Diversity in the rhizobia associated withPhaseolus vulgarisL. in Ecuador, and comparisons with Mexican bean rhizobia

Abstract

Common beans (Phaseolus vulgaris L.) have centers of origin in both Mesoamerica and Andean South America, and have been domesticated in each region for perhaps 5000 years. A third major gene pool may exist in Ecuador and Northern Peru. The diversity of the rhizobia associated with beans has also been studied, but to date with an emphasis on the Mesoamerican center of origin. In this study we compared bean rhizobia from Mexico and Andean South America using both phenotypic and phylogenetic approaches. When differences between the rhizobia of these two regions were shown, we then examined the influence of bean cultivar on the most probable number (MPN) count and biodiversity of rhizobia recovered from different soils. Three clusters of bean rhizobia were distinguished using phenotypic analysis and principal-component analysis of Box AIR-PCR banding patterns. They corresponded principally to isolates from Mexico, and the northern and southern Andean regions, with isolates from southern Ecuador exhibiting significant genetic diversity. Rhizobia from Dalea spp., which are infective and effective on beans, may have contributed to the apparent diversity of rhizobia recovered from the Mesoamerican region, while the rhizobia of wild Phaseolus aborigineus from Argentina showed only limited similarity to the other bean rhizobia tested. Use of P. vulgaris cultivars from the Mesoamerican and Andean Phaseolus gene pools as trap hosts did not significantly affect MPN counts of bean rhizobia from the soils of each region, but did influence the diversity of the rhizobia recovered. Such differences in compatibility of host and Rhizobium could be a factor in the poor reputation for nodulation and N2 fixation in this crop.

Analysis of Rhizobium etli and of its symbiosis with wild Phaseolus vulgaris supports coevolution in centers of host diversification

“…These strains are examples of populations from the United States (n ϭ 5), Mexico (n ϭ 21), Belize (n ϭ 2), Colombia (n ϭ 10), Ecuador (n ϭ 16), Peru (n ϭ 2), Brazil (n ϭ 12), Spain (n ϭ 21), Indonesia (n ϭ 4), and Burundi (n ϭ 1). Some of these strains had been characterized previously for their plasmid profile, DNA fingerprint, and symbiotic ability (8,10,12,(19)(20)(21). The results shown in Table 1 demonstrate that nodC types ␣, ␥, and ␦ were represented in the collection, but, unlike the results of isolates from NWA shown in the preceding section, only eight strains showed nodC type ␦, namely seven strains from Ecuador and one from Peru.…”

“…R. etli bv. phaseoli is the predominant species found associated with both wild and cultivated common beans from Mexico, Colombia, and the southern Andes (7), leading to the proposal of coevolution between bean host and rhizobia, although this has not been fully demonstrated and remains a matter of conjecture (8,9).…”

“…To test whether lineages that were recovered at high frequency from each soil sample are more competitive for nodulation or are just better adapted to soil and numerically dominant, assays for nodulation competitiveness were performed with representative bean plants from the different genetic pools. Although several studies have used agriculturally cultivated beans (8,26), our study was performed mostly with wild common beans, and its resulting data were quantitative and suitable for statistical analysis. We found accessions of wild as well as cultivated common beans that are preferentially nodulated by the lineage nodC type ␣, whereas other accessions nodulate preferentially with the lineage nodC type ␦.…”

Genetic and phenotypic diversity of Rhizobium isolates from Southern Ecuador

“…In Ecuador, an Andean region from which common bean originates (Rodiño et al, 2010), few rhizobia identification studies have been conducted, despite potentially being an important source of rhizobial diversity, which is a key determinant of common bean productivity (Baginsky et al, 2015). Recently, Ribeiro et al (2015) determined the taxonomic affiliations of isolated strains from Ecuadorian soil previously reported by Bernal and Graham (2001) and Mexican strains, showing that all the Ecuadorian isolates corresponded to three novel lineages from the Rhizobium etli group that fall into the R. phaseoli/R. etli/R.…”

“…The few previous studies from the Ecuadorian Andean region have focused only on determining the phylogenetic potential of symbionts (Bernal;Graham, 2001;Ribeiro et al, 2013). However, the effect of rhizobia on plant phenotypic parameters and the variability among isolated strains are rarely studied under laboratory, greenhouse or field conditions.…”