Welcome, fellow Dr. Tatiana fans!
Olivia Judson's latest column includes a good summary of work in my lab on cooperation between soybean plants and the rhizobium bacteria that (typically) provide them with nitrogen. As she points out, "cheating" is less likely to evolve in symbiont populations if they are transmitted in eggs or seeds, relative to symbionts that are acquired from the environment. In the former, if the host dies before reproducing, the symbiont dies, too. Symbionts without brains (bacteria, say) can't anticipate the effects of their actions; it's just that those whose genetically programmed behavior increases host survival become more common over generations.
Similarly, low symbiont diversity within an individual host may favor symbiont investment in costly activities that benefit the host. If each host has many different symbionts, on the other hand, then helping the host indirectly benefits competing symbionts sharing that host.
Rhizobium bacteria reach new host plants through soil, not via seeds, and they can do so even if the host dies without reproducing. Furthermore, each individual plant has multiple strains of rhizobia, which should undermine cooperation. Why then, do most rhizobia use their limited energy supply to fix nitrogen, giving most of it to the host plant? Why not use that energy for their own reproduction, instead?
Although there are several rhizobium strains per plant, they are typically segregated into individual root nodules. So, Toby Kiers and I reasoned, if plants monitor individual nodules and do something nasty to those that provide less nitrogen, that would act as a form of natural selection against cheating rhizobia. A computer model by Stuart West came to similar conclusions. To test this hypothesis, we forced some nodules to cheat, by surrounding them with an argon-oxygen atmosphere lacking nitrogen gas. Control nodules on the same plant got normal air, which is 80% nitrogen. Would rhizobia freed from the burden of fixing nitrogen redirect resources into their own reproduction? Would the plant impose sanctions on nonfixing nodules? If the answers to these questions are yes and yes, what would be the overall effect of cheating on rhizobium reproductive success?
After 10 days, Toby found that the nodules that fixed nitrogen grew bigger and contained up to three times as many rhizobia per nodule. If the number of rhizobia that escape back into the soil (when the plant dies at the end of the season) is proportional to the number of rhizobia inside, then rhizobia that try to cheat their hosts, by diverting resources to their own reproduction at the expense of nitrogen fixation, will actually end up less common in the next generation.
We think this result explains the otherwise surprising (see introduction) level of cooperation between rhizobia and legumes, but it also leads to new questions.
How, physically, do soybeans "punish" nodules that fix little or no nitrogen? As Olivia Judson mentioned, the plant decreased the oxygen supply to the interior of nonfixing nodules. We think this low oxygen may have limited reproduction by rhizobia inside the nodule, either directly or indirectly. As a commentator noted, high levels of oxygen can actually destroy the enzyme that fixes nitrogen. But nodules have a gas diffusion barrier (like insulation, only for oxygen rather than heat), that keeps oxygen inside nodules way below toxic levels. In fact, it's typically down in the range where oxygen is too low for maximum respiration (the oxygen-requiring process by which rhizobia get energy), and it was even lower in our nonfixing nodules. Nodules are red inside because they make hemoglobin to transport oxygen within their rhizobium-infected cells, similar to its role in our blood. (The idea that hemoglobin magically makes oxygen vanish has been obsolete for over 20 years, although you may still see it in textbooks.) But could the relation between low oxygen and low rhizobium reproduction be coincidence? I will be starting experiments soon to find out.
What about legumes other than soybean? Nodules containing nonfixing rhizobia typically grow faster in other species, as Ellen Simms and colleagues have shown with wild lupines. Ryoko Oono, in my lab, started a preliminary experiment today, similar to our soybean experiment, only with alfalfa.
Given the severe effects of sanctions, why are cheating rhizobia still common in some soils? Will Ratcliff, also in my lab, has some ideas and some data on this. Rhizobia can accumulate more resources per cell inside nodules, so numbers aren't the whole story. Also, some rhizobia can manipulate their hosts to give them more resources.
Some cheating rhizobia may prosper by sharing nodules with more altruistic strains. Work in my lab by Alain Chapon, Bob Rousseau, and Lysistrata Munson found that mixed nodules may be more common than we thought.
Thanks to the National Science Foundation for supporting this work; be sure to fund at least one of my submitted proposals so we can continue! For example, the rhizobia that fix nitrogen in some nodules, including those of alfalfa, have lost the ability to reproduce. So why cheat? On the other hand, why fix nitrogen? We think the answer involves kin selection, but it's complicated.