Probiotics for tougher plants

Ancestral varieties of modern corn and wheat might be the key to non-chemical mycotoxin prevention. More specifically, some of the strains of bacteria 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 these researchers are working with the private sector to develop a practical product for grain growers to use.

Finding the right bacterial companion

Manish Raizada is a professor of plant agriculture at the University of Guelph in the province of Ontario. After elevated levels of mycotoxins had raised serious 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 measure for grain growers. 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 explains, often have mutually beneficial relationship with the host plant (some strains can, for example, enhance root growth and nitrogen absorption).

He also states that 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.

Startlingly positive results

Overall, Raizada and his colleagues screened approximately 200 microbial strains. 5 anti-Fusarium bacteria strains were isolated and used in greenhouse trials, with each one dramatically suppressing mycotoxin DON accumulation (up to 97% in corn and 85% in wheat). Applying the endophytes via seed coating was less effective than 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 less impressive, but 3 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. Raizada says they observed how one strain (known as M6, derived from finger millet) responded 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 which they use to seek and destroy pathogens,” he says.

Untreated corn infested with Fusarium (head blight) (left) and corn treated with the promising M6 endocyte (right). Photo: University of Guelph

The overall goal of this research was to develop an in-season spray or a seed coating containing 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 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.

Barriers to commercialisation

There are some notable barriers to commercialisation, though. Delivering endophytes via seed coating – theoretically the 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, although they 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.”

The M6 strain of endophyte, labelled with green florescent protein. It co-exists with maize and wheat plants. Photo: University of Guelph

Raizada adds that inadequate storage during transportation and on the farm are an even greater barrier. As with some other biologicals, poor storage commonly means growers are applying dead or low-activity products. Raizada says the ideal solution to both issues would be improved seed formulations – that is, something that better protects the endophytes in storage and prevents them from being outcompeted in the soil. “If there’s some formulation that allows the microbes to be coated on the seed and can also tolerate poor storage conditions, that would be the best solution.” The cost of the endophytes should not be prohibitive for farmers. “Theoretically, the microbe itself is very inexpensive. We haven’t worked out the exact cost, but it’s certainly competitive with fungicides,” Raizada says.

Variability problems

Apart from the recent greenhouse tests at the University of Guelph, 3 additional field trials were conducted over 2 years. The results were not as positive – year-on-year variability in the corn crop itself was a problem, as was inherently low Fusarium pressure in the wheat plots – but professor Raizada reiterates that these and other variability problems are typical of in-field microbial studies. The most important revelation, he says, is how effective these microbes can be. Indeed, replicated field trials with corn showed 3 promising bacterial endophytes, with one strain in particular reducing (DON) mycotoxin accumulation by up to 65%.