Abstract
Common bean is one of the most important crops for human feed, and the most important legume for direct consumption by millions of people, especially in developing countries. It is a promiscuous host legume in terms of nodulation, able to associate with a broad and diverse range of rhizobia, although the competitiveness for nodulation and the nitrogen fixation capacity of most of these strains is generally low. As a result, common bean is very inefficient for symbiotic nitrogen fixation, and nitrogen has to be supplied with chemical fertilizers. In the last years, symbiotic nitrogen fixation has received increasing attention as a sustainable alternative to nitrogen fertilizers, and also as a more economic and available one in poor countries. Therefore, optimization of nitrogen fixation of bean-rhizobia symbioses and selection of efficient rhizobial strains should be a priority, which begins with the study of the natural diversity of the symbioses and the rhizobial populations associated. Natural rhizobia biodiversity that nodulates common bean may be a source of adaptive alleles acting through phenotypic plasticity. Crosses between accessions differing for nitrogen fixation may combine alleles that never meet in nature. Another way to discover adaptive genes is to use association genetics to identify loci that common bean plants use for enhanced biological nitrogen fixation and, in consequence, for marker assisted selection for genetic improvement of symbiotic nitrogen fixation. In this review, rhizobial biodiversity resources will be discussed, together with what is known about the loci that underlie such genetic variation, and the potential candidate genes that may influence the symbiosis' fitness benefits, thus achieving an optimal nitrogen fixation capacity in order to help reduce reliance on nitrogen fertilizers in common bean.
Highlights
Common bean (Phaseolus vulgaris L.) is cultivated worldwide, constituting a staple in developing countries in East Africa and South America, and in some regions of Asia, where it is the main source of protein [1]
These results suggest that R. etli could have had difficulty in persisting in the soils of Northern Spain when it was brought from America, probably because of the climatic conditions [95]
A more extensive screening of native common bean rhizobial strains in Ethiopian soils revealed that the majority of the isolates belonged to the species R. etli, R. leguminosarum and R. phaseoli, with a minor presence of R. leucaneae and R. giardinii, based on the sequencing of symbiotic and housekeeping genes [55]
Summary
Common bean (Phaseolus vulgaris L.) is cultivated worldwide, constituting a staple in developing countries in East Africa and South America, and in some regions of Asia, where it is the main source of protein [1]. It is accepted that domestication of common bean from wild forms took place several thousand years ago in two main and independent centers of diversification, Mesoamerica (Mexico and Central America) and the Andes (Peru, Bolivia, and Northwest Argentina), resulting in two highly differentiated gene pools [5]. From these centers, the crop was spread all over the American continent, and, after the first voyages of Columbus (1492), common bean was brought to Europe. In addition to genetic background, several abiotic factors can greatly
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