I am excited to welcome back Dr. Tuesday Simmons for another "Tuesdays with Tuesday" monthly issue. Dr. Simmons will be sharing her perspective on the importance of the soil microbiome to agriculture and the development of new technologies over the next few month. You can read her full bio at the end of this post.
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Rooted Partnerships: How Plants Cultivate Microbial Allies
The relationship between plants and microbes is so important that many universities group plant biology and microbiology together in the same department, including at my alma mater, the University of California, Berkeley.
Almost every environment hosts a microbiome—the human gut, deep ocean sediments, even the International Space Station—but the partnership between plants and microbes is something truly special. (I will admit, I’m a little biased.)
Last month, I talked about the broader soil microbiome. It’s an incredibly diverse environment, boasting tens of thousands of species in a single teaspoon of soil. As you travel through the soil and get closer to plant roots however, this diversity starts to decrease.
The soil in the few millimeters surrounding plant roots is known as the rhizosphere, and is made up of microbes from the surrounding soil (aka bulk soil) that are “selected” by plants. Moving in even further, the microbial community living inside plant roots (aka root endosphere) is like a private club where the host plant only allows an elite few access. But how do they get invited in?
Silent conversations: how plants “talk” to soil microbes
Human society functions mainly using verbal and visual communication, though we observe other species communicate using other senses. If you took your dog for a walk this morning, they might have left a message for the neighborhood on the corner fire hydrant. If you’re a cat person, perhaps your cat rubbed up against you to communicate affection.
So how do plants and microorganisms communicate? They secrete chemical signals into the soil, which is not too far off from dogs leaving signals that can be smelled by other dogs. When an animal smells something, it’s a chemical signal sent from the nose to the brain.
Plants exude small molecules (collectively known as exudates) into the soil that can be detected by microbes and other plants. These chemicals can share information such as: plant species, age, and how the plant is “feeling” (1).
Plants aren’t just sending messages out into the ether; microbes are also producing chemical signals that are received by plants. For example, legumes form a special relationship with nitrogen-fixing bacteria that benefits both parties. When the right type of bacteria approaches a legume root, it essentially performs a “secret knock” by releasing a specialized chemical signal that the plant recognizes.
What else can microbes do for plants?
In addition to the relationship between legumes and nitrogen-fixers, microbes are able to perform a whole host of tasks helpful for plants.
- Nutrient cycling is top of mind for most folks in agriculture. While many nutrients are added to the soil in the form of fertilizers, microbes continue to play an important role in macronutrient (N, P, & K) and micronutrient availability. In addition to the famous single-celled nitrogen fixers, Arbuscular Mycorrhizal Fungi (AMF) are gaining recognition for their role in phosphorus, water, and mineral uptake (3).
- Plant hormone production: Some microbes known as Plant Growth Promoting Rhizobacteria (PGPR) directly produce plant hormones (like auxin), which could be likened to them screaming “GROW!!”
- Protection against environmental stresses: When plants are stressed out, their root exudates change, which impacts their rhizosphere and root microbiomes. In these cases, they tend to “recruit” microbes that can help them out (4).
- Pathogen protection: Whether it’s an SOS or preemptive planning, plants can get help fighting off pathogens from beneficial microbes.