Pathogen Challenge Experiment

Fusarium fungus can be a potent pathogen against corn, but not always. It can live without incidence inside the plant, and can even be inherited in its kernels. However, if the plant is unhealthy – stressed by drought for example – the unassuming fungus can go rogue, choosing to turn against its plant host to use it as an energy-rich launching pad for its aerial spores, rather than invest in the plant’s survival. In its battle with the plant, it can release mycotoxins (fumonisins and T-2 toxin) to high levels. Corn kernels rich in these toxins are serious health concern for people and farm animals.

The maize plant doesn’t go without a fight. In addition to its general defenses, it produces anti-fungal benzoxazinoids (BXs) in large quantities, as part of a systemic response[1]. However,  Fusarium has the ability to degrade BX to achieve an upper hand against the plant, and the co-inhabiting microbes that are not BX-tolerant.  Due to the sophisticated abilities of Fusarium, people are looking more to the role of microrganisms to aid the plant in defending against the unruly guest. This includes beneficial bacterial endophyte Bacillus subtilis, which is directly antagonistic to Fusarium [2]. Although a promising inoculant, the ecology of this organism is still little-known. It is not guaranteed to persist on the farm after initial inoculation, requiring a new inoculation each year. Nevertheless, this remains a potential organic biological alternative to soil fumigation, or dousing the seed with fungicides prior to planting, to block out soil-borne Fusarium.

Currently, beneficial microbes are tested, selected and propagated in a controlled environment that’s notably different than the farm where it is intended to be used. Therefore, the question is how we can select and maintain beneficial microorganisms within the farm system itself? I believe this can be accomplished with informed farming practices, including soil management and careful selection of seeds. If we can understand the nature of plant-associated microorganisms, perhaps beneficial microbes can be recruited to protect plants from pathogens, particularly during times of stress.

To understand the nature of beneficial microbes, I will “challenge” maize and its microbes by planting it in a field whose soil contains high numbers of Fusarium pathogens. I will use an organic maize seed variety and a conventional maize seed variety, each with distinct communities of inhabiting microbes. I will treat both of these seed varieties with heat-removal of its microbes,  or the addition of wither commercial fungicides, or biological control organisms. Throughout the growing season I will take a deep look at how the plants and their associated microbial communities interact with pathogens.

It may be that the plant has a genetic predisposition to preventing pathogenesis, in which case it would do equally well with or without its seedborne microbes. If seedborne endophytes do prevent the plant disease, they could either do so by preventing Fusarium from entering the plant in the first place, or tolerating its presence within the plant, but preventing Fusarium from becoming pathogenic. Lastly, seedborne microbes could give tolerance to the disease, in which case the disease would still occur, but with less impact – and perhaps less mycotoxins as well. Seedborne microbes are only part of the story, as the plant recruits hordes of soil microbes during its development. Thus, I will also be careful to characterize the soil microbial community.

To top off this experiment, I will find out which microbes are found back in the seeds, to see which ones were inherited for future generations. If certain microbes (1) assist the plant in staving off disease, and (2) are inherited for future plants against future diseases, then this would be the first demonstration of heritable resistance against disease in plants due to microbes. (Interesting, this is already demonstrated in fruit flies, which carry a maternally-inherited microbe that protects it against a parasitic nematode [3]!)

 

References:

[1]         M. Saunders and L. M. Kohn, “Evidence for alteration of fungal endophyte community assembly by host defense compounds.,” The New Phytologist, vol. 182, no. 1, pp. 229–38, Jan. 2009.

[2]         C. W. Bacon, a. E. Glenn, and I. E. Yates, “Fusarium Verticillioides : Managing the Endophytic Association With Maize for Reduced Fumonisins Accumulation,” Toxin Reviews, vol. 27, no. 3–4, pp. 411–446, Jan. 2008.

[3]         J. Jaenike, R. Unckless, S. N. Cockburn, L. M. Boelio, and S. J. Perlman, “Adaptation via symbiosis: recent spread of a Drosophila defensive symbiont.,” Science (New York, N.Y.), vol. 329, no. 5988, pp. 212–5, Jul. 2010.

 

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