Influence of Plant Age on Symptom Development in Ulmus minor Following Inoculation by Ophiostoma novo-ulmi

Priming of Plant Defenses against Ophiostoma novo-ulmi by Elm (Ulmus minor Mill.) Fungal Endophytes

Some fungal endophytes of forest trees are recognized as beneficial symbionts against stresses. In previous works, two elm endophytes from the classes Cystobasidiomycetes and Eurotiomycetes promoted host resistance to abiotic stress, and another elm endophyte from Dothideomycetes enhanced host resistance to Dutch elm disease (DED). Here, we hypothesize that the combined effect of these endophytes activate the plant immune and/or antioxidant system, leading to a defense priming and/or increased oxidative protection when exposed to the DED pathogen Ophiostoma novo-ulmi. To test this hypothesis, the short-term defense gene activation and antioxidant response were evaluated in DED-susceptible (MDV1) and DED-resistant (VAD2 and MDV2.3) Ulmus minor genotypes inoculated with O. novo-ulmi, as well as two weeks earlier with a mixture of the above-mentioned endophytes. Endophyte inoculation induced a generalized transient defense activation mediated primarily by salicylic acid (SA). Subsequent pathogen inoculation resulted in a primed defense response of variable intensity among genotypes. Genotypes MDV1 and VAD2 displayed a defense priming driven by SA, jasmonic acid (JA), and ethylene (ET), causing a reduced pathogen spread in MDV1. Meanwhile, the genotype MDV2.3 showed lower defense priming but a stronger and earlier antioxidant response. The defense priming stimulated by elm fungal endophytes broadens our current knowledge of the ecological functions of endophytic fungi in forest trees and opens new prospects for their use in the biocontrol of plant diseases.

Complexities underlying the breeding and deployment of Dutch elm disease resistant elms

Dutch elm disease (DED) is a vascular wilt disease caused by the pathogens Ophiostoma ulmi and Ophiostoma novo-ulmi with multiple ecological phases including pathogenic (xylem), saprotrophic (bark) and vector (beetle flight and beetle feeding wound) phases. Due to the two DED pandemics during the twentieth century the use of elms in landscape and forest restoration has declined significantly. However new initiatives for elm breeding and restoration are now underway in Europe and North America. Here we discuss complexities in the DED 'system' that can lead to unintended consequences during elm breeding and some of the wider options for obtaining durability or 'field resistance' in released material, including (1) the phenotypic plasticity of disease levels in resistant cultivars infected by O. novo-ulmi; (2) shortcomings in test methods when selecting for resistance; (3) the implications of rapid evolutionary changes in current O. novo-ulmi populations for the choice of pathogen inoculum when screening; (4) the possibility of using active resistance to the pathogen in the beetle feeding wound, and low attractiveness of elm cultivars to feeding beetles, in addition to resistance in the xylem; (5) the risk that genes from susceptible and exotic elms be introgressed into resistant cultivars; (6) risks posed by unintentional changes in the host microbiome; and (7) the biosecurity risks posed by resistant elm deployment. In addition, attention needs to be paid to the disease pressures within which resistant elms will be released. In the future, biotechnology may further enhance our understanding of the various resistance processes in elms and our potential to deploy trees with highly durable resistance in elm restoration. Hopefully the different elm resistance processes will prove to be largely under durable, additive, multigenic control. Elm breeding programmes cannot afford to get into the host-pathogen arms races that characterise some agricultural host-pathogen systems.

Growth resilience and oxidative burst control as tolerance factors to Ophiostoma novo-ulmi in Ulmus minor

The Dutch elm disease (DED) pathogens, Ophiostoma ulmi (Buisman) Nannf. and the more aggressive Ophiostoma novo-ulmi Brasier, have decimated European elm populations in the last 100 years. Today, the number of tolerant elm varieties available on the market is limited, partly due to the long breeding cycles and expensive facilities they require. Developing a low-cost technique to allow early screening of elm tolerance based on simple morphological and/or biochemical traits would considerably boost elm breeding and research. Within this general aim, we developed an in vitro plant culture system to (i) characterize stress responses to O. novo-ulmi-root inoculation in two Ulmus minor Mill. clones of contrasting susceptibility level to DED (termed 'tolerant' and 'susceptible') and (ii) compare the upward dispersal rate of the pathogen in the two clones. Constitutive xylem anatomy was similar in both clones, indicating that differences in plant responses to the pathogen are not attributable to anatomical factors (e.g., conduit size). Susceptible plantlets suffered a significant delay in apical growth and a decrease in chlorophyll content at 21 days post-inoculation (dpi). The rate of pathogen dispersal from roots to aerial tissues was similar in both clones. However, the tolerant clone showed a marked increase in lipid peroxidation at 1 dpi, while the susceptible clone showed enhanced values of lipid peroxidation during most of the experimental period (1-21 dpi). Despite wide stem colonization by the pathogen, the tolerant clone effectively regulated the oxidative stress levels and showed remarkable resilience to inoculation. These results extend current knowledge on elm defense mechanisms, and the proposed in vitro plant culture system emerges as a promising early screening method for tolerance to improve elm breeding.

