Signalling higher yields
By Carolyn King
Could compounds produced by root bacteria offer a practical, low-cost way to boost canola yields when the crop faces environmental stress? A five-year project at McGill University is working on the answer to that question.
Over the past few decades, research has brought about a revolution in our understanding of the importance of the relationships between plants and microbes. It turns out plants have evolved close and complex relationships with the community of microbes that live in and around them.
“When plants are growing in a field, they are never without microbes. So you really need to consider the plant and its associated microbes,” explains Donald Smith, a professor in McGill’s plant science department who is leading the project. “A plant growing in a field isn’t really an individual; it is a community.”
It also turns out a plant’s microbial community is pretty important to whether the plant is healthy or unhealthy. Some of the microbial species in these plant-microbe communities have evolved to work with certain plant species, so both the plant and the microbe benefit. These beneficial microbes can promote plant growth in various ways such as making nutrients available to the plant, producing plant hormones, suppressing plant pathogens, and improving the plant’s ability to withstand stresses like drought and cold.
Another research advance in recent years has been the discovery by Smith and other scientists that a plant and its microbes communicate with each other by releasing biochemical, “signal compounds.” Smith outlines a classic example of the use of such signal compounds – when a legume species communicates with its associated species of nitrogen-fixing rhizobia bacteria. “The legume plant produces a set of compounds that the bacteria recognize, and in response, the bacteria produce a set of compounds that the plant recognizes. These compounds trigger changes in each other’s gene expression patterns and those kinds of things.” Those changes in the plant and the bacteria initiate the symbiotic, mutually-beneficial nitrogen-fixation process.
Smith’s current project with canola grew out of his extensive research on signal compounds. He says, “I started out working on the legume-rhizobia interaction, and we were eventually able to show that the signals the bacteria produce back to the legumes do act as a signal and allow the symbiosis to go ahead. But we also discovered quite accidentally that the signals also have a way of promoting plant growth, particularly when some stress is present. When we realized that, we tested the compounds on many other types of plants, and found they work on a lot of other plants, not just on legumes.”
The signal compounds released by rhizobia bacteria are called lipo-chitooligosaccharides, or LCOs. The researchers in Smith’s lab have done quite a bit of work with an LCO from radyrhizobium japonicum, a rhizobia bacterium associated with soybeans. They have patented this compound for its growth stimulation activity, and it has been commercialized for various crops.
Smith’s research team has also been investigating whether other types of plant-associated bacteria produce signal compounds that promote plant growth. They discovered one such compound in a strain of the bacterium Bacillus thuringiensis. They have named the compound thuricin 17 and have patented it. It is now being licensed for commercial use.
“Bacillus strains are very common on plant roots; if you isolate bacteria from plant roots, you almost always get acillus types,” Smith explains. “Whether they all produce thuricin 17 is a good question. I suspect not, but some of them may produce it or other compounds that cause these kinds of growth-promoting effects.”
Although the two signal compounds have been widely tested on many crops and in many areas, very little testing had been done with canola until Smith started this research. He led an initial two-year project (from April 2011 to March 2013) to explore the potential of these two compounds for promoting growth in canola and found some promising results. His current project builds on that initial work, with further experiments to look for the most effective ways to use the compounds in canola production.
Smith points out canola is somewhat unusual in terms of its symbiotic microbial relationships. “Canola has its own set of bacteria that are associated with the roots, but it doesn’t have the two classically studied plant-microbe associations – the legume-rhizobia relationship and the relationships that most plants have with mycorrhizal fungi [which help plants capture nutrients from the soil]. So this research project could give us a new, direct way to manage aspects of plant-microbe interactions with canola.”
His current project, now in its third year, involves controlled environment studies and field trials at McGill’s Agronomy Research Centre near Montreal, and it includes several components.
One component is determining how applications of the two compounds influence canola growth under various stress conditions. “From our previous research, we know the effects of these compounds are much larger when there is stress. Generally plants growing in a field are at least somewhat stressed. For instance, if the growing temperature is perfect during the day, then it is probably going to be too cold at night,” Smith says.
Another component is comparing the effects of the compounds applied as seed treatments or as one or more foliar spray applications. The researchers are assessing which options produce the best canola performance and looking into related questions like whether it would make sense to apply the compounds as a tank mix with another product, such as an herbicide.
Smith suspects a seed treatment would help a canola crop get off to a good start in the spring. “We’ve done some work with seed germination at low temperatures because when you plant a crop in the spring, especially on the Canadian Prairies, there is always the risk of cool temperatures. The seed treatment could help quite a lot in that situation.”
Similarly, a foliar application might help the crop deal with weather stress a little later in the growing season. He says, “For instance, canola doesn’t like hot weather; if the temperature rises up into the high 20s when canola is flowering, that can lead to abortion or sterility. If spraying the compound would help the plant deal with heat stress, then it could improve yields.”
The results so far indicate that applying either of these signal compounds tends to improve canola’s germination, growth and yield potential under some conditions. As has been found with other crop types, the compounds’ level of effectiveness on canola varies with the weather conditions. Smith says, “These compounds interact with the weather, so the exact effect in any given year is hard to predict. It depends on if there’s weather stress and when it comes during crop development – that can have a huge impact on how large the effect of these compounds is.”
Another aspect of the project involves testing the two compounds on different canola genotypes. “For example, it might work well on certain genotypes and not very well on others, or perhaps it works reasonably well on all of them but with some minor variability,” he notes.
The project also includes an investigation of the mechanisms underlying canola’s responses to the two compounds. Smith says, “That is a big question. We’ve done some work on that in Arabidopsis, which is a plant in the mustard family and reasonably closely related to canola. What we see with Arabidopsis is a lot of responses related to energy metabolism that is up-regulated when the plants are treated with these signals.”
So far, their research suggests that, as in Arabidopsis, the signal compounds are likely promoting growth in canola by stimulating some pathways related to energy metabolism.
This possible effect on energy metabolism makes sense given that compounds have their greatest effects when a plant is under stress. That’s because stress usually disrupts a plant’s energy pathways. So the compounds could be helping the plant to keep its energy pathways running at a good level when the plant is coping with stress.
Smith’s lab is currently digging deeper to understand the exact biochemical mechanisms occurring in the canola plant as it responds to the thuricin 17 and LCO treatments.
Smith thinks the two signal compounds have potential to be practical options for canola growers. “These compounds are applied at really low concentrations, so they would be inexpensive inputs, perhaps a few dollars per hectare. Generally with canola, getting it established and getting a nice, even crop stand up to about the three-leaf stage is key. If the compounds really help with that consistently, then this could potentially set canola growers up for increased returns over time.”