Higher Activities of Defense-Associated Enzymes may Contribute to Greater Resistance of Manchurian Ash to Emerald Ash Borer Than A closely Related and Susceptible Congener

Distinct metabolites affect the phloem fungal communities in ash trees (Fraxinus spp.) native and nonnative to the highly invasive emerald ash borer (AGRILUS PLANIPENNIS)

Emerald ash borer (EAB, Agrilus planipennis) is an invasive killer of ash trees (Fraxinus spp.) in North America and Europe. Ash species co-evolved with EAB in their native range in Asia are mostly resistant, although the precise mechanism(s) remain unclear. Very little is also known about EAB or ash tree microbiomes. We performed the first joint comparison of phloem mycobiome and metabolites between a native and a nonnative ash species, infested and uninfested with EAB, in conjunction with investigation of larval mycobiome. Phloem mycobiome communities differed between the tree species, but both were unaffected by EAB infestation. Several indicator taxa in the larval gut shared a similarly high relative abundance only with the native host trees. Widely targeted metabolomics revealed 24 distinct metabolites in native trees and 53 metabolites in nonnative trees, respectively, that differed in relative content between infested and uninfested trees only in one species. Interestingly, four metabolites shared a strong relationship with the phloem mycobiomes, majority of which affected only the native trees. Collectively, our results demonstrate a complex interplay between host tree chemistry and mycobiome, and suggest the shared relationships between the mycobiomes of the native host tree and EAB may reflect their shared co-evolution.

Emerald Ash Borer Management and Research: Decades of Damage and Still Expanding

Since the discovery of the ash tree (Fraxinus spp.) killer emerald ash borer (EAB; Agrilus planipennis) in the United States in 2002 and Moscow, Russia in 2003, substantial detection and management efforts have been applied to contain and monitor its spread and mitigate impacts. Despite these efforts, the pest continues to spread within North America. It has spread to European Russia and Ukraine and is causing sporadic outbreaks in its native range in China. The dynamics of EAB's range expansion events appear to be linked to the lack of resistant ash trees in invaded ranges, facilitated by the abundance of native or planted North American susceptible ash species. We review recently gained knowledge of the range expansion of EAB; its ecological, economic, and social impacts; and past management efforts with their successes and limitations. We also highlight advances in biological control, mechanisms of ash resistance, and new detection and management approaches under development, with the aim of guiding more effective management.

Integrative analysis of green ash phloem transcripts and proteins during an emerald ash borer infestation

BACKGROUND

Emerald ash borer (Agrilus planipennis; EAB) is an Asian insect species that has been invasive to North America for 20 years. During this time, the emerald ash borer has killed tens of millions of American ash (Fraxinus spp) trees. Understanding the inherent defenses of susceptible American ash trees will provide information to breed new resistant varieties of ash trees.

RESULTS

We have performed RNA-seq on naturally infested green ash (F. pennsylvanica) trees at low, medium and high levels of increasing EAB infestation and proteomics on low and high levels of EAB infestation. Most significant transcript changes we detected occurred between the comparison of medium and high levels of EAB infestation, indicating that the tree is not responding to EAB until it is highly infested. Our integrative analysis of the RNA-Seq and proteomics data identified 14 proteins and 4 transcripts that contribute most to the difference between highly infested and low infested trees.

CONCLUSIONS

The putative functions of these transcripts and proteins suggests roles of phenylpropanoid biosynthesis and oxidation, chitinase activity, pectinesterase activity, strigolactone signaling, and protein turnover.

Morphoanatomical and biochemical factors associated with rice resistance to the South American rice water weevil, Oryzophagus oryzae (Coleoptera: Curculionidae)

The rice water weevil, Oryzophagus oryzae (Coleoptera: Curculionidae), is an economically important pest of flooded rice paddies throughout South America, and species with similar life histories are present in many rice-producing regions globally (collectively referred to here as RWWs). Plant resistance is a key strategy for management of RWWs; however, the mechanisms responsible for rice resistance to RWWs are poorly understood. We investigated morphoanatomical and biochemical plant traits potentially involved in rice resistance to O. oryzae. Resistance-associated traits were characterized in two cultivars, 'Dawn' (resistant) and 'BRS Pampa CL' ('Pamp' = susceptible), which were selected from among six cultivars on 2-year field screenings. Anatomical and morphological traits of leaf tissues from 'Pamp' and 'Dawn' were similar, which perhaps explains the lack of antixenosis during host plant selection. However, significant antibiosis effects were found. The activities of antioxidant enzymes involved in plant defense, as well the content of hydroxycinnamic and hydroxybenzoic acids derivatives and lignin, were higher in roots of 'Dawn' than in 'Pamp', over the period of larval infestation in the field. Additionally, 'Dawn' exhibited a root sclerenchyma arranged in three layers of lignified cells, which differed from the arrangement of cells in 'Pamp', regardless of larval infestation. Our results provide the first evidence for specific resistance-related traits associated with mortality and malnutrition of RWWs in rice.

