We are all familiar with human partnerships of many kinds, both mundane and fictional. In the mundane category is your typical carpool. In the fictional category are superheroes working together to save the world (in comic books and movies, of course!)
There are even examples of humans working with other species. We use cats for pest control on farms. Many breeds of dogs are used for herding sheep and other farm animals.
Plants can work with other species, too. There are beautiful and intricate patterns of biology behind these partnerships. They hold promise to continue to improve the economic and environmental sustainability of farming. Let’s look at some clear examples of how understanding bacterial and fungal partnerships have improved food security and sustainability.
Rhizobia nodules: The bright red color of the opened nodule is an indication of healthy rhizobia (bacteria) inside. Credit: Jennifer Dean via Creative Commons
The most important microbial partnership known in agriculture is found in legumes. This includes forage crops like alfalfa and clover, vegetables like peas and beans, and agronomic crops like soybeans. Agronomic practices of planting a legume crop with others dates back before the Native Americans planted the “three sisters.”
Legume crops partner up with types of bacteria (Rhizobium, Bradyrhizobium etc.) to form special root structures called nodules. The bacteria live in the nodules provided by the legumes – a safe home. In return, Rhizobia are able to convert nitrogen gas from the atmosphere into ammonium. This process is referred to as nitrogen fixation, and it provides a nutrient the plant needs. The bacteria also receive food from the plant, in the form of sugars. This is a prime example of a cross-species cooperation – both the plant and bacteria benefit from this relationship.
If you want to plant legumes, but your soil has not been grown in them before, the soil may not contain these nitrogen-fixing microbes. The legumes might not thrive. You can buy “seed inoculants” to add at planting to provide these partner microbes. They are available at your garden stores. Current and developing agricultural practice includes seed treatments with bacterial strains tailored for crop productivity.
Researchers discovered an indigenous type of corn in Mexico that makes its own nitrogen in aerial roots via a plant partnership of the corn with microbes. Credit: Jean-Michel Ane’
The importance of nitrogen fixation in ecology and agriculture is reflected in the ongoing search for new forms of microbial partnerships. A surprising and beautiful new example was recently discovered.1 Corn grown in Mexico was found to secrete a sugary mix through their aerial roots. This mixture then encouraged growth of bacteria – which then provided nitrogen back to the corn. Of course, it’s a bit more complex than this, but it’s another example of a plant-bacterial cooperation – outside of the legume family. Corn is in the cereal-grain family.
When we look beyond nitrogen-fixing plant-microbe partnerships, there are other important partnerships. Plant-fungal partnerships are among the most notable. The classic examples of these partnerships are found in plant root systems. Fungal filaments acquire soil nutrients and transport them to the roots. The technical term for these type of fungi is mycorrhiza.
Plants vary in their dependence upon their fungal partners for nutrients. Orchids (like vanilla plants) present an extreme case. The tiny seedlings are totally dependent upon mycorrhizal fungi for the resources need for their initial stages of growth. The most common type of association, VA mycorrhizas, found in most crops, increases capture of phosphorus. This is important because phosphorus binds to many soil particles, and is hard for plants to access. The VA mycorrhizas are “lending a hand” to help plants acquire phosphorus!
Plant-microbial partnerships can suppress diseases that can destroy a farmer’s crop. “Take-all” disease of wheat and other cereal crops is caused by a fungus that infects and blackens roots. It blocks access to water and causes yellow, unproductive plants. Plants partner with and enrich for select strains of Pseudomonas bacteria that fight and kill the take-all fungus. Over years, soils naturally become suppressive to “take-all” disease. We learn from these partnerships to better protect crop plants from disease and pests.
Development of seed inoculants based on existing plant-microbe super-hero teamups holds great promise for agriculture. Enhancing plant-microbial partnerships is key to improving plant nutrient use efficiency and health, sustainably.
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