Microbial Consortium Protect Plant Hosts From Harmful Pathogens

photograph of green plants and their roots in the reddish-brown dirt

Biological Strategy

Bacteria

Mary Hoff

Image: Lidiane-Miotto / Shutterstock / Some rights reserved

Biological Control of Populations, Pests, Diseases

An ecosystem must prevent overpopulation of any given species within it, as well as the spread of pests and diseases, to allow organisms to survive without threatening extermination of other organisms or ecosystems. To control pests and diseases, living systems use such strategies as developing antibodies and killing or repelling pests or disease-causing organisms. Ecosystems have checks and balances to maintain populations, pests, and diseases. For example, in a lake, a balance of predators, prey, and nutrients is crucial for proper functioning of the ecosystem. Predatory fish keep down the numbers of smaller prey fish, which in turn keep populations of algae-eating zooplankton in check. When the levels of any of these organisms change, the change can disrupt the health of the entire ecosystem.

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Manage Disturbance in a Community

When environmental conditions change, they can disrupt an ecosystem’s equilibrium. Excessive rain can cause flooding and drought can cause forest fires. An ecosystem must be resilient to such disturbances. Disturbances are unpredictable in location, size, and intensity, so ecosystems must be able to regrow and must have a variety of duplicate forms, processes, or systems that are dispersed in location. For example, a forest ecosystem can recover from fire because diverse organisms play different roles in different ways and in different locations. Many organisms can resprout or grow from seeds triggered by fire, and their dispersed distribution ensures that an entire population isn’t decimated. Though the recovered ecosystem may look totally different from the pre-fire one, the ecosystem as a whole remains healthy.

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Cooperate Within an Ecosystem

An ecosystem is a community of organisms (plants, animals, and microbes) interacting with one another and the nonliving components of their environment (such as air, water, and mineral soil). This interaction can be passive or active, and can cooperatively enhance the functioning of the ecosystem as a whole. For cooperation to contribute to maintaining communities within an ecosystem, it must be beneficial to at least some members of the community. Cooperation consists of symbiotic relationships, such as mutualism (in which two or more species in an ecosystem benefit) and commensalism (in which one species benefits and the effect on others is neutral). An example of a commensal relationship is that between bromeliad plants and trees: bromeliads live on trees without harming them. Bromeliads have mutualistic relationships with other species, including insects, frogs, and worms. The plants capture water in their base, forming a pond that these organisms join. The nutrients that these organisms excrete in their droppings nourish the bromeliad.

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Cooperate/Compete Between Different Species

From parasitism to mutualisms, there are endless interactions between organisms in nature. Interactions between species that lead to a negative outcome for one species are known as competition. Cooperation occurs when organisms work together for the benefit of each organism or species. While competition often occurs over resources or mates, the more biologists have begun to look for examples of cooperation in nature, the more they have found. For example, around 90% of plants have a beneficial partnership with fungi. Fungi provide the plant with nutrients such as nitrogen and phosphorus, in exchange for sugars from the plant.

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Protect From Microbes

In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Bacteria

Domain Bacteria (“small staff”): L. acidophilus, Staphylococcus

From inside of our own bodies to the deepest parts of the ocean, bacteria have adapted to almost every possible environment. All bacteria are unicellular, microscopic, and lack a membrane-bound nucleus and organelles, and come in three basic shapes: spherical (cocci), rod (bacilli), and spiral (spirilla). Some strains can be deadly, like Staphylococcus aureus (MRSA) while others, like Lactobacillus acidophilus, found in our intestinal microbiome, keep us healthy. Cyanobacteria, also known as blue green algae, caused the Great Oxidation Event 2.4 billion years ago, producing Earth’s first oxygenated atmosphere.

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Plants recruit beneficial soil microbes to suppress disease-causing organisms.

Introduction

The billions of bacteria, fungi, and similar microscopic organisms that make up the microbiome around a plant’s roots are a mixed bag for the plant itself.

Some of the members of this invisible community are harmful. They can infect a plant, robbing it of nutrients, disrupting its ability to function normally––even killing it.

Other soil inhabitants, however, are beneficial, helping the plant take up water or nutrients or producing chemicals that help it grow faster. In exchange, the plant shares some of the sugars it makes from water and oxygen using energy from the sun.

