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

No priming, just fighting-endophytic yeast attenuates the defense response and the stress induced by Dutch elm disease in Ulmus minor Mill

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 this work, the expression of defense-related genes to the causal agent of DED, Ophiostoma novo-ulmi Brasier, was analyzed in in vitro clonal plantlets from two DED-resistant and two DED-susceptible Ulmus minor Mill. 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 discrimination of 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 complex and heterogeneous nature of DED resistance mechanisms and highlight the possibility of using certain endophytic yeasts as biological tools to improve tree resilience against biotic stress.

Deciphering the Genome-Wide Transcriptomic Changes during Interactions of Resistant and Susceptible Genotypes of American Elm with Ophiostoma novo-ulmi

Dutch elm disease (DED), caused by Ophiostoma novo-ulmi (Onu), is a destructive disease of American elm (Ulmus americana L.). The molecular mechanisms of resistance and susceptibility against DED in American elm are still largely uncharacterized. In the present study, we performed a de novo transcriptome (RNA-sequencing; RNA-Seq) assembly of U. americana and compared the gene expression in a resistant genotype, 'Valley Forge', and a susceptible (S) elm genotype at 0 and 96 h post-inoculation of Onu. A total of 85,863 non-redundant unigenes were identified. Compared to the previously characterized U. minor transcriptome, U. americana has 35,290 similar and 55,499 unique genes. The transcriptomic variations between 'Valley Forge' and 'S' were found primarily in the photosynthesis and primary metabolism, which were highly upregulated in the susceptible genotype irrespective of the Onu inoculation. The resistance to DED was associated with the activation of RPM1-mediated effector-triggered immunity that was demonstrated by the upregulation of genes involved in the phenylpropanoids biosynthesis and PR genes. The most significantly enriched gene ontology (GO) terms in response to Onu were response to stimulus (GO:0006950), response to stress (GO:0050896), and secondary metabolic process (GO:0008152) in both genotypes. However, only in the resistant genotype, the defense response (GO:0006952) was among the topmost significantly enriched GO terms. Our findings revealed the molecular regulations of DED resistance and susceptibility and provide a platform for marker-assisted breeding of resistant American elm genotypes.

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.

Sap flow, xylem anatomy and photosynthetic variables of three Persea species in response to laurel wilt

Laurel wilt, a lethal vascular wilt disease caused by the fungus Raffaelea lauricola, affects several tree species in the Lauraceae, including three Persea species. The susceptibility to laurel wilt of two forest tree species native to the southern USA, Persea borbonia and Persea palustris, [(Raf.) Sarg.] and avocado, Persea americana (Mill.) cv Waldin, was examined and related to tree physiology and xylem anatomy. Net CO2 assimilation (A), stomatal conductance (gs), leaf chlorophyll index (LCI), leaf chlorophyll fluorescence (Fv/Fm), xylem sap flow, theoretical stem hydraulic conductivity (Kh) and xylem vessel anatomy were assessed in trees of each species that were inoculated with R. lauricola and in control trees. Laurel wilt caused a reduction in A, gs, LCI, Fv/Fm and blockage of xylem vessels by tyloses formation that negatively impacted Kh and sap flow in all Persea species. However, disease susceptibility as indicated by canopy wilting and sapwood discoloration was less pronounced in P. americana cv Waldin than in the two forest species. Xylem vessel diameter was significantly smaller in P. borbonia and P. palustris than in P. americana cv Waldin. Differences in laurel wilt susceptibility among species appear to be influenced by physiological and anatomical tree responses.

Fasciclin-like arabinogalactan gene family in Nicotiana benthamiana: genome-wide identification, classification and expression in response to pathogens

BACKGROUND

Nicotiana benthamiana is widely used as a model plant to study plant-pathogen interactions. Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), participate in mediating plant growth, development and response to abiotic stress. However, the members of FLAs in N. benthamiana and their response to plant pathogens are unknown.

RESULTS

38 NbFLAs were identified from a genome-wide study. NbFLAs could be divided into four subclasses, and their gene structure and motif composition were conserved in each subclass. NbFLAs may be regulated by cis-acting elements such as STRE and MBS, and may be the targets of transcription factors like C2H2. Quantitative real time polymerase chain reaction (RT-qPCR) results showed that selected NbFLAs were differentially expressed in different tissues. All of the selected NbFLAs were significantly downregulated following infection by turnip mosaic virus (TuMV) and most of them also by Pseudomonas syringae pv tomato strain DC3000 (Pst DC3000), suggesting possible roles in response to pathogenic infection.

CONCLUSIONS

This study systematically identified FLAs in N. benthamiana, and indicates their potential roles in response to biotic stress. The identification of NbFLAs will facilitate further studies of their role in plant immunity in N. benthamiana.

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.