Host responses and metabolic profiles of wood components in Dutch elm hybrids with a contrasting tolerance to Dutch elm disease

Abstract

Background and aims: Changes occurring in the macromolecular traits of cell wall components in elm wood following attack by Ophiostoma novo-ulmi, the causative agent of Dutch elm disease (DED), are poorly understood. The purpose of this study was to compare host responses and the metabolic profiles of wood components for two Dutch elm (Ulmus) hybrids, 'Groeneveld' (a susceptible clone) and 'Dodoens' (a tolerant clone), that have contrasting survival strategies upon infection with the current prevalent strain of DED.

Methods: Ten-year-old plants of the hybrid elms were inoculated with O. novo-ulmi ssp. americana × novo-ulmi. Measurements were made of the content of main cell wall components and extractives, lignin monomer composition, macromolecular traits of cellulose and neutral saccharide composition.

Key results: Upon infection, medium molecular weight macromolecules of cellulose were degraded in both the susceptible and tolerant elm hybrids, resulting in the occurrence of secondary cell wall ruptures and cracks in the vessels, but rarely in the fibres. The (13)C nuclear magnetic resonance spectra revealed that loss of crystalline and non-crystalline cellulose regions occurred in parallel. The rate of cellulose degradation was influenced by the syringyl:guaiacyl ratio in lignin. Both hybrids commonly responded to the medium molecular weight cellulose degradation with the biosynthesis of high molecular weight macromolecules of cellulose, resulting in a significant increase in values for the degree of polymerization and polydispersity. Other responses of the hybrids included an increase in lignin content, a decrease in relative proportions of d-glucose, and an increase in proportions of d-xylose. Differential responses between the hybrids were found in the syringyl:guaiacyl ratio in lignin.

Conclusions: In susceptible 'Groeneveld' plants, syringyl-rich lignin provided a far greater degree of protection from cellulose degradation than in 'Dodoens', but only guaiacyl-rich lignin in 'Dodoens' plants was involved in successful defence against the fungus. This finding was confirmed by the associations of vanillin and vanillic acid with the DED-tolerant 'Dodoens' plants in a multivariate analysis of wood traits.

Keywords: Cellulose degradation; Dutch elm disease; Ophiostoma novo-ulmi; Ulmus; crystallinity; lignin; syringyl to guaiacyl ratio.

Fig. 1.

Scanning electron microscopy images of wood samples separated from the sixth annual ring of infected ‘Groeneveld’ plants (A–C), the sixth annual ring of infected ‘Dodoens’ plants (D–F) and the seventh annual ring of infected ‘Dodoens’ plants (G–I). (A) Tyloses formed in latewood vessels in response to the Ophiostoma novo-ulmi ssp. americana × novo-ulmi inoculation. Cross-section. (B) Tyloses occluding latewood vessel lumens. Tangential section. (C) Fungal hyphae growing inside latewood vessels. Cross-section. (D) Tyloses formed in earlywood vessels in response to fungal inoculation. Cross-section. (E) Formation of many narrowed latewood vessels in response to fungal inoculation. Cross-section. (F) Narrowed latewood vessels as a possible resistance factor limiting the movement of the fungus in the vascular system. Radial section. (G) Reduced formation of large earlywood vessels in the next growing season after fungal inoculation. Cross-section. (H) Continuation of narrowed latewood vessel formation in the next growing season after fungal inoculation. Cross-section. (I) Occasional occurrence of fungal hyphae (arrows) growing inside earlywood vessel. Radial section. Scale bars: (A, D, E, G, H) = 500 μm; (B) = 200 μm; (C, I) = 50 μm; (F) = 100 μm.

Fig. 2.

Size exclusion chromatography of molecular weight distributions of cellulose tricarbanilates. (A) Sixth annual ring of ‘Groeneveld’ plants. (B) Sixth annual ring of ‘Dodoens’ plants. (C) Seventh and eighth annual rings of ‘Dodoens’ plants.

Fig. 3.

Scanning electron microscopy images of secondary cell wall ruptures and cracks. (A) Ophiostoma novo-ulmi ssp. americana × novo-ulmi hypha inside the vascular tracheid (arrowhead) and ruptures in secondary cell walls of wood fibres (arrows). Cross-section of sixth annual ring of an infected ‘Groeneveld’ plant. Scale bar = 10 μm. (B) Fungal hyphae inside an earlywood vessel and radial cracks in the secondary cell wall (arrow). Radial section of sixth annual ring of an infected ‘Groeneveld’ plant. Scale bar = 20 μm.

Fig. 4.

Relationship of cellulose content with the degree of polymerization (DP_w) in (A) non-infected plants and (B) infected plants.

Fig. 5.

Solid-state ¹³C MAS NMR spectra of wood sawdust samples. (A) Sixth annual ring of ‘Groeneveld’ plants. (B) Sixth annual ring of ‘Dodoens’ plants. (C) Seventh and eighth annual rings of ‘Dodoens’ plants.

Fig. 6.

Positions of 25 wood traits on the first and second axes of the principal components analysis (PCA). Traits associated with differential host responses are indicated in grey. Bottom and left axes refer to wood traits; top and right axes refer to annual rings of Dutch elm hybrids. Trait abbreviations: ARA, l-arabinose; CEL, cellulose content; CI, crystallinity index of cellulose; DP_w, degree of polymerization of cellulose; EXT, extractives content; GAL, d-galactose; GLC, d-glucose; HBAC, p-hydroxybenzoic acid; HBAL, p-hydroxybenzaldehyde; HOL, holocellulose content; LIG, lignin content; LOI, lateral order index; MAN, d-mannose; M_n, number-average molecular weight; M_p, peak molecular weight; M_w, weight-average molecular weight; M_z, z-average molecular weight; M_z+1, z + 1-average molecular weight; PDI, polydispersity index; S/G, syringyl:guaiacyl ratio in lignin; SAC, syringic acid; SYR, syringaldehyde; VAC, vanillic acid; VAN, vanillin; XYL, d-xylose.