剂在豆科作物（如大豆、苜蓿和三叶草）上形成固氮根瘤的功效。一些根瘤菌菌株在受控条件下可以增加共生固氮作用。然而，利用这些菌株在农业条件下提高固氮作用的尝试往往失败，通常是由于本地根瘤菌的存在限制了接种菌株的结瘤。这个问题被称为根瘤菌竞争问题，分子遗传学正从两个角度来解决这个问题。首先，正在对根瘤菌的宿主特异性进行表征，其长期目标是培育出能够与非常菌株特异性的宿主豆科植物基因型结瘤的菌株。其次，正在研究几种菌株竞争力的遗传基础。已经分离出结瘤竞争力的遗传决定因素，并开发了将其稳定整合到优良固氮菌株基因组中的机制。已经确定了几种在结瘤竞争力中起重要作用的表型，包括抗菌性、运动性、结瘤速度、细胞表面特征和结瘤效率。这些策略可能会产生几种解决这个问题的方法，并且对特定地点的某些豆科植物有用。
