The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health

The Nuclear Protein Sge1 of Fusarium oxysporum Is Required for Parasitic Growth

Dimorphism or morphogenic conversion is exploited by several pathogenic fungi and is required for tissue invasion and/or survival in the host. We have identified a homolog of a master regulator of this morphological switch in the plant pathogenic fungus Fusarium oxysporum f. sp. lycopersici. This non-dimorphic fungus causes vascular wilt disease in tomato by penetrating the plant roots and colonizing the vascular tissue. Gene knock-out and complementation studies established that the gene for this putative regulator, SGE1 (SIX Gene Expression 1), is essential for pathogenicity. In addition, microscopic analysis using fluorescent proteins revealed that Sge1 is localized in the nucleus, is not required for root colonization and penetration, but is required for parasitic growth. Furthermore, Sge1 is required for expression of genes encoding effectors that are secreted during infection. We propose that Sge1 is required in F. oxysporum and other non-dimorphic (plant) pathogenic fungi for parasitic growth

Effect of Biocontrol Agent Pseudomonas fluorescens 2P24 on Soil Fungal Community in Cucumber Rhizosphere Using T-RFLP and DGGE

Fungi and fungal community play important roles in the soil ecosystem, and the diversity of fungal community could act as natural antagonists of various plant pathogens. Biological control is a promising method to protect plants as chemical pesticides may cause environment pollution. Pseudomonas fluorescens 2P24 had strong inhibitory on Rastonia solanacearum, Fusarium oxysporum and Rhizoctonia solani, etc., and was isolated from the wheat rhizosphere take-all decline soils in Shandong province, China. However, its potential effect on soil fungal community was still unknown. In this study, the gfp-labeled P. fluorescens 2P24 was inoculated into cucumber rhizosphere, and the survival of 2P24 was monitored weekly. The amount decreased from 108 to 105 CFU/g dry soils. The effect of 2P24 on soil fungal community in cucumber rhizosphere was investigated using T-RFLP and DGGE. In T-RFLP analysis, principle component analysis showed that the soil fungal community was greatly influenced at first, digested with restriction enzyme Hinf I and Taq I. However, there was little difference as digested by different enzymes. DGGE results demonstrated that the soil fungal community was greatly shocked at the beginning, but it recovered slowly with the decline of P. fluorescens 2P24. Four weeks later, there was little difference between the treatment and control. Generally speaking, the effect of P. fluorescens 2P24 on soil fungal community in cucumber rhizosphere was just transient

Management of Diseases Induced by Soil- Borne Pathogens, Solarization and Biological Control

Diseases induced by soil-borne plant pathogens are among the most difficult to control. Prophylactic methods aiming at preventing the introduction of pathogens in healthy soils have to be respected, because it is almost impossible to eradicate pathogens from an infested field soil. Even the drastic disinfestation techniques based on the application of biocide molecules such as methylbromide failed to eliminate the pathogens, but are harmful to man and the environment. Growers should prefer new, environment friendly techniques such as solarization and biological disinfestation of soils. These methods induce changes in the microbial balance, reducing the population density of the pathogens and stimulating the activity of some beneficial microorganisms. The study of soils that naturally suppress diseases induced by soil-borne pathogens has led to a better understanding of the interactions between pathogenic and antagonistic microorganisms and has resulted in the selection of bio control agents

Management of Diseases Induced by Soil- Borne Pathogens, Solarization and Biological Control

Diseases induced by soil-borne plant pathogens are among the most difficult to control. Prophylactic methods aiming at preventing the introduction of pathogens in healthy soils have to be respected, because it is almost impossible to eradicate pathogens from an infested field soil. Even the drastic disinfestation techniques based on the application of biocide molecules such as methylbromide failed to eliminate the pathogens, but are harmful to man and the environment. Growers should prefer new, environment friendly techniques such as solarization and biological disinfestation of soils. These methods induce changes in the microbial balance, reducing the population density of the pathogens and stimulating the activity of some beneficial microorganisms. The study of soils that naturally suppress diseases induced by soil-borne pathogens has led to a better understanding of the interactions between pathogenic and antagonistic microorganisms and has resulted in the selection of bio control agents