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  Fighting Microbes with Microbes
Like humans, with their complement of microbes that aid in everything from immune responses to nutrition, plants rely on a vast array of bacteria and fungi for health and defense. Over the last decade, research has revealed many new functional aspects of the crosstalk between human-associated microbes and human cells, but plant biologists are only beginning to scratch the surface of the often surprising ways that soil microbiota impact plants, from underground fungus-wired alarm systems to soil bacteria that can trigger defensive plant behavior or even act as a sort of vaccine. But despite these benefits, microbes are still primarily thought of as harbingers of disease.

“Since the discovery of antibiotics, medical research has been dominated by a ‘bazooka mentality,’” and so has agricultural research, says Alexandre Jousset, a plant scientist at the Georg-August University in Göttingen, Germany. “Traditionally, microbes have been viewed negatively, and focus has been placed on eradication.” Today, scientists and some medical doctors are becoming increasingly aware of their utility, and botanical researchers have also begun to debate whether the same may be true of plants.

While the Human Microbiome Project has discovered that some 10,000 species of microorganisms live in and on the human body, outnumbering our own cells by ten to one, plant scientists have found that any given soil sample contains more than 30,000 taxonomic varieties of microbes. Soil microflora not only provide nutrients for plants, but also suppress disease. In exchange, roots secrete fixed carbon into the soil and feed their bacterial symbionts.

Although the medical community now warns that overprescribing antibiotics kills beneficial organisms and encourages the formation of resistant strains, a similar change in opinion has not occurred in agriculture, where a kill-all approach to plant pathogens has given rise to biocides that indiscriminately wipe out the beneficial along with the pathogenic. “Biocides can nuke the soil, but they never kill everything,” says Mike Cohen, a biologist at Sonoma State University in California. “This creates a biological vacuum that becomes filled by opportunistic survivors and organisms from the surrounding soil.” Biocides create a strong selective pressure: the few pathogens that survive face little competition and proliferate, giving rise to pathogenic communities that can evade standard treatments.

Beneficial soil organisms, however, can protect plants more selectively than biocides do. They displace pathogens and produce toxins that kill pathogenic microbes, and they also trigger plants’ own defense mechanisms. “Native bacteria are the first and most powerful barrier to prevent the establishment of pathogens,” says Jousset. “A diverse community is especially important to keeping pathogens away—this is true in the human gut and in the soil.”

“The idea is that we can reduce pesticide and fungicide use by utilizing the microbiome,” says Harsh Bais, a plant biologist at the University of Delaware in Newark. “But we need to know more about the mechanisms of action; relationships between microbes and plants are very complex.”

Life Underground

According to a recent study published in PLOS One, underground networks of fungi help tomato plants “eavesdrop” on the alarm signals produced by their neighbors.1 Even when plants are not able to communicate with chemical cues released through their leaves, they can link up and share vital information under the soil.

Researchers at South China Agricultural University in Guangzhou inoculated tomato plant leaves with the early blight fungus Alternaria solani, which creates brown and dead patches on leaves and can rot the tomato fruit. They then covered all research plants with airtight plastic bags, which prevented the transmission of airborne signals. Despite being covered, the tomato plants were able to communicate. Uninoculated plants growing several feet away activated defense-related genes and started making disease-fighting enzymes.

Researchers traced communication back to the fungus Glomus mosseae, which forms a symbiotic relationship with plant root hair known as a mycorrhizal network by inserting itself into the root cell’s membrane. Bagged tomato plants grown in soil that lacked this underground network were unable to receive the “activate-defenses!” signal from infected neighbors and did not produce disease-fighting compounds. In contrast, in soils containing Glomus mosseae, uninfected plants detected the warning signs of disease and produced higher levels of six defense-related enzymes, including peroxidase (POD), polyphenol oxidase (PPO), chitinase, β-1,3-glucanase, phenylalanine ammonia-lyase (PAL), and lipoxygenase (LOX). (See diagram below.)

Because the mycorrhizal network can extend from one set of plant roots to another, it’s possible that the network of fungal mycelia acts like telephone wires, allowing the plants to communicate underground. If this hypothesis is proven by identifying compounds that relay the chemical signal through the fungi, it might be possible to prevent plant disease by cultivating an appropriate mix of microbes in the soil. “The problem is that we don’t know how plants and microbes select one another,” says Bais.