Impact of Hemlock Woolly Adelgid (Hemiptera: Adelgidae) Infestation on the Jasmonic Acid-Elicited Defenses of Tsuga canadensis (Pinales: Pinaceae)

Hemlock woolly adelgid is an invasive piercing-sucking insect in eastern North America, which upon infestation of its main host, eastern hemlock ('hemlock'), improves attraction and performance of folivorous insects on hemlock. This increased performance may be mediated by hemlock woolly adelgid feeding causing antagonism between the the jasmonic acid and other hormone pathways. In a common garden experiments using hemlock woolly adelgid infestation and induction with methyl jasmonate (MeJA) and measures of secondary metabolite contents and defense-associated enzyme activities, we explored the impact of hemlock woolly adelgid feeding on the local and systemic induction of jasmonic acid (JA)-elicited defenses. We found that in local tissue hemlock woolly adelgid or MeJA exposure resulted in unique induced phenotypes, whereas the combined treatment resulted in an induced phenotype that was a mixture of the two individual treatments. We also found that if the plant was infested with hemlock woolly adelgid, the systemic response of the plant was dominated by hemlock woolly adelgid, regardless of whether MeJA was applied. Interestingly, in the absence of hemlock woolly adelgid, hemlock plants had a very weak systemic response to MeJA. We conclude that hemlock woolly adelgid infestation prevents systemic induction of JA-elicited defenses. Taken together, compromised local JA-elicited defenses combined with weak systemic induction could be major contributors to increased folivore performance on hemlock woolly adelgid-infested hemlock.

Light Limitation Impacts Growth but Not Constitutive or Jasmonate Induced Defenses Relevant to Emerald Ash Borer (Agrilus planipennis) in White Fringetree (Chionanthus virginicus) or Black Ash (Fraxinus nigra)

White fringetree is a host for the invasive emerald ash borer (EAB) but is of lower quality than the related and highly susceptible black ash. Field observations suggest that host trees grown in full sun are more resistant to EAB than those in shade, however the impact of light limitation on chemical defenses has not been assessed. We quantified constitutive and jasmonate-induced phloem defenses and growth patterns of white fringetree and black ash under differential light conditions and related them to EAB larval performance. White fringetree had significantly lower constitutive and induced activities of peroxidase, polyphenol oxidase, β-glucosidase, chitinase and lignin content, but significantly higher gallic acid equivalent soluble phenolic, soluble sugar, and oleuropein concentrations compared to black ash. Multivariate analyses based on tissue chemical attributes displayed clear separation of species and induced defense responses. Further, EAB performed significantly worse on white fringetree than black ash, consistent with previous studies. Light limitation did not impact measured defenses or EAB larval performance, but it did decrease current year growth and increase photosynthetic efficiency. Overall our results suggest that phenolic profiles, metabolite abundance, and growth traits are important in mediating white fringetree resistance to EAB, and that short-term light limitation does not influence phloem chemistry or larval success.

Convergent molecular evolution among ash species resistant to the emerald ash borer

Recent studies show that molecular convergence plays an unexpectedly common role in the evolution of convergent phenotypes. We exploited this phenomenon to find candidate loci underlying resistance to the emerald ash borer (EAB, Agrilus planipennis), the United States' most costly invasive forest insect to date, within the pan-genome of ash trees (the genus Fraxinus). We show that EAB-resistant taxa occur within three independent phylogenetic lineages. In genomes from these resistant lineages, we detect 53 genes with evidence of convergent amino acid evolution. Gene-tree reconstruction indicates that, for 48 of these candidates, the convergent amino acids are more likely to have arisen via independent evolution than by another process such as hybridization or incomplete lineage sorting. Seven of the candidate genes have putative roles connected to the phenylpropanoid biosynthesis pathway and 17 relate to herbivore recognition, defence signalling or programmed cell death. Evidence for loss-of-function mutations among these candidates is more frequent in susceptible species than in resistant ones. Our results on evolutionary relationships, variability in resistance, and candidate genes for defence response within the ash genus could inform breeding for EAB resistance, facilitating ecological restoration in areas invaded by this beetle.

Feeding, Survival, and Fecundity of Adult Emerald Ash Borer (Coleoptera: Buprestidae) on Foliage of Two Novel Hosts and Implications for Host Range Expansion

