Biological Control Potential of Sphingomonas sp. against Plant Pathogenic Fungi and Root knot Nematodes
The main problem brought about by modern agricultural production is the abuse of pesticides and fertilizers, which has led to serious consequences such as land degradation, worsening of crop diseases and pests, deterioration of ecological conditions, and decline in the quality of agricultural products. Chemical pesticides inevitably remain in the food consumed by humans, posing a threat to human health. Therefore, the world is continuously reducing the use of chemical pesticides in agricultural products, and the public and scientific community strongly hope to seek safer and eco-friendly alternatives to prevent and control plant diseases in agriculture. In recent years, biological control strategies have received widespread attention in controlling plant diseases. Biological control is an effective alternative method for controlling plant diseases, which not only solves the “3R” problem caused by pesticides, but also meets people’s needs for food safety.

Microbial control is a type of biological control that refers to the use of microorganisms or their metabolites to affect or inhibit harmful organisms. It mainly inhibits the growth of pathogens through antagonism, competition, lysis, parasitism, and other interactions between organisms, thereby preventing and controlling plant diseases. This is considered the safest, most promising, and eco-friendly new method. At present, there are many types of microorganisms that can be used to prevent and control plant diseases, among which Chaetomium spp., Trichoderma spp., Gliocladium spp., Aspergillus spp., Bacillus spp., and Pseudomonas spp. are the main antagonistic microorganisms against plant pathogens. There are over 300 species of hairy shell fungi reported globally, belonging to the strict saprophytic type of fungi. They are recognized as an important family for producing carbohydrate active enzymes and antibiotics. They can effectively degrade cellulose and organic matter, and have antagonistic effects on other microorganisms in the soil. Sphingomonas is one of the earliest studied biocontrol bacteria in the field of mycorrhizal fungi, and its use as a biocontrol agent for plant disease control can be traced back to 1954. At that time, Tveit and Moore found that the seeds of certain crops in Brazil, such as oats and barley, when infected by Fusarium graminearum, would have an antagonistic effect on Helminthosporium victoriae and could protect barley and oat seedlings from infection by Fusarium spp. Subsequently, with the continuous deepening of research, we found that Sphingomonas can effectively improve the stress resistance of plants and have very good control effects on various plant diseases. Its secondary metabolites are diverse and structurally novel, with biological activities such as antifungal, nematode killing, and antiviral. It has considerable prospects for biological control of plant pathogenic fungi and root knot nematodes.

 

Plant diseases caused by plant pathogenic fungi and root knot nematodes seriously endanger crop production. The application of biocontrol bacteria and their secondary metabolites is an important strategy for preventing and controlling plant diseases in agriculture. Sphingomonas is the most studied biocontrol bacterium in the genus Sphingomonas, which can effectively inhibit various common plant pathogenic fungi such as Fusarium, Fusarium, Sclerotinia, Alternaria, and Colletotrichum. Sphingolipids, azones, and flavopin are the main active substances of Sphingolipids against plant pathogenic fungi. Among them, chaetoglobosin A and favipin have strong killing effects on the dominant root knot nematodes in China, southern root knot nematodes and Javanese root knot nematodes. Sphingomonas may control plant diseases through seven mechanisms: producing secondary metabolites with antifungal activity, producing cell wall degrading enzymes, inducing systemic resistance in plants, promoting plant growth and development, heavy parasitism, competition, and synergistic antagonism.

Sphingomonas has great potential for biological control, but there are still some problems in its application for biocontrol. Firstly, Sphingomonas and its metabolites are highly dependent on environmental conditions. Research on Sphingomonas and its metabolites is mostly conducted under controlled laboratory conditions. However, when applied in the field, the antagonistic effect may be greatly reduced due to factors such as variable temperature, humidity, pH, and residual pesticides in the field can also have a negative impact on Sphingomonas. Secondly, there are significant differences in the biocontrol effects of different strains of Sphingomonas. Some strains of Sphingomonas have extremely strong selectivity and often only have good control effects on one or a few pathogenic bacteria. There are multiple plant pathogens in the field, and if the antibacterial spectrum of biocontrol bacteria is too narrow, it is difficult to achieve the desired effect. Finally, the large-scale fermentation and product purification system of Sphingomonas is not yet sound. Therefore, it is necessary to further isolate and screen Sphingomonas, evaluate the actual biocontrol effects of Sphingomonas and its secondary metabolites in the field, and construct a rapid and effective fermentation and product purification system for better and more comprehensive development and utilization of Sphingomonas. I believe that with the deepening of research and the gradual improvement of people’s ecological and environmental awareness, the development of more efficient, safe, and environmentally friendly biopesticides has always been a trend in plant disease control in the future. The prospect of using Sphingomonas as a biocontrol agent is also promising.