Role of rhizobacterial secondary metabolites in crop protection against agricultural pests and diseases

Bacteria colonizing the rhizosphere of plants (=rhizobacteria) play different ecological roles in their natural environment. Some are deleterious to plant growth and health, while many others are beneficial and act as both promoters of plant growth (biofertilizers and phytostimulators) and/or controllers of plant pathogens (biocontrollers). The main mechanisms beneath the biocontrol abilities of the latter depend on the production and excretion of secondary metabolites. These metabolites are numerous and diverse and include, among others, antibiotics, siderophores, semiochemical quenchers, and/or elicitors of a plant’s induced systemic resistance (ISR). Owing to their production, other inhabitants of the rhizosphere are killed, inhibited, diminished, or deceived. Since this antagonistic activity affects crop pests and pathogens, plant growth is promoted in an indirect way. Therefore this natural ability of many rhizobacteria has been exploited successfully in the agricultural field to develop efficient biopesticides. These bioproducts, already marketed worldwide, often contain living microorganisms; however, the possibility of making profit from the microbial pure metabolites by developing such products is still in its infancy. Here we review some of the recent advances concerning the mode of action of some microbial secondary metabolites produced by rhizospheric bacteria, emphasizing their potential to develop ecofriendly biopesticides.