Invasive plants can literally swallow your garden in a few weeks. How do they do that? That question is being explored through microbiome research. As with animals and humans, plants also interact symbiotically with microbes in their life.
The reason researchers are focusing on the microbiome surrounding invasive plants is because certain microbes are contributing to the success of invasive plants.
The soil microbiome
The soil microbiome is defined as all microorganisms and their genes found in soil, including archaea, bacteria, viruses, fungi, protists, and other microbial eukaryotes (Fierer, 2017).
The soil microbiome plays a vital role in establishing invasive plants. Therefore, it is essential to understand the soil microbiome's function and composition.
Surprisingly, and contrary to what many people think, the number of microbes in soil is relatively low. For instance, less than 1% of the available soil surface area is typically occupied by microorganisms.
Biotic factors like the number of microbial predators (e.g., protists or nematodes) and abiotic factors such as the amount of available carbon, influence the total amount of microbial biomass found in soil at any given point.
Furthermore, soil humidity and pH can be indicators of microbial richness and bacterial and archaeal community composition, respectively. For instance, the wetter the soil (e.g., rainforest soil), the more significant amounts of microbial biomass can be found.
Illustrates the scale of microbial distribution within the soil. Fungi and bacteria are far more numerous compared to viruses, archaea and protists.
The dominant microbes in the soil are bacteria and fungi. These two groups usually have 102 –104 times more biomass than protists, archaea, and viruses. Unfortunately, many of these taxa remain unknown.
Interestingly, surviving in the soil is not easy. Microbes are faced permanently with limited carbon sources, low water, abrupt changes in dryness and wetness, and a high degree of competition with other microbes.
This can explain why >95% of the total microbial biomass pool is represented by inactive microorganisms at any given time (Blagodatskaya & Kuzyakov, 2013).
Therefore, the soil microbiome can be classified into three general categories: stress tolerators, competitors, and ruderal microbes.
What is a non-native invasive plant?
A non-native invasive plant is a non-native plant whose introduction likely causes disturbance to other plant environments and causes economic and environmental harm (Beck et al. 2008).
Invasive plants adapt well to unfavorable conditions. After germination, invasive plants grow fast, establish populations rapidly, and quickly reach the flowering phase.
They produce numerous seeds that are easily dispersed over long distances and can stay in the soil for several years until conditions are favorable for germination.
Some invasive plants have organs that help them conquer new soils quickly, like rhizomes; therefore, they propagate vegetatively (without a fertilized seed). Furthermore, these invasive plants get in close contact with native soil microbes.
Interactions between invasive plants and microbes
Plants interact closely with soil microbes throughout their lifecycle.
These interactions are complex because they depend on many biotic and abiotic factors, such as the climate where the plant and microbes interact, the plant life cycle stage (young or old), and the soil conditions (e.g., pH, water availability).
Invasive plants can recruit microbes from the soil for their benefit; however, it is mainly unknown which mechanisms are responsible for this selection (Agler et al. 2016).
Two main types of interactions between plants and microbes are known as endophytic and epiphytic. Some of these interactions and their species have been reported for invasive plants.
Endophytic interactions
An endophytic interaction is when microbes live within plant tissues. Endophytic microbes are often restricted to particular organs, usually roots, stems, or leaves. Endophytic microbes can be either bacterial or fungi.
Leaf infected by a fungus representing an endophytic infection, via Canva
One of the best-studied associations between endophytes and an invasive plant like grass is with the Neotyphodium fungi (asexual stage).
Interestingly, about 20%–30% of all grass species are colonized by Neotyphodium endophytes and their sexual relatives Epichloe.
Epiphytic interactions
The epiphytic lifestyle generally refers to microbial development directly on host surfaces. The epiphytic microbes come from the soil, water, seed, animal excrement, or the atmosphere and comprise the breadth of bacterial and fungal diversity.
These epiphytic associations are primarily driven by nutrients released from the plant into the adjacent soil.
For instance, fungal epiphytes such as Fusarium fujikuroi and Fusarium oxysporum have been reported to promote the growth of the invasive plant Ipomoea cairica through hormone interactions (Xu et al. 2021).
Strawberry plant with Fusarium oxysporum, via Canva
Invasion factors
Microbial interactions can favor or inhibit invasive plant success. Advantages and disadvantages depend on the origin of the microbe, direction, prevalence and strength of the microbial association.
However, researchers are still trying to understand, the mechanism and the role of many of these microbial symbioses.
It is also important to highlight that invasive species are generally less dependent on microbial interactions than native plants, which could explain the success rate of invasive plants.
Managing invasive plants through soil microbiome manipulation
Plant invasions are a global concern because they threaten biodiversity and natural resource management, especially in protected areas.
These invasions are generating a significant investment toward control methods. For instance, the crop loss caused by invasive plants is estimated to be more than $100 billion US dollars per year (Swanton et al., 2015).
The practices to control invasive plants include mechanical removal, burning, and using herbicides, which can cause harm.
Researchers believe there is great potential to using the plant/soil microbiomes to aid in invasive plant management. In order to do so, there would have to be intentional manipulations of microbial interactions that are meant to reduce the competitiveness of invasive plants or increase the productivity of non-invasive and native plants.
For example, some fungal endophytes enhance the competitiveness of invasive plants. Finding antagonist species that, through exogenous applications, inhibit the fungal benefits provided to the invasive plant is ideal.
Another strategy includes soil transplant, where the introduction to soils of complex microbiomes from naturally disease suppressive soils improve the invaded soil microbiome (Kowalski et al. 2015).
The strategy requires engineering the soil microbiomes to receive the beneficial healthy plant-associated microbiomes.
For instance, quantification of the rhizosphere microbiome transplant efficacy revealed more than 60% of the donor microbial communities successfully colonized the rhizosphere of recipient plants. This resulted in the recipient plants reaching up to a 47% reduction in disease incidence (Jiang et al., 2022).
Although rhizosphere microbiome transplant (RMT) represents a promising approach for invasive plant management, its research is still evolving, and recipes related to ratios on what is a good or bad soil microbiomes have not been developed yet.
Keywords
Soil microbiome, invasive plants, weeds, invasive plant management, soil microbiome manipulation.
References
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