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Co-evolution of Phaseolus vulgaris and its rhizobia

Background

Beans were domesticated independently in both Andean South America (~4,400BP) and in Mesoamerica (2,800 BP), with multiple domestication events likely in each region. Limited seed trade between the different regions in which domestication occurred, has allowed the fixation of different seed traits in bean plants from each area, including perhaps coevolution of host and rhizobia.

Bean remains in the south-western USA date to 300-700 AD. By 1300 AD beans were grown by native Americans in both the Susquehanna Valley and Knife River regions. Maize, beans and squash were important components of the diet for some Indian tribes including the Hidatsa and Mandans. For example, Lewis and Clark were fed beans in Mandan (ND) in 1804.

Bean distribution to other areas of the world occurred largely after Spanish colonization of Latin America. Post colonization, beans spread from Mesoamerica to the Caribbean and then Brazil, and from Andean South America to Europe and thence Africa. Rhizobia for beans could have been transferred as seed contaminants, but as survival on the seed is low, founder effects would be likely. Since Phaseolus vulgaris is a promiscuous host nodulated by at least six species of rhizobia, introduced plants could also have established symbiosis with rhizobia from Leucaena, Onobrychis, Dalea, etc.

Studies on the diversity of rhizobia from Phaseolus vulgaris in Central and South America

We have shown that rhizobia from bean soils in Ecuador belong primarily to the species R.etli. Despite this they are clearly different from rhizobia of that species collected in Mexico, as shown by their DNA fingerprints obtained using BOXA1R-PCR. Further, there appear to be differences between isolates collected in Northern Ecuador (where most bean production is for export to Colombia) and those from the south of the country (where production is mainly for home consumption, and often employs seed mixtures)(Bernal and Graham, 2001).

Differences between bean cultivars in the rhizobia with which they will associate have been shown. Thus, when bean cultivars representing different domestication events are planted in the same soil, each will recover different rhizobia.

Differences in the diversity of bean rhizobia collected from the different centers of origin of this crop.

Studies on the diversity of rhizobia from Phaseolus vulgaris from bean soils in Central Minnesota

Until recently beans grown in Minnesota were rarely inoculated. Further, bean seed was often treated with captan and streptomycin to control root disease, presumably killing most rhizosphere organisms including rhizobia. We characterized the rhizobia found in the bean soils of this region using BoxA1R-PCR, showing that few of these isolates were apparently similar to reference rhizobia known to nodulate beans. We then tried to identify representative strains using Fatty Acid Methyl Ester (FAME) analysis and 16S rRNA sequence analysis. Each of these approaches identified the majority of these organisms differently, highlighting ongoing problems with species recognition in this genus.

Differences between methods of strain characterization in how each identified strains representative of different groups of bean rhizobia recovered from soil in Staples/Verndale, MN

Because we thought that exposure to captan and streptomycin used as seed protectants might have selected for atypical bean rhizobia we tested rhizobial isolates obtained from soils in the Staples/Verndale area for their tolerance to each antimicrobial compound. Surprisingly the only strain that was resistant to these compounds was one of our inoculant strains R.tropici UMR1899, included as a control. When we then tested additional isolates of this species for tolerance to streptomycin and captan, essentially all isolates from R.tropici Type IIB were resistant, whereas R.tropici Type IIA strains showed very limited tolerance.

We had also hypothesized that rhizobia from Dalea might have transferred to bean after the opening of what was prairie to agriculture, post 1800. When recovery of Dalea rhizobia on beans was shown to be very infrequent, we tested different groups of rhizobia to determine if they were limited in their ability to compete for nodulation sites, and might not be competitive. For this we used the root-tip marking assay to estimate speed in nodulation, and were able to show that the rhizobia from both Dalea and R.giardini were slow to nodulate beans.

Speed in nodulation of bean isolates from Staples and Verndale
compared to R.giardini and a strain from Dalea