Roots Recruit Symbiotic Soil Bacteria

Biological Strategy

Maize

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Image: Waldemar Brandt / Public Domain - No restrictions

Protect From Plants

Living systems must share resources with other living systems. Often, this can be done in cooperative ways but at other times, there is competition for resources such as water, nutrients, and sunshine. By consuming resources, plants can threaten other living systems. In response, threatened living systems must discourage this competition in ways that don’t harm the threatened system. For example, trees in tropical rainforests have strategies to prevent climbing vines from attaching to their trunks, eliminating competition from the vines for sunlight above and nutrients at the roots.

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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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Send Chemical Signals (Odor, Taste, Etc.)

Tastes, odors, and hormones are all chemical signals that can float through the air or water, or be applied to solid surfaces. Chemicals are important for signaling and communication; even humans, with our underdeveloped sense of smell, are influenced by chemicals more than we realize. Chemical signals are often specific to the living system intended to receive them, and are often relatively weak, dispersing upon moving through liquids or gases. To ensure that chemical signals reach their target, living systems create unique chemical signals and methods of dispersal. One example is an orchid that is pollinated by dung beetles, and therefore distributes a dung-smelling aroma to attract them.

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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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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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The roots of maize defend against phytopathogens by releasing a pesticide that also attracts a beneficial microbe that preys on the target pest.

Introduction

Plant roots are surrounded by tens of thousands of species of microbes that collectively make up what is called the rhizosphere. Some of these microbes are phytopathogens that have damaging effects on plants. Others benefit the plant through promoting nutrient uptake, nitrogen fixation, and pathogenic defense systems. Of the rhizosphere microbes that aid in immunity, some function by out-competing pathogens for nutrients, while others actively secrete antibiotics or induce an immune response in the plant.

The Strategy

It has been recently documented that maize is capable of attracting beneficial bacteria to its rhizosphere to promote health. Early stage maize seedlings are especially vulnerable to harm from phytopathogens.

Image: LadyDragonflyCC / CC BY - Creative Commons Attribution alone

Early stage maize seedlings are especially vulnerable to harm from phytopathogens.

They have long been known to secrete a chemical called DIMBOA (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one) from their roots. This compound is a potent biocide that eliminates harmful bacteria, insects, and other plants. Remarkably, a beneficial bacterium called Pseudomonas putida is attracted to DIMBOA and can detoxify it. Once within the rhizosphere or the maize seedling, the bacteria can out-compete phytopathogens for the limited nutrient supply. In this way, maize plants are able to recruit microbial allies to their roots in order to prevent the growth of phytopathogens.

The Potential

To protect our food crops, we often apply pesticides that are harmful to other organisms and the environment. With maize, we see an example of a plant altering the chemical and biological properties of soil to suit its needs. By continuing to study how corn accomplishes this as well as identifying other plants that can affect soil properties, we may be able to plant plants as “pesticides” instead of using harsh chemicals.

Plant Compounds Protect From Competitors

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Compounds in rice plants protect the plant from competitors by suppressing growth in competing plants

photograph of grain plants and cloudy sky during day

Biological Strategy

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Last Updated March 23, 2020

References

“[P]lants interact with a wide range of rhizosphere-colonising bacteria. These are attracted to root surfaces by chemical components in root exudates…This so-called rhizosphere effect supports bacterial cell densities in the root vicinity up to 100-fold greater than in surrounding soil…the plant-microbe interaction can range from deleterious, in the case of phytopathogens, to beneficial, where rhizobacteria can promote plant growth and resistance to plant stress…rhizobacteria can promote growth indirectly by protecting the host plant against pests and diseases. This protection can be based on direct antibiosis or competition for nutrients, but can also result from induced systemic resistance (ISR). Evidence suggests that plant-associating bacteria have evolved the ability to metabolise plant-derived aromatic compounds…Benzoxazinoids (BXs), such as 2,4-dihydroxy-7-methoxy-2H- 1,4-benzoxazin-3(4H)-one (DIMBOA), are heteroaromatic metabolites with benzoic acid moieties…their role in plant defence against above-ground pests and pathogens. BXs are typically produced during relatively early, vulnerable plant growth stages.” (Neal et al. 2012:1)

Journal article

Benzoxazinoids in Root Exudates of Maize Attract Pseudomonas putida to the Rhizosphere

PLoS ONE | 25/04/2012 | Andrew L. Neal, Shakoor Ahmad, Ruth Gordon-Weeks, Jurriaan Ton |

Ching-Hong Yang

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