Are there gamma-rhizobia?
Lionel Moulin has just raised an interesting question in a comment left on my post “What are rhizobia”. He asks about the evidence for rhizobia in the class Gammaproteobacteria. Lionel is particularly well placed to ask that question, of course, since it was he who brought us beta-rhizobia ten years ago (1).
My current view is that gamma-rhizobia probably do exist, but nobody has proved it yet.
Why do I think they exist? Well, until ten years ago, all known rhizobia belonged to certain clades within the class Alphaproteobacteria, and it would have been perfectly rational to have speculated that the nodulation genes needed a specific genetic background, found only in this restricted bacterial group, in order to function. The discovery of effective nodulation by the Betaproteobacteria Burkholderia and Cupriavidus showed that this was not the case.
There are strong similarities between Betaproteobacteria like Burkholderia and Gammaproteobacteria like Pseudomonas. Indeed, until 1992 (2), the bacteria we now know as Burkholderia were considered to be species of Pseudomonas. There are many similarities in phenotype and lifestyle, and mobile elements such as IncP plasmids move frequently between betas and gammas. If Burkholderia can be a rhizobium, why not gammas like Pseudomonas or Xanthomonas? They commonly interact with plants in other ways, so they have ample opportunities to pick up nodulation genes.
There is every reason, therefore, to think that gamma-rhizobia should be possible. However, I agree with Lionel that none of the claims that I am aware of look convincing. I think we should collect all the potential examples that we are aware of, and then evaluate them carefully. Please send me references to publications that report potential gamma-rhizobia. If you like, also tell us what you think about the evidence. Please keep your comments polite, though – I will not publish abusive personal attacks! Once we have a list of the potential evidence, we can discuss the current state of our knowledge, and perhaps decide how to look for definitive evidence.
Right now, I am at Manchester Airport on my way to Cuernavaca, Mexico, where the Centre for Genomic Sciences is celebrating 30 years. This is a major rhizobium research centre, so I shall have more to say once I get there.
(2) Yabuuchi E et al. Microbiol Immunol. 1992;36(12):1251-75.