Rhizobium pisi gains a symbiovar
The concept of a symbiovar is key to understanding the diversity of rhizobia. The genes that determine symbiotic host range are part of the accessory genome that can transfer between strains and between species. The consequence is that different bacterial species (usually closely related) may carry almost identical symbiosis genes and have the same host range, while strains that are in the same species may carry quite different symbiosis genes and have distinct host ranges. Jarvis et al. (1980) were the first to recognise this situation by proposing that clover symbionts formed a biovar of Rhizobium leguminosarum. With remarkable insight for the time, they speculated:
“It seems likely that specific plasmids confer plant specificity on basically similar strains of bacteria and thus provide an alternative mechanism for the acquisition of plant specificity which does not require evolutionary specialization and consequent genetic divergence.
Carl Jordan, writing in Bergey’s Manual (1984), formalised the description of three biovars of R. leguminosarum (bv. viciae, bv. trifolii, bv. phaseoli). Much later, the more specific term “symbiovar” was proposed by Rogel et al. (2011), who documented numerous examples (see my early post about symbiovars).
The species R. pisi was separated from R. leguminosarum because its core gene sequences are sufficiently different to merit species status. The symbiosis genes of its type strain are, however, almost the same as those of the R. leguminosarum type strain. Marek-Kozaczuk et al. (2013) have now described a symbiont isolated from red clover that is R. pisi according to its core gene phylogeny, but has symbiosis genes much the same as those of R. leguminosarum symbiovar trifolii strains. This strain K3.22 is, rather obviously, R. pisi sv. trifolii. This is totally unsurprising – R. leguminosarum and R. pisi are closely related species, and if they can share plasmids carrying sv. viciae symbiosis genes, there is no reason to think they would not also share sv. trifolii genes.
So far, no surprises. No surprise, either, that R. pisi sv. K3.22 nodulates and fixes nitrogen on the clovers Trifolium pratense and T. repens but not vetches, while the type strain R. pisi sv. viciae DSM 30132 is effective on the vetch Vicia villosa, but not on clovers. That is how the same biovars behave in R. leguminosarum. The most unexpected statement in this new paper, however, is that “both strains nodulated pea (P. sativum cv. Iłówiecki) and the Spanish bean cultivar (P. vulgaris cv. Slenderette).” Now, the bean Phaseolus vulgaris is well known as a promiscuous host that often lets the “wrong” rhizobia form nodules, but these nodules fix no nitrogen. That a sv. trifolii strain should form nodules on pea is quite unexpected, though. My first thought was that this was another case of occasional, ineffective nodules, but later the authors state that “K3.22 efficiently nodulated red clover, pea and some bean cultivars”, so it seems that this R. pisi sv. trifolii is truly something new – a clover symbiont that is also effective on a host that is normally only nodulated effectively by sv. viciae. This is certainly the most interesting observation in the whole paper but, frustratingly, those two quotes are the only times this is mentioned, and no data are presented to support this novel claim. We can only hope that work is under way to understand how this strain can form an efffective symbiosis with pea, and more information will be published soon.
R. pisi has gained a new symbiovar, but more interestingly, it seems that sv. trifolii has gained a new host.
Jarvis BDW, Dick AG, Greenwood RM (1980) Deoxyribonucleic acid homology among strains of Rhizobium trifolii and related species. International Journal of Systematic Bacteriology 30, 42-52. http://dx.doi.org/10.1099/00207713-30-1-42
Jordan DC (1984). Family III. Rhizobiaceae. In Bergey’s Manual of Systematic Bacteriology, vol. I, pp. 234–242. Edited by N. R. Krieg & J. G. Holt. Baltimore: Williams and Wilkins Co.
Rogel MA., Ormeño-Orrillo E, Martinez Romero E (2011) Symbiovars in rhizobia reflect bacterial adaptation to legumes. Systematic and applied microbiology, 34(2), 96-104. http://dx.doi.org/10.1016/j.syapm.2010.11.015
Marek-Kozaczuka M et al. (2013) Rhizobium pisi sv. trifolii K3.22 harboring nod genes of the Rhizobium leguminosarum sv. trifolii cluster. Syst Appl Microbiol http://dx.doi.org/10.1016/j.syapm.2013.01.005