This raises a big question: How can a plant fight off harmful bacteria, fungi, and more without destroying the beneficial ones, too?

The Strategy

The way a plant’s root system is structured can favor friendly microbes by providing habitat they prefer more than harmful microbes do. But the friendly microbes themselves turn out to be valuable allies in warding off harm, too.

When a disease organism starts to take over and harm a plant, the plant produces sugars, amino acids, and other chemicals that signal to beneficial microbes that it’s in trouble. The beneficial microbes respond by moving in and multiplying. In some instances, the beneficial organisms simply crowd the harmful ones out. But in other cases, the beneficial microbes actually repel or destroy the attackers.

Perhaps most interesting is the fact that no single type of beneficial microbe has the ability to protect a plant on its own. Rather, many different microbes work together in a complementary way to create optimal disease-suppressing conditions within the soil. The exact mix of the microbes varies from place to place and from time to time, depending on the nature of the threat from plant pathogens in the soil. For example, a mix of 17 different microbes helps potato plants fight off Rhizoctonia solani, a fungus that attacks their underground structures.

The Potential

Plants are not alone in enlisting helpful microbes to stay healthy. Animals—humans included—also get help warding off harmful bacteria and other microorganisms from the community of beneficial microbes that live on and in them.

Understanding how plants signal distress and how beneficial soil microorganisms work together to respond is important for developing strategies to maximize the ability of beneficial microbes in any microbiome to fight off harmful ones. One specific potential application is the development of antiviral and antifungal treatments, soil remediation, and developing recipes for soil inhabitants to boost agricultural yields. The approach could also be applied to boosting the ability of the human immune system to ward off disease-causing organisms or even cancer cells.

Thinking more broadly, we can apply the concept of synergy and customizing the mix of benefactors to fit the unique circumstances more broadly to the search for solutions to just about any type of effort to favor beneficial over adverse outcomes. When no single approach seems able to ensure the “good guys” win, it might be worthwhile to take a lesson from beneficial microbes and look for a combination of many different approaches that just could do the trick.

Mutualism Enhances Pollutant Breakdown

Azobacter

Plants and bacteria break down organic compounds in the soil through a mutualistic relationship.

Last Updated December 29, 2023

References

“Plants host a mesmerizing diversity of microbes inside and around their roots, known as the microbiome. The microbiome is composed mostly of fungi, bacteria, oomycetes, and archaea that can be either pathogenic or benefi cial for plant health and fi tness. To grow healthy, plants need to surveil soil niches around the roots for the detection of pathogenic microbes, and in parallel maximize the services of benefi cial microbes in nutrients uptake and growth promotion. Plants employ a palette of mechanisms to modulate their microbiome including structural modifi cations, the exudation of secondary metabolites and the coordinated action of different defence responses.” (Pascale 2020:1)

Journal article

Modulation of the root microbiome by plant molecules: The basis for targeted disease suppression and plant growth promotion

Frontiers in Plant Science | January 24, 2020 | A. Pascale, S. Proiettei, I.S. Pantelides, I.A. Stringlis

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Reference

“These findings suggest that the complex phenomenon of disease suppressiveness of soils cannot simply be ascribed to a single bacterial taxon or group, but is most likely governed by microbial consortia.” (Mendes et al. 2011:1100)

Journal article

Deciphering the rhizosphere microbiome for disease-suppressive bacteria

Science | May 5, 2011 | R. Mendes, M. Kruijt, I. de Bruijn, E. Dekkers, M. van der Voort, J.H.M. Schneider

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Reference

Journal article

Modulation of the root microbiome by plant molecules: The basis for targeted disease suppression and plant growth promotion

Frontiers in Plant Science | January 24, 2020 | A. Pascale, S. Proiettei, I.S. Pantelides, I.A. Stringlis

embedly preview

Reference

Journal article

Deciphering the rhizosphere microbiome for disease-suppressive bacteria

Science | Mau 5, 2011 | R. Mendes, M. Kruijt, I. de Bruijn, E. Dekkers, M. van der Voort, J.H.M. Schneider

embedly preview

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