Rhizobia just the tip of the iceberg, researchers say

A new field of research looks at dark septate endophytes’ potential for 
helping crops become more efficient and cope with stress

Everybody and their pea-growing grandma knows about rhizobia, the nitrogen-fixing soil bacteria.

But there are a host of other beneficial soil organisms that could potentially boost crop growth, increase stress tolerance, and reduce the need for fertilizer, say scientists at Agriculture and Agri-Food Canada’s Semi-Arid Prairie Agricultural Research Centre in Swift Current.

Dark septate endophytes, a type of fungi, are the focus of a new branch of research because they can grow and thrive in the soil both with or without a plant host.

“The exact mechanisms of how plants benefit from them is less well understood, but they have been associated with drought resistance in plants,” said Keith Hanson, a manager at the centre’s microbial lab.

The work builds on existing knowledge of arbuscular mycorrhizal fungi, a group of organisms that develop a network of hair-like root extenders called hyphae that help plants access nutrients, such as phosphorus, they wouldn’t otherwise be able to reach.

In one study, three dark septate strains were added to chickpea plants, which were then subjected to various stresses. Normally, when a chickpea plant is stressed by drought, flooding, disease or excessive heat, it produces ethylene gas which hinders growth. The plants with the added bacteria produced less ethylene when stressed.

The same may also be true for wheat, said Hanson. Scientists at the centre will use genetic markers to look at the response of specific wheat cultivars to the beneficial micro-organisms.

There are more micro-organisms in a cupful of soil than there are people on Earth. But with only a small portion of dark septate endophytes identified by scientists, their full potential remains unknown. To better understand their role in crop growth, the scientists are also studying a collection of beneficial fungi found in Prairie soil, with the idea they could be added to soil to promote plant growth. For example, endophytes might be part of a powder-based seed inoculant or side banded in granular form — although Hanson notes micro-organisms aren’t like regular inputs.

“Because it is a living organism, it’s not the same as putting down 80 pounds of nitrogen per acre,” he said.

But it can be done. Penicillium bilaii, a phosphorus-extracting soil fungus discovered by AAFC researchers and now sold by Novozymes under the name JumpStart, has been on the market for years. Other commercially available beneficial fungi products are appearing.

Devin Bryant, a technical support adviser for Premier Tech Biotechnologies, said that Myke Pro, which contains live spores of arbuscular mycorrhizal fungi, has been applied to about 30,000 acres of field crops such as potatoes and wheat in Manitoba this spring.

“The mycorrhiza is extracting from the soil what you have in there, sucking in the tied-up phosphate and micronutrients such as zinc, copper, boron and manganese, you name it,” he said, adding that it effectively provides a “20 per cent extension via a secondary root system.”

Available in granular, liquid, and peat powder for use as a seed treatment, the cost ranges from $13 to $16 per acre. A pail of peat-based inoculant weighing four kilograms can cover 40 acres, while the granular mixed with zeolite is applied at five pounds per acre.

“We’re still in the early stages of seeing where it is going to work best,” said Bryant.

Yield response in soft white wheat, for example, has ranged from two to 14 bushels per acre.

Although registered for use by organic growers, conventional farmers using commercial fertilizer are seeing the most benefits, and the biggest boost appears to come from high-yielding crops, he said.

Although mycorrhizal fungi colonize most plants, canola actually depletes populations of the native fungus.

“This could be an option to replenish them and benefit pulse and wheat crops following canola,” said Bryant.

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