For as long as plants have been growing on land, their roots have shared a symbiotic relationship with microbes in the soil. Arbuscules — the microscopic nutrient-gathering hairs of mycorrihizae — have been found connected to 400-million-year-old plant root fossils from the Rhynie chert in Scotland.
A paper by Winfried Remy et al, published in Proceedings of the National Academy of Sciences in 1994, reported on the finding, noting that arbuscule-forming fungi — specifically vesicular arbuscular mycorrhizae (VAM) — are now “nearly universal in their association with flowering plants.” (As an aside, canola is one of the exceptions.)
And this “association” is complex. “While it is well known that VAMs considerably increase the uptake of specific nutrients by the host, other functions attributed to the fungi include production of plant growth hormones, protection of host roots from pathogens, and increased solubility of soil minerals,” the paper reported.
The take-home is just how little plant roots can do on their own. Relationships with microbes are essential for plant life and food production. The plant provides its microbiome buddies with carbohydrates from photosynthesis, and the microbiome provides the plant with nutrients from the soil.
VAM is not alone in this jungle.
With refined microscopes and continually improving tools to analyze organisms through their DNA, researchers can better explore the ancient, complex and massive root microbiome with its billions of organisms per tablespoon of soil. They are just getting started on what this knowledge could provide for agriculture.
One of Canada’s leading researchers of the root microbiome is Donald Smith, professor in the plant science department at McGill University in Montreal. Smith says the most significant advances in managing the microbiome have been with rhizobia inoculants for pulse crops, the basics of which were put into practice over a century ago. Many other products followed, including Penicillium bilaii that makes soil and fertilizer phosphate more available to the plant (sold as JumpStart), plant growth-promoting rhizobacteria (PGPR) and various hormone products.
Putting ancient LCOs to work
Smith says lipochitooligosaccharides (LCOs) represent a promising next stage. LCOs are natural molecules that friendly microbes express as a signal to plant roots that they’re present and ready for coupling. They help in a number of ways.
LCOs trigger nodule-forming between rhizobia and pulse crops, so adding LCOs to inoculants could boost nodulation rates — especially in a stressed situation. Smith gives a soybean example. “Soybeans are originally from south-coastal China, a warm climate. Soybeans in Canada are going into relatively low-temperature soils at planting. With LCOs, nodulation happens faster in this higher-stress situation.”
LCOs also trigger mycorrhizal symbiosis and work as general plant growth stimulators for crops such as canola that do not form rhizobia nodules or have mycorrhizal relationships. As with nodulation, “LCOs provide this stimulation to a much greater degree when plants are stressed,” Smith says.
Because they are “signals” and not nutrients, LCOs operate at very low concentrations, Smith adds, meaning that an effective amount could be added through seed treatment.
The BioAg Alliance, a partnership between Monsanto and Novozymes, has several LCO products on the market or in the pipeline. TagTeam LCO for pea and lentil crops in Western Canada combines rhizobium inoculant for nodulation, while P. bilaii (JumpStart) and LCO prompts pea and lentil roots to receive nodulating rhizobia much faster. LCO is also found in combination with rhizobium in the soybean product Optimize, which is sold in Canada and around the world. The BioAg Alliance will follow with an LCO seed treatment for Monsanto’s corn hybrids in 2019. Products for canola and wheat will come later.
“We have seen a lot of response from plants given an LCO application,” says Colin Bletsky, vice-president of BioAg at Novozymes. Bletsky works out of the Saskatoon office and also farms at Canora, Sask. He says LCO speeds up the relationship between plant roots and beneficial micro-organisms in the soil — “like a handshake.”
But what about canola?
But what is canola shaking hands with if it doesn’t make mycorrhizae or rhizobium relationships? Smith says the LCOs will have some direct effects on the plants — related to stress responses, for example. “They could also affect the way roots respond to other microbes,” Smith says, but adds, “I don’t think there is much published around this.”
With LCOs or any other probiotic-type product sold to boost nutrient uptake or improve stress response, growers need to set reasonable expectations.
“We know inoculants work well for pulses. Beyond that, the best these products can provide are small incremental advantages in certain situations — such as high stress,” says Curtis Rempel, vice-president of crop protection and innovation with the Canola Council of Canada. “Unfortunately, it has become a buyer-beware minefield for farmers.”
Free-living nitrogen fixers
Driving the market is this underlying motivation: farmers would like a low-cost way to boost yield and reduce their fertilizer bill, especially for nitrogen. But there is nothing yet that can replace nitrogen fertilizer. It will be huge news when it happens.
“If someone had that… we’d all know about it,” Bletsky says. “The organization would be a $10 billion company instantly.”
Free-living nitrogen-fixing microbes are “a promising frontier,” Rempel says. Free-living nitrogen-fixing fungi or bacteria are not nodule-forming and they don’t rely on a relationship to specific plant species. They are generalists, taking nitrogen from the air — which is 78 per cent nitrogen — and converting it to a plant-available form. They work in close proximity to or on the surface of the root, working in exchange for carbohydrates from the plant roots.
“Many perennial shrubs and sedges use free-living nitrogen as a major source of nitrogen, and sugar cane has made big strides to utilize free-living nitrogen fixers,” Rempel says. “The trick is to identify these nitrogen-fixing species.”
Once researchers discover microbes that work for Canadian annual crops, the next step is finding rotations and application techniques that help this beneficial microbe thrive in our climate and cropping systems, Rempel says.
In the meantime, Bletsky says there are “lots of other good things happening” with microbes. He thinks the use of beneficial micro-organisms will become a fourth pillar in crop inputs, joining seed, chemistry and fertilizer.
Smith shares that view. “There are several examples that demonstrate the vast potential of the plant root microbiome to serve as a source of extremely effective, very inexpensive and environmentally friendly crop production inputs that can, with some further research effort, become in the 21st century as important to crop production as chemical inputs were for crop production in the 20th century,” he says.
These are exciting times for root microbiome explorers. Smith and his fellow researchers have 400 million years — or more — of relationships to discover, and they’re just getting started.