Abstract
Verticillium dahliae, a vascular-colonizing fungus, causes economically important wilt diseases in many crops, including olive trees. Trichoderma spp. have demonstrated an effective contribution as biocontrol agents against this pathogen through a variety of mechanisms that may involve direct mycoparasitism and antibiosis. However, molecular aspects underlaying Trichoderma–V. dahliae interactions are not well known yet due to the few studies in which this pathogen has been used as a target for Trichoderma. In the present study, Trichoderma atroviride T11 overgrew colonies of V. dahliae on agar plates and inhibited growth of highly virulent defoliating (D) V. dahliae V-138I through diffusible molecules and volatile organic compounds produced before contact. A Trichoderma microarray approach of T11 growing alone (CON), and before contact (NV) or overgrowing (OV) colonies of V-138I, helped to identify 143 genes that differed significantly in their expression level by more than twofold between OV and CON or NV. Functional annotation of these genes indicated a marked up-regulation of hydrolytic, catalytic and transporter activities, and secondary metabolic processes when T11 overgrew V-138I. This transcriptomic analysis identified peptidases as enzymatic activity overrepresented in the OV condition, and the cpa1 gene encoding a putative carboxypeptidase (ID number 301733) was selected to validate this study. The role of cpa1 in strain T11 on antagonism of V-138I was analyzed by a cpa1-overexpression approach. The increased levels of cpa1 expression and protease activity in the cpa1-overexpressed transformants compared to those in wild-type or transformation control strains were followed by significantly higher antifungal activity against V-138I in in vitro assays. The use of Trichoderma spp. for the integrated management of plant diseases caused by V. dahliae requires a better understanding of the molecular mechanisms underlying this interaction that might provide an increase on its efficiency.
Highlights
The use of Trichoderma species for the biocontrol of plant diseases has been related mainly to their antagonistic abilities against phytopathogenic fungi and oomycetes (Howell, 2003)
The ability of T11 as an antagonist of V. dahliae strains differing in lineage, race and pathotype was tested under three experimental conditions: (i) direct confrontation in dual cultures, (ii) antibiosis triggered by non-volatile compounds such as hydrolytic enzymes or antibiotics, and (iii) antibiosis triggered by volatile organic compounds (VOCs)
T11 appears to bear higher antagonistic potential against V. dahliae compared with that reported for Trichoderma asperellum strains assayed in vitro under conditions identical to those used in this present work (Carrero-Carrón et al, 2016)
Summary
The use of Trichoderma species for the biocontrol of plant diseases has been related mainly to their antagonistic abilities against phytopathogenic fungi and oomycetes (Howell, 2003). The mechanisms of Trichoderma-based plant disease biocontrol rely mainly on the production of antibiotics and/or hydrolytic enzymes as well as competition for nutrients (Lorito et al, 2010). Functional characterization of individual Trichoderma genes has provided valuable insight into mycoparasitism of fungal pathogens by Trichoderma spp. and their role in the biocontrol of plant diseases. A recent study has suggested that lateral gene transfer is linked to the ability of Trichoderma to parasitize taxonomically related fungi and this may have allowed Trichoderma fungi to modify their lifestyles (Druzhinina et al, 2018). Following an RNA-seq approach, mycoparasitism-related genes of Trichoderma harzianum have been identified in response to Sclerotinia sclerotiorum (Steindorff et al, 2014)
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