Cell-free filtrates from plant pathogens: Potential new sources of bioactive molecules to improve plant health

Abstract

Lack of alternatives to reduce the use of and risk by agrochemicals makes necessary to search for environmentally friendly and health-safe options to increase crop production. The use of beneficial microorganisms in agriculture offers a sustainable alternative to the use of chemicals. However, only a few microbe-based commercial products are available on the market due to limitations associated with the microbial growth in artificial media, survival, and performance in different environmental conditions. Use of microbial cell-free broth cultures (known as cell-free filtrates: CFFs) from plant pathogens offers several advantages over, and reduces the limitations of, traditional microbe-based products. A large diversity of secondary metabolites and bioactive molecules are secreted by plant pathogens and such metabolites represent a large reservoir of compounds with potential for use in crop growth and crop protection. The objective of this review is to provide an updated compilation and discussion of the published literature on CFFs from phytopathogenic microorganisms. Different growth conditions of microorganisms and ways of applying their CFFs in the studies are shown, since the accumulation of bioactive compounds in CFFs depends on factors such as the composition of the culture medium or the culture temperature. Mechanisms and molecules related to CFFs bioactivity are discussed, evidencing the complexity of the filtrate-plant interaction network. This review underlines the potential of CFFs as an alternative source to plant health in the sustainable crop production system of the future, and it opens the door for their application in other unexplored fields.