Hyper-proteolytic mutant of Beauveria bassiana, a new biological control agent against the tomato borer

Abstract

The world tomato production is threatened by the invasive tomato borer Tuta absoluta. Difficulties in managing this pest were imposed mainly by the development of resistance in strains treated with conventional chemical insecticides. Resistance problems were even reported to insecticides of natural origin, leading to search for other control alternatives. P2 is a spontaneous mutant of the entomopathogenic fungus Beauveria bassiana. It was previously selected from a local strain (P1) and was characterized as hyper-producer of extracellular proteases. Here, the insecticidal potential of P1 and P2 strains was evaluated against T. absoluta larvae under laboratory conditions. Both strains were effective but P2 showed stronger effect than P1; median lethal concentration of P2 is tenfold lower than that of P1. Enzymatic assay analysis showed that extracellular enzymes are differently expressed by the two strains, especially proteases and chitinases which are known as cuticle degrading enzymes. The major expressed subtilisin-like protease (SBP) was upregulated at the transcriptional level in P2 strain. Proteomic analysis revealed four SBP isoforms which are highly over-expressed in this strain compared to P1. Post-translational regulation, most probably phosphorylation, was further suggested to control the SBP protease expression in B. bassiana P1 and P2 strains. The enzymatic profile in the two strains might explain their different insecticidal potential against the tomato borer. This is the first report showing such efficiency of Beauveria strains against this dangerous pest. Particularly, P2 strain showed high virulence reaching almost total larval mortality within 5 days post-application. It thus should be recommended as a new tool for the biocontrol of T. absoluta.