Rhizobacteria are one of the most important key constituents of the soil food web. The different types of rhizobacteria and the numerous roles they carry out are so vast and varied that only a few are known to us even after decades of intense research. Some rhizobacteria fix nitrogen, some help with nitrogen assimilation, and others specialize in denitrification, amongst many other things. Their roles are so diverse that they can range from making nutrients more available to plants (giving them the title they are well-known by, PGPR or Plant Growth Promoting Bacteria), to competing out or outright killing pathogenic bacteria and fungi. They are also known to be involved in regulating plant hormones, but the mechanism through which this is accomplished in not yet clearly understood.
In other words, rhizobacteria are exceptionally interesting microbes that have multiple personalities. A single rhizobacteria strain can be used to boost plant growth, or to protect plants from pathogenic bacteria or fungi, or to help regulate plant metabolism under duress due to various abiotic stressors.
That having been established, it is important to note that a microbial product’s functions heavily depend on the diverse traits of the microbes included in the product. For example, bacillus subtilis, a type of rhizobacteria, are renowned to be excellent plant growth promoters while also being antagonistic towards pathogenic bacteria and fungi. As a result, when you have a plant fertilizing supplement that contains bacillus subtilis as the active ingredient, you can expect to see prevention of some diseases and mitigation of abiotic threats to plants and soils as extra benefits on top of plant growth.
To further examine the effects and effectiveness of rhizobacteria, we conducted an experiment with a commercial product marketed as a plant growth promoting supplement that contains bacillus subtilis and bacillus amyloliquefaciens. Our question for this specific experiment was if bacillus subtilis and bacillus amyloliquefaciens, even when narrowly marketed as a plant growth promoting supplement, would also act as an antagonistic agent against pathogenic fungi that cause root rots. The results were surprisingly positive:
1. Clearing zone around bacillus subtilis / bacillus amyloliquefaciens inhibiting Pythium Aphanidermatum
2. Clearing zone around bacillus subtilis / bacillus amyloliquefaciens inhibiting Fusarium Oxysporum
3. Clearing zone around bacillus subtilis / bacillus amyloliquefaciens inhibiting Phytophthora Capsici
As evidenced by this experiment, rhizobacteria marketed as plant growth promoting supplements also produce antagonistic effects against the three most common pathogenic fungi that generate root rots.
Our investigation illustrates that rhizobacteria can promote plant growth through not a single venue, but through a holistic approach- by suppressing pathogens, mitigating abiotic stressors, etc. on top of the basic functions we are already aware of, such as increasing nutrient availability and boosting plant growth.
Rhizobacteria is a microbe that is full of mystery, yet with so many benefits to soils and plants. This is a microbe we should all be paying more attention to!