Pseudomonas and other Microbes in Disease-Suppressive Soils

Abstract

Soil-borne phytopathogens cause extensive damage to cultivated plants worldwide, resulting in yield loss worth billions of Euros each year. Soil fumigation is the most effective chemical treatment but is too expensive for many crops, and fumigants like methyl bromide are being phased out for environmental reasons. In this context, much is to be learned from disease-suppressive soils, where susceptible plants are protected from soil-borne pathogens by the indigenous microbiota, because these microbial interactions may be exploited to design sustainable crop protection strategies for ordinary farm soils. However, our knowledge of plant-protecting microorganisms and biocontrol mechanisms involved in soil suppressiveness remain very fragmented, as most knowledge on disease suppressive soils comes from studies restricted to individual plant-protecting microbial populations, mostly fluorescent Pseudomonas species. The phenomenon of disease suppressiveness remains therefore poorly understood, even in the most studied cases such as suppressiveness to wheat take-all. We reviewed the respective biocontrol contributions of fluorescent pseudomonads and other plant-protecting microorganisms in disease-suppressive soils. The ability to inhibit soil-borne pathogens and to protect plants occurs both in Pseudomonas and non-Pseudomonas microorganisms, including diverse bacteria and fungi, and both play important roles in soil suppressiveness. In Pseudomonas, antibiosis and competition were shown to be important mechanisms of pathogen suppression, though direct effects on plant, e.g. induced systemic resistance, phytohormone interference and plant-growth promotion, were also reported. These types of mechanisms occur also in non-Pseudomonas biocontrol microbes, some of them also displaying hyperparasitism in certain types of suppressive soils. This review shows that in suppressive soils where Pseudomonas play an important role, the roles of non-Pseudomonas microorganisms were often neglected, and vice versa. Yet, Pseudomonas and other microorganisms may interact with each other in the rhizosphere and with the plant, and some recent studies indicate that disease suppressiveness is an emerging soil property that can typically result from these multiple interactions. In conclusion, we propose that a parallel assessment of Pseudomonas and non-Pseudomonas microorganisms in suppressive soils, e.g. using microarrays or metagenomics, may bring a better understanding of disease suppressiveness.