By Ada Hagan
George Washington Carver, probably without realizing it, was one of the first proponents of plant probiotics. Carver was a faculty member at the Tuskegee Institute in the early 1900’s and re-introduced the concept of crop rotation with peanuts, soy, and other legumes to U.S. agriculture. By alternating corn and cotton crops with peanuts, farmers could replenish the nutrients in the soil but continue harvesting a cash crop. Legumes are an intriguing type of plant since they rely on bacteria, such as Rhizobia, that grow in specialized nodules on their roots to provide them with nutrients, like nitrogen. In return, the plants supply the bacteria with sugars and oxygen for growth, a symbiotic exchange for nutrients the legumes cannot produce themselves.
Supplementing legumes, especially soybeans, with additional bacteria every planting season is an example of biofertilizer use. Biofertilizers contain a mixture of bacteria helpful for plant growth, sometimes called plant probiotics. Biofertilizer use is increasingly popular in agriculture, particularly organic farming, as they are less expensive, cause less pollution than industrial fertilizers and can make industrial fertilizers more effective. But the symbiosis between legumes and bacteria is only the tip of the proverbial iceberg. Researchers have learned that supplementing a number of crops, including rice, coffee, rubber, and coconuts, with different plant probiotics benefit plant growth and increase crop yield.
The rhizosphere, the soil around a plant’s roots, is packed with microbes like bacteria and fungi. In fact, there can be 1011 (one hundred billion) microbial cells, the number of stars in the Milky Way galaxy, all packed into a teaspoon of rhizosphere dirt. And just like there are healthy microbes in your gut, there are certain kinds of bacteria (and fungi) beneficial for plants in the rhizosphere. Biofertilizers apply some of these microbes to crops, helping plants access important nutrients, encouraging root growth, and protecting them from plant pathogens.
Helping plant nutrition
Like legumes, most plants have trouble acquiring certain nutrients from the soil, even when directly applied in fertilizer. Plants can only take up 10-40% of the minerals and nutrients in fertilizer on their own, so rainwater frequently washes away the remaining fertilizer. If a mixture of fertilizer and plant probiotics is given to plants, however, it has the same effect on plant health as fertilizer alone, but requires much less.
This is because the microbes in biofertilizers have a number of nutrient-gathering tricks up their sleeves. Plants need to absorb nitrite and nitrate from the soil to help build proteins and replicate DNA. Some bacteria convert, or fix, atmospheric nitrogen to forms that a plant can use. By adding nitrogen-fixing bacteria in a biofertilizer, farmers waste less fertilizer and populate the soil with probiotics for future crops. Like nitrogen, phosphorous is difficult for plants to access. Required for DNA, phosphorus cannot be accessed by plants without a slightly acidic soil pH. Some inhabitants of the rhizosphere (like the fungi mycorrhiza) can help lower the pH, thus solubilizing phosphorus and making it easier for the plant roots to absorb. These are only two examples of many; some rhizosphere bacteria even help plants access more complex compounds like vitamins.
Enhancing plant root growth
In order for a plant to gather nutrients like nitrogen, phosphorous, and even water, it needs extensive, healthy roots. This is where plant growth promoting rhizobacteria (PGPR) come in handy by encouraging plants to invest in strong root systems. PGPRs secrete chemicals that mimic plant hormones or stimulate seeds to germinate and grow. In both cases, plants respond by making more roots or increasing the number of root hairs on each root. This increases the surface area plants have to access water, nutrients, and (most importantly to the microbe) provide more sugar for the helpful microbes. Consider two radishes one “a” is grown with the PGPR Pseudomonas corrugate, but the second radish, “b” is not. After 17 days, the PGPR treatment enabled radish “a” to take up more nutrients and will look healthier and more developed than radish “b”, with a budding radish and a stronger root system.
Protection from plant pathogens
Another way plant probiotics help plants is by stopping phytopathogens – microbes that attack and damage plants like the fungi that cause tomato blights. In addition to sugars, plants secrete chemicals like amino acids, fatty acids, and vitamins that promote bacterial growth, probiotics and pathogens alike. Probiotics prevent phytopathogen infections by staking claim to the resources provided by the plant and secreting chemicals to inhibit or kill competing microbes. Some species of Bacillus (a genus of bacteria often found in the rhizosphere) can produce antibiotics to help control phytopathogens. B. thuringiensis prevents the phytopathogen Erwinia carotovora from degrading plant cell walls by producing an enzyme that interrupts E. carotovora’s chemical communication. Plant probiotics can also activate the plant’s immune response to make the plant better able to fight off invaders. In one example, when a carnation has a particular strain of Pseudomonas in its rhizosphere, it is no longer susceptible to wilt caused by Fusarium.
Improving biofertilizers
While research and experience have taught agriculture much about plant probiotics, a lot of progress is still left to be made. For instance, many of the biofertilizers currently available don’t work well in mountainous regions, partially because the probiotic bacteria they contain don’t grow well at colder temperatures. To help address this need, researchers isolated bacteria from glaciers(!), looking for new PGRPs that could stand the cold. The authors found a number of bacteria that could grow in a range of low temperatures from 4°C (in a refrigerator) to 30°C (temperature of a comfortable room) plus perform important probiotic activities like solubilize phosphorous, enhance root growth and block phytopathogens. They hope these bacteria can be used to generate “cold-active biofertilizers” as an environmentally friendly option for agriculture.
Using biofertilizers to improve crops’ health and yield is an exciting idea. But there are a number of unanswered questions before biofertlizers reach their full potential, particularly regarding the factors involved in plant/biofertilizer interactions. What types of soil and environments are optimal for a given probiotic? Which crops will they benefit the most? Are there certain combinations of probiotics that yield more, or less, effective biofertilizers? Perhaps rice and wheat have different probiotic needs. But only more research, by scientists and farmers together, will reveal the answers.
George Washington Carver’s lesson about legumes was a catalyst for the development of agriculture technologies harnessing the power of microbes. But it was only the beginning, and there’s much more to learn in the design of effective biofertilizers. While it is as complex a process as the number of bacteria in the rhizosphere is large, the promise of biofertilizers for increasing crop yield and decreasing fertilizer pollution is one worth striving for.
About the author
Ada Hagan is a doctoral student here at the University of Michigan in the department of Microbiology and Immunology. She does recon on the sneaky ways bacteria find nutrients (like iron!) when they are invading our bodies. Originally hailing from the mountains of East Tennessee, Ada earned both her B.S. and M.S. in Microbiology from East Tennessee State University. In her spare time, Ada spends time with her pets and husband, cooking, fishing & the occasional Netflix binge. Follow her on Twitter and see other posts on LinkedIn.
Read more posts by Ada here.
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