At the European Nitrogen Fixation Conference, Annet Westhoek and colleagues reported that they knocked out nifH, a gene key to nitrogen fixation, in rhizobia that normally supply pea plants with fixed nitrogen. They found that pea plants didn’t use partner choice to discriminate against nonfixing mutants — the mutants occupied just as many root nodules as one would expect from their frequency in the inoculum. They concluded that partner choice is not a reliable way to protect plants from rhizobial cheaters — postinfection sanctions are needed.
I agree, but sanctions may not be a complete solution either — rhizobia fixing at only 50% of potential may escape sanctions. Also, even somewhat-unreliable partner choice could be useful, as discussed below.
Rhizobia are soil bacteria that can infect legume roots and multiply inside root nodules, where they typically “fix” nitrogen gas into ammonia or other nitrogen compounds that plants can use to make protein. Some rhizobia fix little or no nitrogen, however. Some are cheaters diverting resources from nitrogen fixation to their own reproduction, while others are simply defective.
Legumes often impose sanctions on less-beneficial nodules, however, reducing the reproduction or rhizobia inside. So we need to distinguish between potential cheaters (those that divert resources from nitrogen fixation to their own reproduction, but thereby trigger sanctions that reduce their reproduction) and successful cheaters, which divert resources and yet escape sanctions.
Westhoek’s research and previous work by others shows that plants can’t reliably tell how efficiently a strain will fix nitrogen (if at all) until it’s already established in a nodule. But there could be location-specific associations between nitrogen-fixation efficiency and signal molecules that the plant could use for partner choice. I would not expect such associations to last very long, because less-beneficial rhizobia with mutations that mimic the signals of more-beneficial ones would increase over years.