Not all microbes are bad. We know this because those yogurt commercials tell us we need some in our gut. In the plant world, there are genes that decide which microbes are allowed to live inside leaves to keep plants healthy – their own version of Jamie Lee Curtis.
Scientists at Michigan State University were the first to show a causal relationship between plant health and the microbial community in the phyllosphere (the above-ground portion of a plant). Microbiome studies of the human gastrointestinal tract have become more mainstream recently, as have large-scale plant microbiome studies. The field is only about a decade old, according to Sheng Yang He, a member of the MSU-DOE Plant Research Laboratory. He and his researchers wanted to know if plants need a properly assembled phyllosphere microbiome.
“In nature, plants are bombarded by zillions of microbes,” said He, of the Plant Biology and Microbiology and Molecular Genetics departments at MSU. “If everything is allowed to grow in the plants, it would probably be a mess. We want to know if the numbers and types of microbes matter, if there is a perfect composition of microbes. If so, do plants have a genetic system to host and nurture the right microbiome?”
Their work shows plants do. One genetic network involves the plant immune system; another controls hydration levels inside leaves. The networks collaborate to select which microbes survive inside of plant leaves. When the researchers removed both networks from a plant, they saw symptoms of damage – “conceptually like those associated with inflammatory bowel disease in humans,” He said. “This is probably because the genes involved are ancient, in evolutionary terms. These genes are found in most plants, while some even have similarities to those involved in animal immunity.”
One of their experiments involved extracting a community of bacteria from sick plants and introducing them to healthy plants, and vice-versa. They found both the microbiome composition and the plant genetic systems are required for plant health. A plant with defective genetics could not take advantage of a microbiome transplanted from a healthy plant; it slowly reverted to the state that caused sickness. A healthy plant exposed to a sick plant’s microbiome suffered too. Although it had the genetic tools to select the right microbes, microbe availability was limited and abnormal.
The sick plants in the study had 100 times more microbes in a leaf than a healthy plant, but the population was less diverse. To understand why, the researchers did thousands of one-on-one bacteria face-offs to figure out which strains were more aggressive.
In sick plants, proteobacteria strains (of which many are harmful to plants) jumped from 67% of a healthy microbiome to 96% in the abnormal population. Fermicutes strains (which may be helpful) went down in numbers. “Perhaps, when the population of a microbiome is abnormally higher in that sick plant, the microbes are physically too close to each other,” He said. “They fight over resources, and the aggressive – in this case harmful – ones unfortunately win. Healthy plants seem to prevent this takeover from happening.”
So, diversity is shown yet again to be important in supporting healthy living systems. Each type of microbe might impart different benefits to plants, such as increased immunity, stress tolerance or nutrient absorption, according to this study.
Now scientists want to be able to manipulate plants’ genetic systems to reconfigure microbiomes. Plants could become more efficient at selecting their microbial partners and experience improved plant health, resilience and productivity.
“Microbiome research tends to focus on human gut bacteria. But many more bacteria live on plant leaves, the lungs of our planet,” He said. “It would be wonderful to understand how microbes impact the health of the phyllosphere in natural ecosystems and crop fields.”