To try to answer that question, Bais and his colleagues turned to Arabidopsis thaliana plants and Bacillus subtilis, a bacterium known to improve plant health. Despite the plant’s antimicrobial defenses, B. subtilis somehow becomes established in the soil. The team found that B. subtilis secretes an antimicrobial peptide that temporarily suppresses toxins secreted by the root, allowing the beneficial bacterium to colonize the soil around the roots.2 The peptide secreted by B. subtilis may also help ward off soil-borne pathogens while the plant’s defenses are compromised, says Bais.

In a prior study, Bais and his colleagues found that plants can pick and choose the beneficial bacteria species recruited during pathogen attacks.3 The team infected Arabidopsis seedlings with the bacterium Pseudomonas syringae pv. tomato, which causes bacterial speck—a major disease of tomato crops. Plant roots soon began secreting L-malic acid, a food source for B. subtilis. As a result, B. subtilis colonized the roots, which in turn triggered production of the plant’s defense chemical salicylic acid, helping it fight the bacterial infection. “This isn’t a typical symbiotic relationship,” Bais says, “but there is an interesting reciprocity here.” (See diagram below.)

Plants may even be able to recruit different bacterial species as their need for food and water changes. Researchers from Ain Shams University in Cairo, Egypt, recently dissected the root systems of drought-sensitive pepper plants (Capsicum annuum) grown with varying amounts of water.4 After comparing the structure and diversity of bacterial communities in the rhizosphere, the team found that plants grown in the desert with little water have larger populations of plant growth–promoting (PGB) bacteria which can enhance photosynthesis and biomass synthesis by as much as 40 percent under drought stress. Although PGB’s mechanism of action has not been worked out, the bacteria are known to alleviate salt stress by reducing the production of ethylene in tomato seedlings.

Surprisingly, there is some evidence that the effects of beneficial bacteria can endure across generations. Even after a bacterial community wanes, the biochemical pathways developed by the plants in response to bacterial colonization remain intact. “This suggests the bacteria function as a vaccine of sorts,” says Bais. This heightened disease response can then be passed to the next generation of plants. For example, even when progeny are not exposed to B. subtilis, they are better able to fight disease if parent plants fostered a relationship with the bacterium. “The bacteria help prime the plant to respond more quickly to disease, and they pass this memory to the next generation,” says Bais. The effects appear to last the duration of the offspring plant’s life, but are not passed on to a third generation.

Although most microorganisms that are beneficial to plants reside in the soil, their effects are not always localized to the roots. In a third study, published in The Plant Journal,5 Bais and his colleagues showed that beneficial soil microbes encourage the closure of stomatal pores in the leaves of Arabidopsis plants. Stomata allow carbon dioxide to diffuse into the leaf and release expired oxygen and water into the air. Hot and dry conditions are known to trigger stomatal closure to preserve a plant’s water, but Bais was the first to show that soil bacteria can trigger the response—an important finding, as some pathogenic bacteria, such as P. syringae pv. tomato, enter the plant through the stomata.

To see if root microbes could help counteract already established plant infections, Bais and his colleagues grew plants infested with P. syringae and then inoculated the soil with the beneficial B. subtilis. As the roots recruited new colonies of B. subtilis, the plants began producing abscisic acid—a chemical known to regulate stomatal closure. After three hours, only 43 percent of stomata were open in B. subtilis-treated plants. In control groups, 56 percent of stomata remained open. “This difference was significant and helped reduce disease,” says Bais.

Unearthing the mechanism of action

It’s much harder for pathogens to take over the human gut when beneficial microflora coat its surface. A similar mechanism is at play in the soil. When it comes to preventing plant disease, some microbes kill pathogens directly; others consume resources, taking up the niches that invading bacteria might otherwise inhabit.

“When a community is composed of species that use distinct resources, there is less free room for invading species,” says Jousset. He and his colleagues set out to determine whether a broader genetic diversity of beneficial bacterial strains was more important than simply cultivating a large variety of bacteria, regardless of their genetic makeup.

The researchers grew 95 microbial communities, each containing between one and eight strains of Pseudomonas fluorescens bacteria—another species known to improve plant health. Each group had varying degrees of genetic similarity. The team then exposed the colonies to the invading bacterial species Serratia liquefaciens, which colonizes soil, water, and even the human gut and urinary tract, where pathogenic strains cause infection. After 36 hours, S. liquefaciens was able to invade communities that contained genetically similar species, but it was not able to gain a foothold in more genetically diverse communities. Indeed, as genotypic dissimilarity increased threefold, researchers saw a linear decrease in the colonization by S. liquefaciens.6

However, the number of beneficial species in the soil was nearly as important as the degree of genetic dissimilarity between them. Communities with four to six species were better able to ward off invasion. Interestingly, communities were more susceptible to invasion by S. liquefaciens when a lower or higher number of bacterial species was present. Most likely, says Jousset, this is due to the variety of toxins produced. The colonies containing too many species produced a large amount of toxins, some of which also harmed beneficial strains of bacteria, whereas communities with too few species had low levels of toxin production, thus making invasion more likely.

