Plants are surrounded by and contain abundant microorganisms. Some of these are beneficial plant growth-promoting bacteria (PGPB), which form a mutualistic symbiotic relationship with plants and promote their growth and development. These microorganisms are mainly divided into rhizosphere growth-promoting bacteria, phyllosphere growth-promoting bacteria, and endophytic growth-promoting bacteria.
The concept of “rhizosphere” was first proposed by German microbiologist Lorenz Hiltner. It refers to the interaction area between plant roots and soil microorganisms, including the inner rhizosphere, root surface, and outer rhizosphere. In the plant rhizosphere, there is rich nutrition and diverse microorganisms. Rhizosphere microorganisms can be classified as symbiotic or free-living based on their relationship with plants.
Some rhizosphere microorganisms not only promote plant growth by utilizing nutrients secreted by plant roots like organic acids, sugars, and amino acids but also enhance plant disease resistance and stress tolerance. These are known as plant growth-promoting rhizosphere bacteria (PGPR). PGPR comes in a wide variety of types and has diverse functions, making it widely used in the research and development of biofertilizers and biopesticides. Well-known PGPRs include bacteria such as Bacillus, Pseudomonas, Azospirillum, Rhizobium, and fungi like Trichoderma and Penicillium.
The application of PGPR shows significant advantages in resisting soil-borne diseases. Soil-borne pests and diseases pose a major threat to crop growth and development. As a biological control agent, PGPR is more environmentally friendly and has good application potential.
PGPR exerts its biocontrol effect by stably colonizing and forming biofilms in the plant rhizosphere. Its mechanisms include producing siderophores, degrading enzymes, antibacterial substances, and detoxification factors, while reducing the survival ability of pathogens through competitive colonization. Additionally, since plants lack an adaptive immune system, the presence of PGPR can be identified as a potential threat, activating the plant’s defense system and increasing its resistance to pests and diseases.
PGPR has obvious advantages in improving the soil environment. Compared to traditional soil remediation methods, it is more environmentally friendly and can preserve the natural properties of the soil. It can improve soil nutrients, remediate pollution, improve microbial communities, and remediate heavy metals.
However, in practical applications, some PGPR strains may be pathogenic, limiting their commercial use. Also, the application effect of PGPR is regionally specific. The same strain may perform differently in different environments and may not show the expected growth-promoting effect. Field trials are needed to collect more data.