Ten years ago, Ryoko Oono, Toby Kiers, and I published a “Tansley Review” in New Phytologist, titled “Controlling the reproductive fate of rhizobia: how universal are legume sanctions?” Much of it focused on differences between legume hosts where rhizobia in nodules retain the ability to reproduce as they differentiate into the nitrogen-fixing bacteroid form versus hosts where bacteroids are swollen and nonreproductive. We wrote then that:
Our working hypothesis is that plant traits leading to rhizobial dimorphism (with swollen bacteroids), loss of bacteroid reproduction, and inability of bacteroids to accumulate PHB are newly derived in several legume lineages. We further hypothesize that only dimorphism and bacteroid swelling are imposed directly by these legume hosts, probably by interfering with bacterial cell wall growth (Sutton & Paterson, 1983). Individual plants somehow benefit immediately from bacteroid swelling (e.g. increased nitrogen fixation or easier retrieval of nutrients from senescing bacteroids). Loss of reproductive potential of bacteroids could then be a side effect of swelling.
Ten years on, the hypotheses in boldface above have been supported by subsequent research (click on links). But Google Scholar doesn’t seem to know about the original paper — as of September 5, it wasn’t showing up in the Google-Scholar pages for any of the authors. Google-Scholar seems to have deleted the ability of authors to edit their pages (due to abuse?), but I’m hoping linking in the first paragraph to a PDF version will fix this. Also in this 2009 paper:
However, growth with single-strain inoculation is an imperfect proxy for differences in mutualism among strains under field conditions, where each plant is
infected by multiple strains. With single-strain inoculation, a poorly nodulating strain with high nitrogen-fixation efficiency might result in less plant growth than a less efficient strain that forms more nodules. Plant growth with single-strain inoculation would incorrectly identify this as a less beneficial strain. In the field, however, the poorly nodulating strain might only occupy 10% of the nodules on a plant, while providing 20% of the nitrogen. A better method might be to compare the growth of plants with two different strains in their nodules, in different proportions. The more beneficial strain is the one that increases plant growth as its nodule representation increases. A benefit:cost ratio would also be a better measurement to assess a strain’s symbiotic qualities. For example, the ratio of nitrogen fixation rate to respiration rate of a single nodule or the ratio of host shoot biomass to total nodule mass would be a more accurate evaluation of the strain’s symbiotic quality, independent of differences in nodulation efficiency.