Category: Microbes

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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.

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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. ((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))

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.

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The Cretaceous roots of agriculture

A comment on a long but fascinating post on yeast genetics and evolution at The Loom sent me to a New Scientist article from a couple of years back which is perhaps more immediately relevant to our agricultural biodiversity focus here.

Some time in the distant past Saccharomyces cerevisiae, to give it its full name, developed a chemical trick that would transform human societies. Some anthropologists have argued that the desire for alcohol was what persuaded our ancestors to become farmers and so led to the birth of civilisation.

The article goes on to describe how brewer’s yeast evolved its somewhat surprising abilities. It turns out that its peculiar habit of carrying out anaerobic respiration even in the presence of oxygen — at a steep energetic cost, and resulting in the production of what is usually a poison, alcohol — dates back to an accidental duplication of its genome back in the Cretaceous. Eighty million years ago later, bakers and brewers are daily taking advantage of a genetic mistake that took place in a microscopic fungus when dinosaurs ruled the Earth. Isn’t agrobiodiversity wonderful?

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Roman antibiotics

Also from Tangled Bank comes news of a study looking at the evidence for various infectious diseases from the skeletons of people killed at Herculaneum by the eruption of Vesuvius in 79 AD. ((That’s the one that also destroyed Pompeii, though in a somewhat different way.)) Among the diseases was brucellosis, evidence for which was also gleaned from the carbonized cheeses found at the site. Herculaneum was apparently famous for its goat cheeses, which seem, however, to have been badly infected. Which is all amazing enough. But one of the commenters on the article points to another paper which adds a twist to the story.

It seems the inhabitants of Herculaneum, despite their brucellosis and tuberculosis, were relatively free of non-specific bone inflammations. And that may be because:

Pomegranates and figs, consumed by the population, were mainly dried and invariably contaminated by Streptomyces, a bacterium that produces natural tetracycline, an antibiotic.

Is there similar evidence from contemporary populations of the protection conferred by natural antibiotics?

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Fungal agricultural biodiversity

More today about fungi as important constituents of agricultural biodiversity. Following the recent post on the microsymbiotic Frankia, I ran across a couple of papers on other fungi and their interactions with crop plants in agricultural systems. 

First, there’s Trichoderma. According to a recent review in Soil Biology and Biochemistry ((Francesco Vinale, Krishnapillai Sivasithamparam, Emilio L. Ghisalberti, Roberta Marra, Sheridan L. Woo and Matteo Lorito, Trichoderma-plant-pathogen interactions. Soil Biology and Biochemistry. In Press, Uncorrected Proof.)):

Trichoderma spp. are among the most frequently isolated soil fungi and present in plant root ecosystems. These fungi are opportunistic, avirulent plant symbionts, and function as parasites and antagonists of many phytopathogenic fungi, thus protecting plants from disease. So far, Trichoderma spp. are among the most studied fungal BCAs [bio-control agents] and commercially marketed as biopesticides, biofertilizers and soil amendments. Depending upon the strain, the use of Trichoderma in agriculture can provide numerous advantages: (i) colonization of the rhizosphere by the BCA (“rhizosphere competence”) allowing rapid establishment within the stable microbial communities in the rhizosphere; (ii) control of pathogenic and competitive/deleterious microflora by using a variety of mechanisms; (iii) improvement of the plant health and (iv) stimulation of root growth.

Then there’s arbuscular mycorrhizal fungi (AMF). Another paper ((Christine Picard, Elisa Baruffa and Marco Bosco, Enrichment and diversity of plant-probiotic microorganisms in the rhizosphere of hybrid maize during four growth cycles. Soil Biology and Biochemistry. In Press, Uncorrected Proof.)) in the same journal suggests that different maize genotypes had quite different effects on the AMF population in the soil in which they were grown, stimulating “their own adapted phylogenetic AMF subgroups.” According to the authors:

Several new sets of data obtained in this way would be necessary to have a significant view of the actual beneficial interactions between rhizospheric microorganisms and plant roots; but we are confident that such an effort will lead to the definition of new criteria for the rapid breeding of sustainable varieties.