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Rhizobia, pesticides, and peer review

I have some comments on a recent paper that's only tangentially related to evolution. Actually, it's more relevant to science fair projects, the topic of my last post.

One type of science fair project my fellow judges and I are really sick of is "The effect of X on plants", where X is mouthwash, vinegar, cola, etc. The obvious question, which we always ask, is "how often are plants in the field exposed to high concentrations of mouthwash?" Unfortunately, whoever reviewed this paper in Proceedings of the National Academy of Science, claiming that "Pesticides reduce symbiotic efficiency of nitrogen-fixing rhizobia and host plants" apparently failed to ask this question.

The general hypothesis they proposed was plausible enough to be worth testing. Rhizobium bacteria infect the roots of some important crops, including soybeans and alfalfa, and provide them with nitrogen. This is my own area of research. Successful infection involves the exchange of chemical signals between bacteria and roots. It's not inconceivable that pesticides, especially any whose chemical structure is similar to the usual signal, could interfere with signaling. Of course, soil (especially the rhizosphere, right around the root) naturally contains thousands of chemicals released from roots and produced by microbes. So signal exchange between bacteria and plants has been subject to millions of years of selection for resistance to interference by extraneous chemicals. Still, maybe some pesticides are different enough from anything natural that crop plants haven't yet evolved such resistance, especially since crop evolution is been partly under our control since before the invention of synthetic pesticides. So, testing some chemicals commonly used on alfalfa or soybeans, to see their effects on the benefits plants get from rhizobia, is a reasonable idea.

But the key "science fair question" is which chemicals and at what concentrations? The authors don't seem to have given a lot of thought to this. Neither, apparently, did the reviewers who recommended that the paper be accepted for publication in this once-prestigious journal.

The chemical that reduced alfalfa growth the most was pentachlorophenol, a wood preservative. This is nasty stuff. I remember when our cat fell into an open bucket of it; he recovered, but was pretty sick for a while. Its use has since been restricted, and rightly so. But it has never been applied to agricultural fields.

Another chemical on the list, DDT, has been banned in the US for decades. It's much more toxic to insects (until they evolve resistance!) than to humans, but it breaks down very slowly in the environment, and can be a serious threat to birds. Limited, carefully targeted use for control of malaria-spreading mosquitoes may save lives, but use on crops should be, and is, prohibited in most countries.

Methyl parathion is the only chemical on the list that is applied to crops. It is more toxic to humans, so it's mostly restricted to use on crops that aren't eaten, like cotton. I haven't heard of it being used on alfalfa.

The authors don't present any evidence that these chemicals are commonly found in alfalfa fields, at the concentrations used in their experiments. They mention that "long-term agricultural studies have shown SNF [symbiotic nitrogen fixation] is markedly lower in legume crops that are treated with N fertilizer and pesticides." However, that is known to be true for nitrogen fertilizer alone (Denison and Harter, 1995, for example), so such studies provide no evidence of a separate pesticide effect.

If I had judged this paper as a grade-school science fair project, I would have wondered why they didn't include other chemicals that are found only at trace concentrations in agricultural soils, such as mouthwash and vinegar. Maybe the "peer reviewers" at PNAS made them take the data on those chemicals out of the published manuscript? Also puzzling is their apparent failure to test any of the pesticides that are commonly used on alfalfa, at realistic concentrations, of course.


The results with the pentachlorophenol reminded me of a joke that was popular here in Mexico and that I always tell my friends that it's very pertinent to reaching conclusions in science. It's about some researchers measuring the influence of the number of legs on a flea on jumping distance. For this, they sing to the flea: "Jump, little flea, jump!" measure the distance it jumps, then remove one of the (remaining) legs and repeat the procedure. At the end, when the flea doesn't have any legs left and doesn't jump after they sing "Jump, little flea, jump!", they write in their notebooks: "Flea with no legs becomes deaf".

Nice example!

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