Further insights into the components of resistance to Ophiostoma novo-ulmi in Ulmus minor: hydraulic conductance, stomatal sensitivity and bark dehydration

Dutch elm disease (DED) is a vascular disease that has killed over 1 billion elm trees. The pathogen spreads throughout the xylem network triggering vessel blockage, which results in water stress, tissue dehydration and extensive leaf wilting in susceptible genotypes. We investigated the differences between four Ulmus minor Mill. clones of contrasting susceptibility to Ophiostoma novo-ulmi Brasier regarding morphological, anatomical and physiological traits affecting water transport, in order to gain a better understanding of the mechanisms underlying DED susceptibility. We analyzed the differential response to water shortage and increased air vapor pressure deficit (VPD) to investigate whether resistance to water stress might be related to DED tolerance. Sixteen plants per clone, aged 2 years, were grown inside a greenhouse under differential watering. Stomatal conductance was measured under ambient and increased VPD. Growth, bark water content and stem hydraulic and anatomical parameters were measured 22 days after starting differential watering. Vessel lumen area, lumen fraction and hydraulic conductance were highest in susceptible clones. Stomatal conductance was lowest under low VPD and decreased faster under increased VPD in resistant clones. We found a negative relationship between the decrease in stomatal conductance at increased VPD and specific hydraulic conductance, revealing a narrower hydraulic margin for sustaining transpiration in resistant clones. The effect of water shortage was greater on radial stem growth than on leaf area, which could be explained through an extensive use of capacitance water to buffer xylem water potential. Water shortage reduced stomatal conductance and vessel lumen area. Bark water content under conditions of water shortage only decreased in susceptible clones. Higher hydraulic constraints to sap flow in resistant clones may determine higher stomatal sensitivity to VPD and so contribute to DED resistance by limiting pathogen expansion and reducing water loss and metabolic impairment in cells involved in fighting against infection.

Gene expression trade-offs between defence and growth in English elm induced by Ophiostoma novo-ulmi

Wilt diseases caused by vascular pathogens include some of the most damaging stresses affecting trees. Dutch elm disease (DED), caused by the fungus Ophiostoma novo-ulmi, destroyed most of North American and European elm populations in the 20th century. The highly susceptible English elm, also known as Atinian clone, suffered the highest mortality rates during the last pandemic event, probably due to its lack of genetic diversity. To study the DED pathosystem, we inoculated English elm ramets with O. novo-ulmi and evaluated xylem anatomy, molecular response, and disease symptoms. The high DED susceptibility of the clone was linked to xylem structure. The transcript levels changed significantly for 1,696 genes during O. novo-ulmi invasion. Genes covering different steps of the plant immune system were identified, many of which showed homology with Arabidopsis thaliana genes involved in systemic acquired resistance. Induction of several pathogenesis-related proteins and repression of fasciclin-like arabinogalactan proteins and other cell wall biosynthesis pathways evidence unbalanced costs between growth and defence mechanisms far from the inoculation point. This study sheds light on elm molecular defence mechanisms against DED.

Massive sequencing of Ulmus minor's transcriptome provides new molecular tools for a genus under the constant threat of Dutch elm disease

Elms, especially Ulmus minor and U. americana, are carrying out a hard battle against Dutch elm disease (DED). This vascular wilt disease, caused by Ophiostoma ulmi and O. novo-ulmi, appeared in the twentieth century and killed millions of elms across North America and Europe. Elm breeding and conservation programmes have identified a reduced number of DED tolerant genotypes. In this study, three U. minor genotypes with contrasted levels of tolerance to DED were exposed to several biotic and abiotic stresses in order to (i) obtain a de novo assembled transcriptome of U. minor using 454 pyrosequencing, (ii) perform a functional annotation of the assembled transcriptome, (iii) identify genes potentially involved in the molecular response to environmental stress, and (iv) develop gene-based markers to support breeding programmes. A total of 58,429 putative unigenes were identified after assembly and filtering of the transcriptome. 32,152 of these unigenes showed homology with proteins identified in the genome from the most common plant model species. Well-known family proteins and transcription factors involved in abiotic, biotic or both stresses were identified after functional annotation. A total of 30,693 polymorphisms were identified in 7,125 isotigs, a large number of them corresponding to single nucleotide polymorphisms (SNPs; 27,359). In a subset randomly selected for validation, 87% of the SNPs were confirmed. The material generated may be valuable for future Ulmus gene expression, population genomics and association genetics studies, especially taking into account the scarce molecular information available for this genus and the great impact that DED has on elm populations.

Chemical changes in Ulmus minor xylem tissue after salicylic acid or carvacrol treatments are associated with enhanced resistance to Ophiostoma novo-ulmi

Application of endogenous plant hormone salicylic acid (SA) or essential oil component carvacrol (CA) in elms enhances tree resistance to the Dutch elm disease pathogen, although the effect of these compounds on tree metabolism is unknown. The chemical changes induced by SA or CA treatments in Ulmus minor were studied through gas chromatography-mass spectrometry (GC-MS) analysis of xylem tissues. Treatments consisted of fortnightly irrigating seedlings with water, SA or CA at 600 mg L⁻¹. The chemical composition of the xylem tissues sampled from treated trees was significantly altered depending on the treatment type. SA treatment induced an accumulation of the sinapyl alcohol, a precursor of lignin and other phenylpropanoid-derived products. CA treatment induced an accumulation of the methyl esters of palmitic, linoleic and stearic acids. Both treatments resulted in early bud burst and SA significantly reduced sapwood radial growth, possibly as a consequence of a trade-off between tree growth and tree defence. The enhanced resistance provided by these treatments is discussed.

Using citizen science programs to identify host resistance in pest-invaded forests

Threats to native forests from non-native insects and pathogens (pests) are generally addressed with methods such as quarantine, eradication, biological control, and development of resistant stock through hybridization and breeding. In conjunction with such efforts, it may be useful to have citizen scientists locate rare surviving trees that may be naturally pest resistant or tolerant. The degree of resistance of individual trees identified in this way can be tested under controlled conditions, and the most resistant individuals can be integrated into plant breeding programs aimed at developing pest-resistant native stock. Involving citizen scientists in programs aimed at identifying rare trees that survive colonization by pests provides a low-cost means of maximizing search efforts across wide geographic regions and may provide an effective supplement to existing management approaches.