Fate of Trichoderma harzianum in the olive rhizosphere: time course of the root colonization process and interaction with the fungal pathogen Verticillium dahliae

Trichoderma harzianum Rifai is a well-known biological control agent (BCA) effective against a wide range of phytopathogens. Since colonization and persistence in the target niche is crucial for biocontrol effectiveness we aimed to: (i) shed light on the olive roots colonization process by T. harzianum CECT 2413, (ii) unravel the fate of its biomass upon application, and (iii) study the in planta interaction with the soil-borne pathogen Verticillium dahliae Kleb. Fluorescently-tagged derivatives of CECT 2413 and V. dahliae and confocal laser scanning microscopy were used. In vitro assays showed for the first time mycoparasitism of V. dahliae by T. harzianum, evidenced by events such as hyphal coiling. In planta assays revealed that CECT 2413 profusely colonized the rhizoplane of olive roots. Interestingly, biomass of the BCA was visualized mainly as chlamydospores. This observation was independent on the presence or absence of the pathogen. Evidence of inner colonization of olive roots by CECT 2413 was not obtained. These results suggest that CECT 2413 is not able to persist in a metabolically-active form when applied as a spore suspension. This may have strong implications in the way this BCA should be introduced and/or formulated to be effective against Verticillium wilt of olive.This work was supported by European Regional Development Fund-cofinanced grants from the Spanish Ministry of Economy and Competitiveness [Project number BIO2012-33904] and ‘Junta de Andalucía’ [Project number AGR-6038].Peer reviewe

What Determines Successful Colonization and Expression of Biocontrol Traits at the Belowground Level?

Editor: Antonieta De Cal, Paloma Melgarejo, Naresh Magan.Plants and their associated microbial communities are continuously interacting. The consequences of this complex, multi-actor dialogue are of utmost relevance for the health and development of the plant holobiont. The plant-associated microbiota is a natural source of microorganisms with plant growth-promoting abilities, including biological control agents (BCA). Plant roots are the main entrance for soilborne phytopathogens, which are also components of the belowground plant-associated microbiota. Successful root colonization by BCA is thus key for effective biocontrol against these pathogens. Colonization and biocontrol are complex processes influenced by many (a)biotic factors. Upon colonization of the target niche, effective biocontrol is then the result of diverse modes of action that BCA may display, thereby eliminating pathogens’ propagules, reducing their number, or alleviating their deleterious effects. These mechanisms are not mutually exclusive and can operate either individually or in combination, varying in time and space. Inconsistencies usually observed in biocontrol effectiveness is a consequence of numerous (a)biotic and environmental factors affecting BCA performance. A more comprehensive knowledge of colonization processes and biocontrol modes of action of BCA, and that of the diverse factors influencing them, is now possible with the support of the currently-available -omics approaches. They will definitively offer a more holistic perspective that will help to overcome the lack of success sometimes observed when implementing biocontrol measures, particularly at the field scale.Our work is currently supported by grant AGL2016-75729-C2-1-R from the Spanish Ministerio de Economía, Industria y Competitividad/Agencia Estatal de Investigación, co-financed by the European Regional Development Fund (ERDF), and by the Horizon 2020 Project ‘Microbial Uptakes for Sustainable Management of Major Banana Pests and Diseases’ (MUSA; grant number 727624)