Underground Heroes: Plants and Microbes Partner to Shape Our World

Did you know that microbes and plants can help each other survive? Microbes—like bacteria and fungi, for example—can help plants find food and water and can even make them healthier during stressful times. In return, plants give microbes food and a place to live. The world as we know it would not exist without plants, microbes, and their partnerships. Unfortunately, changes to climate will also change our environments. Therefore, studying how plants and microbes partner will help us predict environmental changes to our planet and its inhabitants. In this article, we discuss how microbes and plants partner to support life on Earth.

. What are these microbes doing, exactly? Are they helping or hurting plants? Most microbes that we know of help their plant hosts by increasing the amounts of nutrients, like sugars, proteins, and water, that are available to plants. Microbes can also strengthen a plant's ability to fight o infections. Without a microbiome, plant health su ers.

Figure Figure
The below-ground compartment around the roots is called the rhizosphere. Research has shown that the rhizosphere facilitates close interactions between and among microbes and mycorrhizal fungi, allowing them to exchange nutrients. Although fungi can often be seen by the human eye, powerful microscopes are needed to see the billions of bacteria and other microbes in the rhizosphere.
Although research on plants and microbes has come a long way since its start in the late s, we still have plenty to learn. Researchers are currently working on ways to use microbes to produce larger crops, more food for our growing population, and healthier forests for future generations [ ]. Learning more about how microbes interact with each other and with plants brings us one step closer to accomplishing these goals.
Where exactly do plants and microbes interact? One main area of research focuses on the underground space around the plant root, called the rhizosphere. The rhizosphere is home for many types of RHIZOSPHERE The soil within mm of plant roots. microbes, and it is a hotspot for plant-microbe interactions. We still do not completely understand how microbes communicate with each other or share food, so these are important questions to answer. If we understand the rules that plant microbiomes play by, then we will be better at predicting the ways that climate change may a ect plants and their microbes.
Research has already helped us engineer plants that can withstand drought and increased temperatures, and that can grow larger and faster. But we are still unsure how microbes will interact with plants as Earth's climate changes. Will some microbes lose their ability to help plants grow? Will plants be overtaken by pathogens, becoming sick and stunted? Will we be unable to supply food to Earth's growing human population?

BACTERIA AND FUNGI: KEY PLAYERS IN PLANT MICROBIOMES
Bacteria and fungi make up more than % of a plant's microbiome. As a result, much research focuses on how these microbes interact with plants.
How can bacteria help plants? Bacteria have been living with plants for hundreds of millions of years, and for centuries bacteria have been known to help plants grow. One of the most well-studied groups of bacteria is rhizobia, which convert nitrogen gas in the atmosphere into ammonia. Plants can use ammonia to make amino acids, the building blocks of proteins ( Figure A). We call this conversion nitrogen fixation. Nitrogen-fixing bacteria help plants receive the resources

NITROGEN FIXATION
The conversion of atmospheric nitrogen to ammonia. they need to remain healthy. However, nitrogen fixation is not the only way that bacteria help their plant hosts. Bacteria also balance plant  (myco = fungus and rhiza = root), that partner with more than % of all plant species on Earth. When mycorrhizal fungi colonize plant roots, they exchange resources each needs for their growth like carbohydrates, amino acids, and water. Research has shown that mycorrhizal fungi take up organic matter from the soil and give plants

ORGANIC MATTER
Carbon molecules that are produced from the decomposition of living matter such as leaf litter or fallen tree branches. nutrients like phosphorus, nitrogen, and water ( Figure B). When and how mycorrhizal fungi identify their plant host are important questions that still need complete answers.
Bacteria and fungi work together to help plants. Bacteria called mycorrhizal helper bacteria have been shown to support the

MYCORRHIZAL HELPER BACTERIA
Bacteria that increase mycorrhizal colonization, growth, and abundance. relationships between plants and mycorrhizal fungi [ ]. The more nutrients mycorrhizal fungi can provide to their plant host, the higher the chances that the plant will survive and reproduce. It is still unclear how bacteria and mycorrhizal fungi interact, but evidence suggests that mycorrhizal fungi provide nutrients and shelter to some bacteria.

MICROBES SUPPORT THE HEALTH OF FORESTS AND FARMLANDS
Earth has roughly three trillion trees and more than million farms. This means that the microscopic interactions between plants, fungi, and bacteria impact the fate of our forests and food. Research has shown that microbes can be sprayed on plant seeds or soils to boost crop yields [ ] and to remove toxic compounds from forest soils [ ]. We still do not know how much these microbes will help plants during stressful times, like heatwaves and droughts. Therefore, much research focuses on how climate change will impact plants and their microbiome.
What about the microbes that may not help plants, like pathogens? Researchers predict that global warming will increase the number of plant pathogens [ ] and decrease the number of mycorrhizal fungi. We know that the climate controls much of how plants and microbes come together [ ]. Changes to Earth's climate could cause some trees and crops to become less abundant or even extinct. Manufactured goods such as wood and paper, and foods such as rice and vegetables, would be more di cult to produce. It is possible that changes to food production will prevent us from feeding the Earth's growing population in the next years. Sustainable solutions must be created! kids.frontiersin.org February | Volume | Article | Although these predictions may seem frightening, research continues to focus on ways to preserve our forests and farms by using our knowledge of microbes. How, exactly? Researchers around the world are using DNA sequencing and satellite imaging to find out which microbes pair with which plant types in forests and farmlands (Figure ). With this information, we can identify the proper microbes and spray them on plant seeds or mix them into the soil, to help plants during heatwaves, droughts, and floods.  How can you help? Read, explore, question, and repeat. From articles to stories, pictures, and videos, there are many resources related to plant-microbe interactions online and at local libraries.
You will quickly find that there is a ton of free information available (for example, the soil biodiversity atlas). The more you learn about your topic of interest, the better you will be at asking interesting questions and developing interesting experiments to answer those questions.
If you liked this article and want to learn more about plants and microbes, you could also contact a researcher who could teach you more about plant-microbe interactions. You could start by writing an email with your science teacher, parent, guardian, and/or friends to the authors of this article or to other researchers who study plant-microbe interactions. Researchers are often excited to share their findings and to help young people who are interested in science-so do not hesitate to ask questions!

ACKNOWLEDGMENTS
We thank Kyle Alston for his assistance with graphic design. This work was funded, in part, by a National Science Foundation PRFB Grant No. awarded to LB and NSF CAREER Award DEB to KP. The funders did not contribute to the design of the experiments, data collection, analyses, decision to publish, or the preparation of the manuscript.

CONFLICT OF INTEREST:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
COPYRIGHT © Berrios, Van Nuland, Alvarez Manjarrez, Yeam, Saarunya Clarke, Clarke and Peay. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.  AARON CLARKE I am a doctoral candidate of microbiology at the University of California Santa Cruz. I study how microbes survive in the mammalian stomach. My topics of interest include how bacteria tolerate antibiotics, how bacteria navigate and colonize the gastrointestinal tract, and how non-antibiotic medications influence the growth and distribution of bacteria in the stomach. I have researched many topics related to bacteriology, ovarian cancer, and physiology.

YOUNG REVIEWER
KABIR G. PEAY I am an associate professor in the Department of Biology at Stanford University. I have an extreme fondness for fungi and mushroom hunting. Research in my lab is focused on understanding how plants and fungi come together to form mycorrhizal symbiosis. By studying these interactions in roots and across biomes, I am hoping to understand how this symbiosis has shaped our planet and how it might be used to create practices that sustain a healthy planet.