Designing a synthetic microbial community devoted to biological control: The case study of Fusarium wilt of banana

Fusarium oxysporum f. sp. cubense (Foc) tropical race 4 (TR4) isthreatening banana production because of its increasing spread. Biologicalcontrol approaches have been widely studied and constitute interestingcomplementary measures to integrated disease management strategies. Theyhave been based mainly on the use of single biological control agents(BCAs). In this study, we moved a step forward by designing a syntheticmicrobial community (SynCom) for the control of Fusarium wilt of banana(FWB). Ninety-six isolates of Pseudomonas spp., Bacillus spp., Streptomycesspp., and Trichoderma spp. were obtained from the banana rhizosphere andselected in vitro for the antagonism against Foc TR4. In pot experiments,a large community such as SynCom 1.0 (44 isolates with moderate tohigh antagonistic activity) or a small one such as SynCom 1.1 (seven highlyeffective isolates) provided similar disease control (35% symptom severityreduction). An in vitro study of the interactions among SynCom 1.1 isolatesand between them and Foc revealed that beneficial microorganisms notonly antagonized the pathogen but also some of the SynCom constituents.Furthermore, Foc defended itself by antagonizing the beneficial microbes.We also demonstrated that fusaric acid, known as one of the secondarymetabolites of Fusarium species, might be involved in such an interaction.With this knowledge, SynCom 1.2 was then designed with three isolates:Pseudomonas chlororaphis subsp. piscium PS5, Bacillus velezensis BN8.2,and Trichoderma virens T2C1.4. A non-simultaneous soil application of theseisolates (to diminish cross-inhibition) delayed FWB progress over time, withsignificant reductions in incidence and severity. SynCom 1.2 also performedbetter than two commercial BCAs, BioPakRand T-Gro. Eventually, SynCom1.2 isolates were characterized for several biocontrol traits and their genomewas sequenced. Our data showed that assembling a SynCom for biocontrolis not an easy task. The mere mixtures of antagonists (e.g., SynCom 1.0and 1.1) might provide effective biocontrol, but an accurate investigationof the interactions among beneficial microorganisms is needed to improvethe results (e.g., SynCom 1.2). SynCom 1.2 is a valuable tool to be furtherdeveloped for the biological control of FWB.