ANALYSIS OF CROSS-TALK BETWEEN TRICHODERMA ATROVIRIDE AND PSEUDOMONAS FLUORESCENS

Application of mixtures of biocontrol agents has been shown to more effectively control the level of several plant pathogens. However, very little is known about possible interactions between co-inoculated biocontrol strains at the molecular level. In this study, we have analyzed the effects of two biocontrol agents, Trichoderma atroviride and Pseudomonas fluorescens, in the control of take-all, a disease caused by Gaeumannomyces graminis. The results showed that T. atroviride combined with one of several strains of P. fluorescens isolated from wheat fields in north-west Iran [including stains Z7, B119 (DAPG-producing), P4 (Phenazine-producing), P6, and P21] significantly reduced the severity of take-all in wheat, albeit with different efficiencies. While combination of T. atroviride with bacterial isolates Z7 or B119 improved take-all suppression in comparison to treatment with the individual biocontrol agents, combination of the P4, the P6 or the P21 isolate with T. atroviride had a negative effect on biocontrol activity compared to the individual agents. Since chitinases and DAPG are known to be important biocontrol substances produced by T. atroviride and P. fluorescens, respectively, we have studied changes in their levels of expression under co-inoculation conditions. The results showed that combined inoculation of T. atroviride and P. fluorescens represses expression levels of the Ech42 endochitinase gene both at the transcriptional and translational levels. In contrast, in the presence of the bacterial strains, expression of Nag1, another chitinase gene of T. atroviride, was either slightly enhanced or remained unchanged. Interestingly, when co-inoculated with T. atroviride, PhlD-positive strains of P. fluorescens produced significantly higher levels of DAPG antibiotic. Our results provide further evidence to support the hypothesis that mixing antagonists can have either positive or negative effects on the expression of their key biocontrol genes.