The presence of Rhodotorula sp. in Ulmus minor plantlets moderates the response to Ophiostoma novo-ulmi in roots of tolerant clones

Dutch elm disease (DED) is caused by the fungus Ophiostoma novo-ulmi and is one of the most devastating forest diseases of the last century. However, there are some elm genotypes able to tolerate this disease. Although some mechanisms involved in the tolerance have been already characterized, the molecular basis of the tolerance is still unknown. In addition, the role of the tree microbiome in elm tolerance to DED is poorly known. In this work, we aimed to observe the molecular responses of tolerant and susceptible Ulmus minor genotypes after the inoculation of either the pathogen or one endophytic fungus of the genus Rhodotorula which was found to be more abundantly present in tolerant than susceptible genotypes. We worked with 1 month-old plantlets grown in vitro. The roots were submerged in a solution of spores of each fungus and later the plants were grown for a week preserving the sterility. The expression of 7 genes related to local defense responses was analyzed in the roots. Both fungi produced a strong overexpression of phenylalanine ammonia lyase (PAL) and pathogenesis-related protein 4 (PR4) genes. However the pathogen induced a higher response than the endophyte. In a second experiment, we explored the ability of the endophyte to induce resistance in the elm against O. novo-ulmi. Then, a new treatment included the inoculation with Rhodotorula prior to the pathogen inoculation. In comparison with the single inoculation of O. novo-ulmi, the double inoculation showed a reduction of the PAL overexpression in tolerant genotypes, while no effect or increased expression was observed in the susceptible ones. These results suggest a relationship between the tolerance to DED and the association of certain endophytes to some elm genotypes