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.

Now, those same researchers are working with the private sector to bring a practical product to grain growers.

Finding the right bacterial companion
Manish Raizada is a long-time professor of plant agriculture at the University of Guelph in the province of Ontario. After elevated levels of mycotoxins spurred prominent grain marketing issues in recent years (most notably in 2018), Manish 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 what appeared to be much older and more-resistant varieties of each crop. More specifically, that looked at microbial endophytes (bacteria that live between plant cells) isolated from ancient and landrace corn varieties, as well as finger millet (a very old African crop with natural fusarium resistance).

These microbes are very numerous in the natural world, and as Raizada describes, often have mutually-beneficial relationship with the host plant (some strains can, for example, enhanced root growth and nitrogen absorption).

He also reiterates the ability of fungi to rapidly develop resistance to commonly-used fungicides 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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Enormous success in the lab
Overall, Raizada and his colleagues screened approximately 200 microbial strains. Five anti-Fusarium bacteria strains were isolated and used in greenhouse trials, with each one dramatically suppressing mycotoxin DON accumulation (up to 97 per cent in corn and 85 per cent in wheat).

Applying the endophytes via seed coating was less effective then direct foliar applications, but the results overall were startlingly positive.

“We had huge success here. It’s the best fusarium control in a study ever reported in corn,” says Raizada. Indeed, he says 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 per cent.

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 (known as 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 edophyte can capture and kill the pathogen.

“It’s actually mobile […] Some of these microbes have little tails they use to seek and destroy pathogens,” he says.

The most important revelation, Raizada says, is how effective these microbes can be. Indeed, replicated field trials with corn showed three bacterial endophytes hold promise, with one strain in particular reducing (DON) mycotoxin accumulation by up to 65 per cent.

Field trials

Three additional field trials were conducted over the preceding two years. The results were not as positive as those achieved in the greenhouse setting – year to year variability in the corn crop itself was a problem, as was inherently low fusarium pressure in the wheat plots – but Raizada reiterates these and other variability problems are typical of in-field microbial studies.
In-season spray or seed coating with the microbes

The overall goal of this research was to develop an in-season spray or 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, though grain growers could also employ it in conjunction with chemical solutions for a multi-pronged attack strategy. Currently, he and his university colleagues are working with the private sector to make this happen.

Barriers to commercialisation
There are some notable barriers to commercialisation, though. As previously noted, Raizada says 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-naturalized microbes already present in the soil. “If we spray directly onto the plant, we do see more success,” says Raizada.

Regardless of delivery method, he adds insufficient storage during transportation and on the farm are an even greater barrier. As with some other biologicals (e.g. rhizobium bacteria inoculants used in soybeans), improper storage commonly means growers are applying dead or low-activity products.

Solutions to commercialisation
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 out-competed in the soil.

“If there’s some formulation that lets the microbes to be coated on the seed […] that can tolerate poor storage, that would be the best solution,” he says. “If there’s no technical solution, then we need at the last stage […] If it’s a spray, it’s still then an on-farm application and you need to have good inoculant locally.”

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.