Fungi helps some plants access nutrients and trigger their defence mechanisms when under attack by disease
Now, an Agriculture Canada scientist has been researching one soil fungus phylum — the Glomeromycota — and found an amazing variety of species each with a different habitat and activity in the soil. She compares them to the weed populations that coexist with particular crops and farming systems.
They were called VAM fungi, now the official term is arbuscular mycorrhizae because some lack vesicles, but all have tree-like (arbuscular) branches and form mycorrhizae in plants. They have a symbiotic relationship with plants, obtaining sugars and possibly other substances from them and transporting nutrients and water from the soil to the plant. The fungus is made up of very fine branches or hyphae, some of which penetrate the much larger root hairs of a plant while other hyphae spread through the soil and absorb nutrients and moisture.
Chantal Hamel is trying to understand the complex ecology of mycorrhizae in Prairie soils. Mycorrhizae have been found in soil around most crops, other than crucifers such as canola and mustard. They act as extensions of the plant’s roots, pipelines to deliver moisture and nutrients from a greater volume of soils than the roots. But, she says, the community of mycorrhizae seems to depend more on the soil than the crop.
Hamel has found higher mycorrhizae populations under organic systems and says the lower nutrient levels in organic soils boost populations of the fungi.
“A hungry plant loses more sugars into the soil around its roots than a plant with good supply of nutrients,” says Hamel. “As well as enhancing mycorrhizal activity, the sugars stimulate decomposition and mineralization to make nutrients more available to the plant. Nitrogen use efficiency is higher in organic fields.”
Sugars secreted by plant roots stimulate mycorrhizal hyphae to intrude into roots to set up the symbiotic relationship between plant and fungus. Along with delivering water and nutrients, the fungus provides some stress resistance to the plant, acting almost like a vaccination and helping the plant’s defence mechanisms turn on quickly when the plant is attacked by disease or some other stressor. The old idea that tillage destroys the mycorrhizae is not entirely true, says Hamel.
“In Western Canada, tillage is no more than eight centimetres. That has very little impact on mycorrhizae. In other areas, where farmers till deeper, with more aggressive implements, mycorrhizae are suppressed.”
The diversity of mycorrhizae species is greater under native grass than under annual crops. The most abundant and diverse populations Hamel has found are in ditches — it seems they have an environment that supports cropland species and native grassland communities. Also, some species are more abundant in spring, then others take over later in the year.
Not all species of mycorrhizae are equally effective at helping plants. Most are beneficial, but a few reduce plant growth. Hamel’s team found one species was abundant in every cultivated field they looked at in Saskatchewan. She thinks it’s probably an invasive species, like a weed species.
The type of crop also affects the value of the fungus to the plant.
“Wheat seems to have evolved without taking advantage of mycorrhizae,” says Hamel. “We sometimes see a drop in growth when it’s associated with the fungus. Perhaps wheat’s extensive network of fine roots delivers nutrients so efficiently any extra delivered by the fungus doesn’t justify the cost in sugars lost to the mycorrhizae. Peas are the other extreme — they have relatively few roots and support more vigorous mycorrhizal populations.”
Hamel and her team have found differences among crop genotypes. Some genetic lines of wheat rich in nitrogen and potassium do better with mycorrhizae, with higher levels of potassium and nitrogen in leaves when the plants are grown with mycorrhizae. Potassium makes photosynthesis more efficient by boosting transport of nutrients and carbohydrates produced in leaves.
As soil fungi go, mycorrhizae are very big — each individual can be 200 to 800 microns, big enough to see on a glass — and they grow fast, but they are very difficult to identify. They don’t grow in culture and researchers haven’t found the sexual stage that scientists use to identify fungal species.
“We have so much to learn about soil fungi,” says Hamel. “Plant pathologists are so busy with leaves and the fungi on them, but we know very little about what’s happening under the ground.”