Abstract
The emerald ash borer (EAB, Agrilus planipennis), an ash-tree wood-boring beetle, has caused widespread mortality of ash. Asian ash, which coevolved with EAB, is considered more resistant than its North American and European congeners. Although some compounds and proteins related to resistance to EAB have been identified, the underlying ash resistance mechanism to EAB still needs further study. The Asian ash species, Fraxinus chinensis var. rhynchophylla, is highly resistant to EAB. In this study, metabolic and transcriptional profiling of the phloem of this species was investigated, and differentially expressed metabolites and genes were analyzed by comparing them with those of the susceptible F. pennsylvanica. Four hundred and twenty-eight metabolites were detected in both species, and several coumarins and lignans, which were exclusive to F. chinensis var. rhynchophylla, were identified. Compared with susceptible F. pennsylvanica, genes related to phenylpropanoid biosynthesis, ethylene (ET), and jasmonic acid (JA) biosynthesis and signaling in F. chinensis var. rhynchophylla were found to be up-regulated. It was hypothesized that coumarins, lignans, and ET and JA signaling might contribute to greater resistance to EAB in F. chinensis var. rhynchophylla. This study suggests candidate metabolites and genes for biomarker development in future ash-breeding programs.
Highlights
Ash trees are an economically important tree species and are widespread in temperate and sub-tropical regions, from North America to Eurasia
Eight WRKYs were differentially expressed between the two species, five of which were higher in F. chinensis var. rhynchophylla than in F. pennsylvanica (Figure 4)
Several studies have identified that it is more resistant to emerald ash borer (EAB) than Manchurian ash, which is generally used in the research on ash resistance mechanisms to EAB
Summary
Ash trees are an economically important tree species and are widespread in temperate and sub-tropical regions, from North America to Eurasia. Rhynchophylla and F. pennsylvanica Multivariate statistical analyses showed that the resistant and susceptible species were clearly divided into two separate groups in PCA analysis (Figure S1). A total of 17 metabolites were detected in the resistant but not in the susceptible species, including 2 coumarins (fraxetin diglucoside and fraxin), 1 lignan (forsythialan B), 1 phenylethanoid (calceolarioside A), 2 amino acid derivatives (L-cystathionine and acetyltryptophan), 1 organic acid (coumalic acid), and 10 phenolic acids (Table 1). Among these metabolites, the amounts of coumarins and lignans, especially fraxidin, esculin hydrate, fraxidin 8-Oglucoside, fraxetin, isofraxetin, syringaresinol-4 -O-β-D-monoglucoside, daphnetin, esculetin, and isofraxidin, were greater in the resistant than in susceptible species (Table 1). Compared with gene expression levels in F. pennsylvanica, a total of 8387 DEGs were identified in F. chinensis var.
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