Probiotics for tougher plants

Ancestral varieties of modern corn and wheat might hold a key to non-chemical mycotoxin prevention. More specifically, some of the bacteria strains which naturally developed alongside those varieties have been shown by Canadian researchers to be highly effective against fusarium head blight and gibberella ear rot. Those same researchers are working with the private sector to bring a practical product to grain growers.

Biological control for grain growers

Manish Raizada is a long-time professor of plant agriculture at the University of Guelph. After elevated levels of mycotoxins spurred prominent grain marketing issues in recent years (most notably in 2018), Raizada was asked to head a team of researchers charged with developing a more effective biological control for grain growers.

Microbial endophytes

To do so, they turned to much older, apparently more-resistant varieties of each crop. They looked at microbial endophytes: bacteria that live between plant cells. These were isolated from ancient and landrace corn varieties, as well as finger millet (a very old African crop with natural fusarium resistance).

These microbes are numerous in the natural world and often have a mutually-beneficial relationship with the host plant. Some strains can, for example, enhance root growth and nitrogen absorption.

Raizada also reiterates the ability of fungi to rapidly develop resistance to commonly-used fungicides, which continues to be a growing concern. But because probiotic microbes can evolve with the pathogen, the right endophyte could provide farmers with a longer-term mycotoxin management tool.

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Spraying fungicides on wheat may be (partly) replaced by spraying endocytes, or by coating seeds with these natuarlly occurring organisms. – Photo: Mark Pasveer

Enormous success in the lab

Out of approximately 200 microbial strains screened, 5 anti-fusarium bacteria strains were isolated and used in greenhouse trials. Each one dramatically suppressed mycotoxin DON accumulation (up to 97% in corn and 85% in wheat). Applying the endophytes via seed coating was less effective then direct foliar applications, but the results overall were startlingly positive.

“It’s the best fusarium control in a study ever reported in corn,” says Raizada. Some endophyte strains almost eliminated the ear rot pathogen. The results for wheat were lower, but three of the tested strains still managed to reduce pathogen levels by 60%.

Mobility

Why some of the endophytes were so successful in suppressing fusarium and ear rot, has to do with mobility. In particular, Raizada says, they observed one strain (M6, derived from finger millet) respond to infection by leaving the root system to coat the exterior of the plant. It also promoted root hair growth.

Both factors combined, says Raizada, create an ideal habitat within which the endophyte can capture and kill the pathogen. “It’s actually mobile; some of these microbes have little tails they use to seek and destroy pathogens.”

In-season spray or a seed coating with the microbes

The overall goal of this research was to develop an in-season spray or a seed coating with the microbes that could prevent and suppress the establishment and spread of mycotoxins. If commercialised, such a tool would also have greater longevity than standard fungicides. Grain growers could also employ it in conjunction with chemical solutions for a multi-pronged attack strategy. Currently the university is working with the private sector to make this happen.

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Barriers to commercialisation

There are some notable barriers to commercialisation, though. Delivering endophytes via seed coating – the theoretically ideal system – is inherently less effective than in-season applications made directly to corn ear silks and wheat heads. Indeed, bacteria delivered through seed coating did not appear to effectively colonise the plant in the field.

The researchers are not entirely sure why this is the case, though suspect it’s due to a combination of pressures – specifically fluctuating environmental conditions and the sheer volume of more competitive, already-naturalised microbes present in the soil. “If we spray directly onto the plant, we do see more success.”

Raizada adds that insufficient storage during transportation and on the farm are an even greater barrier. As with some other biologicals, improper storage commonly means growers are applying dead or low-activity products.

Looking for solutions

Raizada says the ideal solution to both issues would be improved seed formulations – that is, something that better protects the endophytes in storage and from being outcompeted in the soil. “If there’s some formulation that lets the microbes to be coated on the seed and that can tolerate poor storage, that would be the best solution.” The physical cost of endophytes, too, should not be prohibitive for farmers. “Theoretically, the microbe itself is very inexpensive. We haven’t worked out the exact cost, but Its certainly competitive with fungicides,” Raizada says.

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The M6 strain of endophyte, labelled with green florescent protein. It co-exists with the corn or wheat plants. – Photo: University of Guelph