The Power of Plant-Microbe Signaling and Strigolactones: Enhancing Crop Health with SOURCE

The relationship between plants and microbes has developed over thousands of years, and these organisms have learned to communicate their needs to each other in subtle yet effective ways. SOURCEⓇ takes inspiration from this to help plants create mutually beneficial connections with beneficial soil organisms like arbuscular mycorrhizal fungi and other key microbes. 

Written by Rachel Sim, with expertise provided by Tamara Meragelman, Senior Director of Research, and Jessica Pupo, Senior Agronomist. 

Over the millennia, plants and microbes have developed mutually beneficial relationships, largely based on photosynthesis, through which plants are able to draw carbon out of the atmosphere and turn it into forms microbes and other life can use. In turn, microbes trade plant-available nutrients to plants in exchange for these sugars and carbohydrates. 

Still, plants need a way to communicate that they are ready to trade. To do this, plants produce various molecules to recruit beneficial microbes, but molecules called strigolactones make up one of the more important groups. 

“These are fairly complex natural products that nature designed very specifically to signal the microbes of interest,” says Tamara Meragelman, Senior Director of Research at Sound.

Strigolactones differ from the carbohydrates and sugars that plants release through their roots for microbes to consume; they are specifically signaling and recruiting molecules, not providing food. If you imagine a bakery or a café, strigolactones are kind of like the smell of fresh baked cookies or coffee, attracting hungry microbes to trade with the plant, but the actual treat comes later. 

It’s important to remember that strigolactones aren’t actually what microbes eat though — they’re not the cookies. ​“It’s those sugars and carbohydrates that are a byproduct of photosynthesis from the plant that are energy for microbes, not strigolactones,” says Jessica Pupo, Senior Agronomist with Sound. ​“So another way to think about it is that strigolactones are the ​‘open’ sign, letting microbes know the plant is open for business.”

These signals need to be powerful and efficient. Because making a complex molecule requires significant energy, strigolactones are produced in very small but potent amounts. When plants want to trade, they may not have a lot of extra resources to spend, but the complexity of strigolactones is part of why they work. 

“Plants need to create a highly sophisticated and specific molecule to avoid attracting harmful microbes or pathogens and only signal the microbes they want,” explains Tamara. Since plants have to put so much energy into making these complex molecules, strigolactones need to be super potent and work in very tiny amounts. 

“If we smell fresh chocolate chip cookies, it doesn’t take a lot to know we want to eat it and track down where the smell is coming from; that’s essentially what strigolactones do for microbes,” says Tamara. 

To develop SOURCE, Sound researchers looked to the powerfully potent microbe signaling strigolactones for inspiration. Because they’re made in such small amounts, using the strigolactones plants made themselves didn’t make any sense, so instead the researchers looked at what they could mimic in a lab. 

“We looked at the molecular structure of natural strigolactones to see which essential parts would give us the same biological effect and if we could make something that microbes wouldn’t recognize as a different molecule,” says Tamara. The result was a much simpler molecule that mimics what these natural compounds do and that is indis­tin­guish­able from natural strigolactones to both plants and microbes. That compound is the heart of SOURCE, but over the years, tweaks have been made to the formulation to improve SOURCE’s signaling ability. Though SOURCE is not itself a strigolactone, when it is foliar applied, it produces an identical effect in the plant rhizosphere and the surrounding soil microbiology.

“While many categories of plants produce some type of strigolactone, they can differ very slightly based on the crop that’s producing them,” says Jessica. 

Today, SOURCE comes in designed formulations for different crops. Tamara says the research team is always looking for better ways to communicate with microbes, and formulation design are the result of those efforts.

“Slightly different species of microbes will have better symbiotic effects when working with particular plants,” Tamara explains. ​“When we tweak the formulation, it’s because we’ve done studies in the lab to understand how best to communicate with microbes, whether it’s with a different concentration or another molecule that targets a particular microbe.” 

Take legumes like soy, for example, which develop small nodules on the roots where specialized microbes convert atmospheric nitrogen into a plant-available form for the crop. This is an incredibly unique relationship that exists only in legumes, ensuring that nitrogen is provided directly to the crop without any risk of environmental loss. 

“We had to maximize the value of that incredible symbiotic partnership and think about how the SOURCE formulation could help the plant communicate directly with the bacteria living in those nodules,” says Tamara. 

When talking about strigolactones, it’s important to remember how nutrients get to the plant. 

“The size of the root zone is limited by how far the roots can reach,” says Jessica, ​“so in order for nutrients to get to those roots and for plants to take them up, they need to enter through mass flow, diffusion or root interception.” Unfortunately, each of these has limitations; mass flow is limited by water availability, diffusion is limited by the chemical balances within the soil and root interception is limited by the plant’s root mass. 

That’s why forming relationships with microbes is so important. ​“When plants are able to form a relationship with the microbes in the soil, they have access to nutrients well beyond the rhizosphere because the microbes can move,” she says. ​“Without that relationship, the plant is limited to only the nutrients that the roots themselves can access.”

While plants naturally make strigolactones on their own, there’s still potential for big benefits when using SOURCE on the farm. When they form relationships with microbes in the soil, crops don’t just get nitrogen and phosphorus. For one, a healthy soil microbiome improves other soil qualities like water retention, organic matter content and other markers of soil health. But nutrient reduction can also improve crop resilience.