Insect herbivores are more likely to successfully use a novel host if the plant is closely related to the ancestral host and the insect is polyphagous. Emerald ash borer (EAB), Agrilus planipennis (Fairmaire) (Coleoptera: Buprestidae), is a specialist wood borer of ash (Fraxinus spp., Lamiales: Oleaceae) trees and one of the most destructive forest pests in North American forests. Recent studies have found that larvae can develop in stems of two ash relatives; white fringetree (Chionanthus virginicus (L.) [Laminales: Oleaceae]) and cultivated olive (Olea europaea (L.) [Laminales: Oleaceae]). For EAB adults, the ability to consume, successfully mate, and lay viable eggs on foliage of these hosts is unknown. Thus, we conducted two no-choice assays with adult EAB on foliage of white fringetree and olive paired with positive controls of susceptible ash. Larval performance was also examined in a reciprocal study with cut stems of white fringetree and green ash (Fraxinus pennsylvanica Marshall) to determine whether adult diet impacted the success of progeny. Longevity, consumption rates, and fecundity of adults were similar on white fringetree and ash foliage. In contrast, adults consuming olive died quickly, consumed more over time, and females laid far fewer eggs compared to those on ash. Adult diet did not impact larval success, but larvae in white fringetree stems grew slower. These results indicate that white fringetree is a suitable host for EAB to complete its lifecycle, although larvae perform more poorly on this host than in susceptible ash species. In contrast, the more distantly related olive appears to be a poor host for adult EAB, although some viable eggs were produced by females.

Larval Performance of a Major Forest Pest on Novel Hosts and the Effect of Stressors

Novel hosts lacking a coevolutionary history with herbivores can often support improved larval performance over historic hosts; e.g., emerald ash borer [Agrilus planipennis (Fairmaire) Coleoptera: Buprestidae] on North American ash (Fraxinus spp.) trees. Whether trees are novel or ancestral, stress on plants increases emerald ash borer preference and performance. White fringetree [Chionanthus virginicus (L.) Lamiales: Oleaceae] and olive [Olea europaea (L.) Lamiales: Oleaceae] are closely related non-ash hosts that support development of emerald ash borer to adulthood, but their relative suitability as hosts and the impact of plant stress on larval success has not been well studied. In a series of experiments, survival and growth of emerald ash borer larvae on these novel hosts were examined along with the impact of stress. In the first experiment, larvae grew more slowly in cut stems of olive than in green ash [Fraxinus pennsylvanica (Marshall) Lamiales: Oleaceae] and several adults successfully emerged from larger olive stems. In two experiments on young potted olive with photosynthesizing bark, larvae died within a week, but mechanical girdling increased the rate of gallery establishment. The final two experiments on field-grown fringetrees found increased larval survivorship and growth in previously emerald ash borer attacked and mechanically girdled plants than in healthy stems or stems treated with the defense hormone, methyl jasmonate. Our results demonstrate that these non-ash hosts are less suitable for emerald ash borer than preferred ash hosts, but previous emerald ash borer attack or girdling led to better survival and growth demonstrating the importance of stress for larval success. In potted olive, high mortality could be due to higher loads of toxic compounds or the presence of chlorophyllous tissue.

Detection of Emerald Ash Borer Infestations in Living Green Ash by Noninvasive Electronic-Nose Analysis of Wood Volatiles

The emerald ash borer (EAB) has been the most destructive and costly nonnative insect to threaten the health of ash (Fraxinus) species in North America for at least the past 25 years. The development of methods for detecting visually-hidden EAB galleries at early stages of infestation would provide a useful tool to more effectively facilitate the planning and implementation of targeted EAB pest-suppression and management activities. We tested the efficacy of using a dual-technology electronic-nose (e-nose)/gas chromatograph device as a means for detection of EAB infestations in green ash trees in different EAB-decline classes by analysis of VOC emissions in sapwood. We found significant differences in VOC profiles for trees from the four decline classes. The VOC composition, quantities, and types of volatile metabolites present in headspace volatiles varied considerably across sample types, and resulted in distinct e-nose smellprint patterns that were characteristic of each unique chemical composition. In addition, specific VOC metabolites were identified as potential healthy and EAB-infestation biomarkers, indicative of the health states of individual trees. Few significant differences in major bark phenolic compounds were found between ash decline classes using LC-MS. The e-nose was effective in discriminating between uninfested and EAB-infested trees based on sapwood VOC emissions.

Oxidative enzymes activity during abiotic and biotic stresses in Zea mays leaves and roots exposed to Cu, methyl jasmonate and Trigonotylus caelestialium

The activities of antioxidative enzymes, i.e. superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX), in the leaves and roots of Zea mays L. plants exposed to abiotic (methyl jasmonate, MJ, or/and copper, Cu) and biotic (Trigonotylus caelestialium) factors were examined. The contribution of MJ as a signal molecule in the defense mechanism against abiotic and biotic stresses was studied. All plants were cultivated hydroponically and divided into three groups: not treated by abiotic factors (control), treated by MJ only (MJ) and by MJ and Cu (MJ + Cu) and in each group half of the plants were exposed to T. caelestialium attack. The enzymatic activities of SOD, CAT, APX, and GPX in the leaves were higher in the insect-treated than non-insect-treated control plants, but lower in both MJ + Cu- or MJ- and insect-treated plants. In the roots, the enzyme activities were elevated in all insect-treated plants with the highest rise in MJ + Cu, in comparison with the MJ-treated plants. The results showed that MJ and MJ + Cu were efficient in reducing the activity of the antioxidative enzymes in the leaves under the insect influence by elevating enzyme activity in the roots.