“This suggests we might be able to encourage disease-fighting bacterial communities by selecting for the right number and combination of species,” says Jousset. Like the gut, the soil is an open system that allows bacteria to come and go, and competition for food and nutrients determines community structure. By manipulating food sources and growing conditions in the soil, it may be possible to select for genetically diverse communities. In a recent field study, Jousset sampled the disease-fighting genes found in the soil and discovered that a diverse mixture of planted herbs and grasses gives rise to the best ratio of disease-fighting genes and helps suppress soil-born pathogens. (See “Down and Dirty,” The Scientist, September 2012.)

Creating healthy soil

Just as antibiotics indiscriminately kill both good and bad bacteria in the gut, fungicides and biocides impede the soil’s innate defenses. Studies have shown that gentler practices such as crop rotation, tillage, and fertilization can influence ecological processes in the soil, and may encourage the establishment of microbial communities capable of suppressing disease.

In search of a way to supplement the soil that encourages the growth of beneficial bacteria, Mike Cohen of Sonoma State University joined colleagues at the US Department of Agriculture to test rapeseed (Brassica napus) meal—a waste product from processing rapeseed into cooking oil or biodiesel.

The researchers split the roots of an apple tree seedling so that the plant had roots potted in two different containers. They then introduced the pathogen Rhizoctonia solani, which causes root rot, into one container. Rapeseed meal was incorporated into the soil of the other container at about 0.5 percent of the total volume, whereas the soil inoculated with the pathogen was left untreated. “This allowed us to test the indirect impacts of seed meal on the plant,” says Cohen.7

The rapeseed meal reduced root rot by about 50 percent relative to control groups grown without the treatment. In fact, the researchers observed that the entire plant benefited from the rapeseed meal even though only half of the roots were exposed. Cohen and colleagues think that rapeseed meal fosters colonization by species of beneficial Streptomyces, known to trigger systemic defenses in plants. There were 10 times as many Streptomyces bacteria in soils amended by rapeseed meal, a finding that was later corroborated by field trials.

Indeed, when the researchers directly inoculated the split-root soil with Streptomyces instead of rapeseed meal, they found that Streptomyces encouraged plant defenses much as the seed meal did. “We can’t say for sure how Streptomyces benefit the plant,” says Cohen, “but some evidence indicates it’s related to induction of the jasmonic acid signaling pathway,” a hormonal signaling system that triggers plant defenses. (See “How Plants Feel,” The Scientist, December 2012.)

Unfortunately, seed meal can also nourish pathogenic organisms. In some studies, disease-causing microbes proliferated in soils treated with seed meal. However, combining seed meals from mustard, rapeseed, and other plants can help minimize the growth of pathogenic microbes, says Cohen. This is because seed meals contain glucosinolates—chemicals that release pathogen-killing fumigants as they break down in water. As the chemicals released by rapeseed may be slightly different than those of mustard seed,“seed meals are more promising when used in combination,” says Cohen. “One seed meal might target a pathogen, while another will help build beneficial communities of Streptomyces.”

As gastroenterologists are now reporting the efficacy of transplanting gut bacteria from healthy individuals into human patients suffering from intestinal inflammation and infection, plant researchers may also find that multiple treatments with different concoctions of beneficial microorganisms will have a great impact on soil ecology. Even if one species doesn’t curtail a pathogen, a full remake of the microbial community might help kick the problem. The goal is to gradually build the soil over time to establish a favorable microbial ecosystem. In rich, healthy soil, the microbial community may be more resistant to disease.

Researchers are now turning to field experiments to test the best combinations of species, and treatments like seed meal are already being used on organic farms in Northern California. If greater microbial diversity improves plant health in large-scale field trials, it could eventually help reduce chemical loads on industrial farms. “It might not work exactly the same way in the gut, but the mechanisms in the soil are very similar,” says Jousset. “If we can protect and cultivate the soil microbiome rather than kill important species, we might need fewer chemicals in the field.” 
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