Just as humans naturally have beneficial microbes on and in our body, so do plants. While hundreds of species can exist on the outside of root hairs, others exist in the crevices between plant cells, and still others live WITHIN the plant cells. Some microbial species happily coexist with the plant, munching on the sugars exuded by plant roots. Others form more complex partnerships where they help fertilize the plant or protect it from disease. The root environment can contain hundreds of different species of microbes.
Azospirillum species thrive on the outside of roots. These short rod shaped bacterial cells anchor themselves to the roots of certain plants. There they harmlessly feed on the small amount of sugar made by the plant that seeps into the nearby soil. Roots are covered in a mucus of plant exudate, or anything a plant exudes, like sap, nectar, gum, latex, resin, which is a wonderful surface for microbes like Azospirillum. These bacterial cells are shaped like short rods and can promote plant growth by producing hormones--molecules that stimulate the plant’s growth. They are commonly used as a commercial bio-fertilizer because it has been shown to increase water and nutrient uptake by the roots as well. Happy roots, happy plants!
Rhizobium species are endophytic bacteria, or the bacteria that live inside the plant cells. These are specifically associated with legume plants like soybeans and clover. The bacteria are “y” and club shaped when they are inside the plant. They force the plant to make large, pink nodules, which then house the bacteria on the inside of the plant. These root nodules can be seen with the naked eye. In return for this home, these bacteria take unusable nitrogen from the air and change it into a form that can be used to fertilize the plants. The next time you have a bean product -- from edamame, to soy milk -- you can thank these root microbes for helping fertilize them.
Glomeromycota species represent a large group of fungal species that are mostly dependent on plants for their carbon (sugars) and energy. The fungal cells form long networks that puncture plant cells and form networks within them. These networks help the fungus pass sugars and nutrients back and forth with the plant. The tiny filaments of Glomeromycota cells grow throughout the soil and act as an extended root network for the plant, pulling in nutrients and water. This partnership is so successful that most plant species have these cryptic associates cloaked beneath the soil.