No abstract

One century after the first report of Dutch Elm Disease (DED), there is still no practical solution for this problem threatening European and American elms (Ulmus spp.). The long breeding cycles needed to select resistant genotypes and the lack of efficient treatments keep disease incidence at high levels. In the present work, the expression of defense-related genes to the causal agent of DED, Ophiostoma novo-ulmi, were analyzed in in vitro clonal plantlets from two DED-resistant and two DED-susceptible U. minor trees. In addition, the effect of the inoculation of an endophytic pink-pigmented yeast (Cystobasidium sp.) on the plant’s defense system was tested both individually and in combination with O. novo-ulmi. The multifactorial nature of the resistance to DED was confirmed, as no common molecular response was found in the two resistant genotypes. However, the in vitro experimental system allowed to discriminate the susceptible from the resistant genotypes, showing higher levels of oxidative damage and phenolic compounds in the susceptible genotypes after pathogen inoculation. Inoculation of the endophyte before O. novo-ulmi attenuated the plant molecular response induced by the pathogen and moderated oxidative stress levels. Niche competition, endophyte-pathogen antagonism, and molecular crosstalk between the host and the endophyte are discussed as possible mechanisms of stress reduction. In sum, our results confirm the multifactorial nature of DED resistance mechanisms and highlight the possibility of using certain endophytic yeasts as biological tools to improve tree resilience against biotic stress.

Certain fungal endophytes are known to improve plant resistance to biotic stresses in forest trees. In this study, three stem fungal endophytes belonging to classes Cystobasidiomycetes, Eurotiomycetes and Dothideomycetes were selected among 210 isolates for their potential as enhancers of Ulmus minor resistance to Ophiostoma novo-ulmi. We evaluated phenotypic traits of these endophytes that could be beneficial for O. novo-ulmi inhibition in the host plant. Under in vitro conditions, the Dothideomycetous isolate YCB36 strongly inhibited O. novo-ulmi growth, released antipathogen VOCs, chitinases and siderophores, and overlapped with the pathogen in nutrient utilization patterns. These functional traits could explain the 40% reduction in leaf wilting due to O. novo-ulmi in elm trees pre-inoculated with this endophyte. Ulmus minor trees responded to endophyte inoculation with increased stomatal conductance and higher amounts of flavonoids and total phenolic compounds in xylem tissues, suggesting induction of defence metabolism.

One century after the first report of Dutch Elm Disease (DED), there is still no practical solution for this problem threatening European and American elms (Ulmus spp.). The long breeding cycles needed to select resistant genotypes and the lack of efficient treatments keep disease incidence at high levels. In the present work, the expression of defense-related genes to the causal agent of DED, Ophiostoma novo-ulmi, were analyzed in in vitro clonal plantlets from two DED-resistant and two DED-susceptible U. minor trees. In addition, the effect of the inoculation of an endophytic pink-pigmented yeast (Cystobasidium sp.) on the plant's defense system was tested both individually and in combination with O. novo-ulmi. The multifactorial nature of the resistance to DED was confirmed, as no common molecular response was found in the two resistant genotypes. However, the in vitro experimental system allowed to discriminate the susceptible from the resistant genotypes, showing higher levels of oxidative stress and phenolic compounds in the susceptible genotypes after pathogen inoculation. Inoculation of the endophyte before O. novo-ulmi attenuated the plant molecular response induced by the pathogen and moderated oxidative stress levels. Niche competition, endophyte-pathogen antagonism, and molecular crosstalk between the host and the endophyte are discussed as possible mechanisms of stress reduction. In sum, our results confirm the multifactorial nature of DED resistance mechanisms and highlight the possibility of using certain endophytic yeasts as biological tools to improve tree resilience against biotic stress.