Competition in the Phaseolus vulgaris-Rhizobium symbiosis and the role of resident soil rhizobia in determining the outcomes of inoculation

Abstract

Background and Aims Inoculation of legumes with effective N2-fixing rhizobia is a common practice to improve farming profitability and sustainability. To succeed, inoculant rhizobia must overcome competition for nodulation by resident soil rhizobia that fix N2 ineffectively. In Kenya, where Phaseolus vulgaris (common bean) is inoculated with highly effective Rhizobium tropici CIAT899 from Colombia, response to inoculation is low, possibly due to competition from ineffective resident soil rhizobia. Here, we evaluate the competitiveness of CIAT899 against diverse rhizobia isolated from cultivated Kenyan P. vulgaris. Methods The ability of 28 Kenyan P. vulgaris strains to nodulate this host when co-inoculated with CIAT899 was assessed. Rhizosphere competence of a subset of strains and the ability of seed inoculated CIAT899 to nodulate P. vulgaris when sown into soil with pre-existing populations of rhizobia was analyzed. Results Competitiveness varied widely, with only 27% of the test strains more competitive than CIAT899 at nodulating P. vulgaris. While competitiveness did not correlate with symbiotic effectiveness, five strains were competitive against CIAT899 and symbiotically effective. In contrast, rhizosphere competence strongly correlated with competitiveness. Soil rhizobia had a position-dependent numerical advantage, outcompeting seed-inoculated CIAT899 for nodulation of P. vulgaris, unless the resident strain was poorly competitive. Conclusion Suboptimally effective rhizobia can outcompete CIAT899 for nodulation of P. vulgaris. If these strains are widespread in Kenyan soils, they may largely explain the poor response to inoculation. The five competitive and effective strains characterized here are candidates for inoculant development and may prove better adapted to Kenyan conditions than CIAT899.

The Beneficial Effects of Inoculation with Selected Nodule-Associated PGPR on White Lupin Are Comparable to Those of Inoculation with Symbiotic Rhizobia

“…Moreover, in general, PGPR are effective with different crops, and both L1 and L12 presented the necessary PGPR characteristics to improve plant growth in a variety of soils as they were able to grow in a wide pH and temperature range, plus they were tolerant to salt stress and to several heavy metals. The fact that they provided the same effects as efficient symbiotic bacteria can be of special interest when resident soil rhizobia are not highly effective in nitrogen fixation, which is relatively frequent for some legumes and soils, especially common bean [ 64 ]. Efficient nitrogen-fixing rhizobial inocula must compete with rhizobia present in the soil that nodulate efficiently but fix nitrogen poorly.…”

“…Efficient nitrogen-fixing rhizobial inocula must compete with rhizobia present in the soil that nodulate efficiently but fix nitrogen poorly. This competition might lead to a low response to inoculation [ 64 ]. In such cases, inoculation with powerful PGPR, such as strains L1 and L12, might represent a feasible advantageous alternative to boost crop yield.…”

Prevalence, diversity and applications potential of nodules endophytic bacteria: a systematic review

“…The effectiveness of nodulation relies on the efficiency and competitiveness of the rhizobia strains ( Andrews and Andrews, 2017 ). However, under stressful environmental conditions, competition between rhizobia strains and the rhizospheric bacterial population can result in ineffective nodulation ( Mwenda et al, 2023 ). In this context, Legumes, like other plants, interact with a diverse microbiome, including plant growth-promoting rhizobacteria (PGPR), to mitigate environmental stress impacts.…”

Conventional management has a greater negative impact on Phaseolus vulgaris L. rhizobia diversity and abundance than water scarcity

“…This is especially important in common bean under drought conditions as it is a very drought-sensitive crop ( Embrapa, 2018 ; Nawaz et al., 2021 ) with a high frequency of inefficient symbiotic relationships ( Michiels et al., 1998 ; Mwenda et al., 2023 ) that affect productivity. Considering the high nutritional value of common bean and that it is the grain legume for human consumption with the highest production worldwide ( Beebe et al., 2013 ; FAO, 2021 ), the search for strategies to improve its productivity under conditions of low water availability is a challenge of great interest.…”