Resistance to Dutch elm disease reduces presence of xylem endophytic fungi in Elms (Ulmus spp.)

Abstract

Efforts to introduce pathogen resistance into landscape tree species by breeding may have unintended consequences for fungal diversity. To address this issue, we compared the frequency and diversity of endophytic fungi and defensive phenolic metabolites in elm (Ulmus spp.) trees with genotypes known to differ in resistance to Dutch elm disease. Our results indicate that resistant U. minor and U. pumila genotypes exhibit a lower frequency and diversity of fungal endophytes in the xylem than susceptible U. minor genotypes. However, resistant and susceptible genotypes showed a similar frequency and diversity of endophytes in the leaves and bark. The resistant and susceptible genotypes could be discriminated on the basis of the phenolic profile of the xylem, but not on basis of phenolics in the leaves or bark. As the Dutch elm disease pathogen develops within xylem tissues, the defensive chemistry of resistant elm genotypes thus appears to be one of the factors that may limit colonization by both the pathogen and endophytes. We discuss a potential trade-off between the benefits of breeding resistance into tree species, versus concomitant losses of fungal endophytes and the ecosystem services they provide.

Figure 1. Location of the two study areas in central Spain.

Figure 2. Accumulation curves of elm endophytic fungi.

Accumulation curves indicating the number of endophyte morphotaxa isolated per number of twigs processed (four twigs per tree, and four leaf, bark and xylem samples per twig) in each tree group [P (R) = resistant U. pumila clones from Puerta de Hierro Forest Breeding Centre; M (R) = resistant U. minor clones from Puerta de Hierro Forest Breeding Centre; M (S) = susceptible U. minor clones from Puerta de Hierro Forest Breeding Centre; and M (F) = U. minor trees from Rivas-Vaciamadrid field site].

Figure 3. Endophyte frequency and diversity in elms.

Mean values of endophyte frequency (a, c, e) and endophyte diversity (b, d, f) of leaf (a, b), bark (c, d), and xylem (e, f) tissues from different groups of elm trees: P (R) = resistant U. pumila clones from Puerta de Hierro Forest Breeding Centre; M (R) = resistant U. minor clones from Puerta de Hierro Forest Breeding Centre; M (S) = susceptible U. minor clones from Puerta de Hierro Forest Breeding Centre; and M (F) = U. minor trees from Rivas-Vaciamadrid field site. Different letters indicate differences among groups of trees (P<0.05), and bars represent standard errors.

Figure 4. Two-dimensional ordination using non-metric multidimensional scaling (MDS) based on Jaccard’s similarity measures.

Each point represents the fungal endophyte community of an individual tree. Endophytes were isolated from leaf (a), bark (b) or xylem (c) tissues. Groups of elm trees: P (R) = resistant U. pumila clones from Puerta de Hierro Forest Breeding Centre; M (R) = resistant U. minor clones from Puerta de Hierro Forest Breeding Centre; M (S) = susceptible U. minor clones from Puerta de Hierro Forest Breeding Centre; and M (F) = U. minor trees from Rivas-Vaciamadrid field site.

Figure 5. Relation between endophytes and susceptibility to DED in elms.

Relations between the mean susceptibility to DED (% leaf wilting) of each elm genotype at the Breeding Centre and its endophyte frequency (a) and diversity (b) in xylem tissues. Solid lines are linear regressions and dotted lines are 95% confidence limits. Wilting values were obtained from a previous susceptibility test .

Figure 6. Separation of elm trees on basis of tissue specific phenolic profiles.

Discriminant function analysis score scatter plot for the HPLC chromatogram peaks of samples taken from leaf (a), bark (b), and xylem (c) tissues from different groups of trees: P (R) = resistant U. pumila clones from Puerta de Hierro Forest Breeding Centre; M (R) = resistant U. minor clones from Puerta de Hierro Forest Breeding Centre; M (S) = susceptible U. minor clones from Puerta de Hierro Forest Breeding Centre; and M (F) = U. minor trees from the Rivas-Vaciamadrid site.

Figure 7. Quantitative patterns of a rosmarinic acid derivative in elms.

Mean peak area (AU×10⁻⁵) of one of the HPLC chromatogram peaks (RT = 24.47 min) of xylem samples that was important in discriminating between tree groups: P (R) = resistant U. pumila clones from Puerta de Hierro Forest Breeding Centre; M (R) = resistant U. minor clones from Puerta de Hierro Forest Breeding Centre; M (S) = susceptible U. minor clones from Puerta de Hierro Forest Breeding Centre; and M (F) = U. minor trees from Rivas-Vaciamadrid field site. Different letters indicate differences between groups of trees (P<0.05); bars represent standard errors [n = 4 for M(R) and M(S), 2 for P(R) or 7 M(F)].