It’s well documented that when crops are fed directly, they stop relying on natural soil systems and relationships, Jessica explains. ​“There are soil tests that will show how dependent a field is on nitrogen fertilizer compared to how efficiently the nutrients in the soil are being used,” she says. ​“If we feed crops with too many external resources, the plants become more dependent on accessing nutrients through fertilizers and other inputs instead of utilizing what’s already in the soil.” 

When plants have too much easy access to nutrition, they simply stop signaling to microbes that they’re open for business. After all, there’s little benefit to making a complex and energy-intensive molecule to attract nitrogen-fixing and phosphorus-solubilizing microbes if the plant is getting all it needs. 

“Every organism is lazy and will spend the least possible amount of energy,” says Tamara. ​“Synthetic fertilizer is the simplest form of nutrient for the plant, so if we overuse it, the plant doesn’t need to recruit or signal microbes and the microbes get no sugar from the plants either.” 

In high enough quantities, this overuse can have serious soil fertility impacts, Tamara says. ​“At some point, the plant stops recruiting and feeding microbes. Over time, the soil becomes less fertile and loses a lot of that biological potential that comes with a healthy microbe community.’ 

This can be particularly true if nutrients are front-loaded during the season. When nutrition is delivered right to the crop, the plants have no reason to develop relationships with beneficial microbes around them or form particularly large root systems. But as the season progresses, the plants’ access to nutrients begins to wane as nutrients either leach or are tied up in the soil by various chemical reactions. 

“At this point, the plant will either try to expand its root zone in search for more nutrients or water, or it’s too late and the plant had switched to focusing its energy on reproducing and therefore energy is concentrated in above-ground growth and the nutrient access is limited to where the already established root zone can reach,” says Jessica. 

As hungry plants desperately look for nutrition, strigolactones are an important way of letting microbes know the plants are open for business. But if plants have had easy access to nutrition before and are trying to develop these microbial relationships from scratch, they struggle.

“If a crop is started off really dependent on external inputs, its ability to access nutrients from the soil may be poor,” says Jessica.

To maximize the benefits of this strigolactone signaling and support the crop in forming these microbial relationships, SOURCE’s recommended timing window is generally right before a period of crucial nutrient uptake for the crop. Remember, strigolactones are naturally produced in small amounts, so SOURCE’s signal boost is very welcome.

“SOURCE works so well at that time because the plant is trying to find as much nutrition and resources as it can before it shoots off into a massive growth period,” says Jessica. ​“Even if the crop is already producing strigolactones, we’re giving it that extra boost to help it create more relationships with microbes and access more nutrients. This is what will help carry the crop in a more sustainable and resilient way through this critical moment.”

Jessica often recommends growers reduce nutrient application before applying SOURCE in order to really set their crop up for success.

“We want the plant to be fully utilizing those natural resources, building up its root system, and forming those connections in the soil,” she says. ​“If nutrients are applied at the same time, it can counteract the work that the strigolactones are trying to do.”

Tamara says success is not found in eliminating fertilizer application entirely, it’s in finding a balance between the plant’s natural abilities to use what’s around it and what growers can give it. 

“There is a perfect balance that we can find where synthetic fertilizers allow us to feed the large number of humans on our planet without losing the value of what nature has already developed with these symbiotic relationships,” she says. ​“With SOURCE, it’s about trying to find that equilibrium.” 

In the short term, SOURCE can benefit growers by helping reduce fertilizer crops, but in the long term, it can also help develop healthier soils. Tamara says one of the interesting things about the soil microbiome is that microbes help each other. 

“As we help to re-establish the soil’s balance, we see that healthy soils not only have just the few species of microbe that are important for plant nutrient cycles, but also different species of microbes, bacteria and fungi of different kinds that help improve the quality of our soil,” she says. 

In fact, it’s well documented that strigolactones are particularly important for attracting arbuscular mycorrhizal fungi (AMF). AMF provide a huge variety of benefits to plants, including extending the reach of the root system, increasing access to nutrients and improving access nutrients, especially phosphorus. 

“Plants will use strigolactones to form relationships with mycorrhizal fungi,” says Jessica. ​“In nature, typically plants in phosphorus stressed environments are found to produce more strigolactones because they are so attractive to AMF, which can bring the plants phosphorus.” 

To take advantage of this relationship, Sound recently launched a new product called BLUEPRINT™ to promote the relationship between crops and AMF. SOURCE has a variety of positive effects in the soil, including signaling AMF and recruiting other beneficial microbes, so BLUEPRINT was a logical next step. And all these organisms working together can have results that are bigger than the sum of their parts. 

“These products may have primary effects like fixing nitrogen, recruiting AMF or signaling bacteria, but once they’re working together, we see improved root size, both in terms of biomass and reach, and with the help of AMF, the plants can reach different areas of the soil to attract nutrients and water,” says Tamara. 

“At Sound, both sustainability and our understanding of how to improve soil health are coming together,” says Tamara. ​“We’ve learned that over-use of a single solution like synthetic fertilizer for too long can be a problem. For us as a research team, our goal is to support growers in finding balance so they can focus on what’s important to them, whether that’s revenue or crop yield and quality. They don’t need to worry about anything else, because we’ve taken care of the rest.”