Modern soybeans cheated by lousy fixers

Ah, synchronicity. While Luigi was fleetingly confused about rhizobia and other bacterial symbionts of pigeonpeas, I was pondering one of the more interesting blog posts — and papers — I have read in a long time, also about rhizobia. Those are the bacteria that “infect” leguminous plants, forming nodules on the roots. In the nodules the bacteria “fix” nitrogen gas, from the air, into a form plants can use. In exchange, as it were, the plants supply the bacteria with a safe home and some of the food the plants have photosynthesized. Some rhizobia do a better job than others, and many are completely useless at fixing nitrogen. Better yet, the plants know, and send more food to the nodules fixing the most nitrogen.

Now, the tricky part.

Modern agriculture does not usually apply nitrogen to leguminous crops. But there can be considerable carry-over from the preceding crop. So, two possibilities arise. Maybe soybeans no longer respond to better nitrogen-fixing bacteria by sending more food their way, because they don’t really need the nitrogen. Or maybe more soil nitrogen means that the plant can afford to starve out all but the very best nitrogen fixers.

But why am I repeating all this? You cannot possibly do better than head over to Ford Denison’s blog, where he does a much better job than me of explaining the significance of his results. The paper is also discussed in Nature News.

Spoiler (aka don’t bother me with the details): modern varieties do very poorly when inoculated with a mixture of good and bad nitrogen fixers. It is as if they simply cannot tell the difference and feed both equally.

Stunning new idea: If modern varieties tolerate low quality rhizobia, then low quality rhizobia are going to proliferate in the soil, doing nobody any good. So why not deliberately breed legume crops to impose very strict sanctions against poorly-performing rhizobia strains? Long term this would enrich the soil with top-notch fixers.

Bacterial infection causes fungal resistance

Some root colonizing bacteria have been found to have beneficial effects on plant growth, and have thus been dubbed plant growth promoting rhizobacteria (PGPR). Now Indian researchers have grown pigeonpea with and without a couple of different strains of PGPR, and also with and without rhizobium infection, and have then infected the plants with the fungus that causes wilt.1

It turns out that pigeonpea plants infected with either PGPR or rhizobium developed “induced systemic resistance” to the fungus. But the resistance was actually best when both were present. I found this pretty amazing, but actually some googling reveals that it’s not that weird. It may have something to do with the increased levels of phenols in the leaves of bacterized plants. Or the reduced production of fusaric acid by the pathogen. In any case, “the results promise the combined use of PGPR and rhizobia for induction of systemic resistance against fusarial wilt in pigeon pea.” They are also another pretty amazing example of the interactions among agrobiodiversity.

  1. S. Dutta, A.K. Mishra and B.S. Dileep Kumar. Induction of systemic resistance against fusarial wilt in pigeon pea through interaction of plant growth promoting rhizobacteria and rhizobia. Soil Biology and Biochemistry, In Press, Uncorrected Proof, Available online 11 October 2007 []