Natural rhizobial diversity helps to reveal genes and QTLs associated with biological nitrogen fixation in common bean

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.

Rhizobial–Host Interactions and Symbiotic Nitrogen Fixation in Legume Crops Toward Agriculture Sustainability

“…Some legumes display a rigid regime of symbiotic bacteria preference while others tend to be flexible. Common bean (Phaseolus vulgaris) and soybean (Glycine max), for example, can be nodulated by a large number of taxonomically different rhizobia including members of genus Burkholderia, while in pea (Pisum sativum) and chickpea (Cicer arietinum) the symbiosis is restricted to a fairly narrow range of rhizobial strains (Andrews and Andrews, 2017;Muñoz-Azcarate et al, 2017;Ramírez et al, 2019; Figure 2).…”

“…A very high specificity within IRLC legumes is associated with a greater N fixation efficiency on the basis of per unit carbon utilized (Oono and Denison, 2010;Kereszt et al, 2011). In contrast, common bean which is a promiscuous host to rhizobial strains is considered as a weak nitrogen fixer compared to other grain legumes (Muñoz-Azcarate et al, 2017).…”

Phylogeography and Symbiotic Effectiveness of Rhizobia Nodulating Chickpea (Cicer arietinum L.) in Ethiopia

“…This could indicate that diversity is limited at the country level compared with global populations. On the other hand, it is conceivable that regions, such as Ethiopia with high chickpea diversity and long cultivation history, harbour genetically diverse rhizobia [25,26]. Also, local contrasts in environmental factors such as pH, temperature, moisture, salinity, elevation and the presence of host plants are known to influence the distribution of the rhizobia [27][28][29] and may determine patterns of genetic diversity in Ethiopian Mesorhizobium species that can nodulate chickpea [23].…”

Response of soybean (Glycine max L.) to application of lime and phosphate fertilizer in an acid soil of western Kenya

“…It can also be used to produce oil and biofuels among other uses and hence its economic returns are higher than those of common grain legumes (Mathu et al, 2010). Soybean is also reputed to be better at fixing atmospheric nitrogen and therefore has a higher potential to replenish soil fertility than the common bean which is inherently a poor nitrogen fixer (Fageria et al, 2014;Muñoz-Azcarate, 2017). However, soil acidity and phosphorus deficiencies, which often occur concurrently, constrain soybean production in western Kenya with the average yields of 0.93 t ha -1 reported from farmers' fields against a potential of more than 3.5 t ha -1 for improved varieties under optimal growth conditions (Mahasi et al, 2010;Verde et al, 2013).…”