Alieta Eyles

Induced resistance to pests and pathogens in trees

Tree resistance can be enhanced by a variety of biotic and abiotic inducers, including nonpathogenic and pathogenic microbes, and herbivores, resulting in enhanced protection against further biotic injury. Induced resistance (IR) could be a valuable tool in sustainable pest management. IR has been actively studied in herbaceous plant species, and, in recent years, in woody plant species, and is fast emerging as an intriguing, eco-friendly concept for enhancing tree resistance. However, before application of IR becomes possible, there is a need to increase our knowledge of the mechanisms of defence in forest trees. A richer understanding of these phenomena will play a critical role in developing sustainable integrated pest management strategies. This review summarizes our current knowledge of IR in forest trees, focusing on inducible defence mechanisms, systemic induction of resistance and phytohormone signalling networks. We conclude by discussing the potential advantages and limitations of applying IR-based management tools in forest systems.

Phenolic Metabolites in Leaves of the Invasive Shrub, Lonicera maackii, and Their Potential Phytotoxic and Anti-Herbivore Effects

Lonicera maackii is an invasive shrub in North America for which allelopathic effects toward other plants or herbivores have been suspected. We characterized the major phenolic metabolites present in methanol extracts of L. maackii leaves. In addition, we examined the effects of methanol–water extracts of L. maackii leaves on seed germination of a target plant species and on feeding preference and growth rate of a generalist insect herbivore. A total of 13 individual major and minor compounds were detected in crude leaf extracts by high-performance liquid chromatography coupled to electronspray ionization-tandem mass spectrometry (ESI-MS/MS). Extracts were dominated by two major flavones, apigenin and luteolin, and their glucoside derivatives, apigenin-7-glucoside and luteolin-7-glucoside. Quantities of these compounds, along with chlorogenic acid, varied between two sampling points. Leaf extracts that contained these compounds were inhibitory to seed germination of Arabidopsis thaliana. In addition, treatment of artificial diet with leaf extracts deterred feeding of the generalist herbivore, Spodoptera exigua, in choice experiments but had no effect on growth rate in short-term no-choice bioassays. Purified apigenin tended to deter feeding by S. exigua and inhibited seed germination of A. thaliana. We conclude that leaves of L. maackii contain phenolic compounds, including apigenin and chlorogenic acid, capable of having biological effects on other plants and insects.

Comparative Phloem Chemistry of Manchurian (Fraxinus mandshurica) and Two North American Ash Species (Fraxinus americana and Fraxinus pennsylvanica)

Recent studies have investigated interspecific variation in resistance of ash (Fraxinus spp.) to the exotic wood-boring beetle, emerald ash borer (EAB, Agrilus planipennis). Manchurian ash (Fraxinus mandshurica) is an Asian species that has coevolved with EAB. It experiences little EAB-induced mortality compared to North American ashes. Host phloem chemistry, both constitutive and induced, might partly explain this interspecific variation in resistance. We analyzed the constitutive phloem chemistry of three ash species: Manchurian ash and North American white (Fraxinus americana) and green (Fraxinus pennsylvanica) ash. Analysis of the crude phloem extracts revealed the presence of an array of phenolic compounds including hydroxycoumarins, a monolignol, lignans, phenylethanoids, and secoiridoids. Both qualitative and quantitative differences were observed among the three ash species. Hydroxycoumarins and the phenylethanoids, calceloariosides A and B, were present only in the phloem of Manchurian ash and might represent a mechanism of resistance against EAB.

Shifts in biomass and resource allocation patterns following defoliation in Eucalyptus globulus growing with varying water and nutrient supplies.

In woody species, potential mechanisms to compensate for tissue loss to herbivory and diseases have been related to post-event shifts in growth, biomass and internal resource allocation patterns, as modulated by external resource limitations. We examined the interactive effects of belowground resource limitations by varying nutrient and water availability, and aboveground carbon limitation imposed by a single defoliation event (40% leaf removal) on stem growth, whole-tree and within-tree resource allocation patterns (total non-structural carbohydrate and nitrogen) and below- and aboveground biomass allocation patterns in 8-month-old, field-grown Eucalyptus globulus Labill. saplings. Two months after treatments were imposed, the direction of the stem growth response to defoliation depended on the abiotic treatment. Five months after defoliation, however, we found little evidence that resource availability constrained the expression of tolerance to defoliation. With the exception of the combined low-nutrient and low-water supply treatment, saplings grown with (1) adequate water and nutrient supplies and even with (2) low-water supply or (3) low-nutrient supply were able to compensate for the 40% foliage loss. The observed compensatory responses were attributed to the activation of several short- and longer-term physiological mechanisms including reduced biomass allocation to coarse roots, mobilization of carbohydrate reserves, robust internal N dynamics and increased ratio of foliage to wood dry mass.

Stable and Extreme Resistance to Common Scab of Potato Obtained Through Somatic Cell Selection

Somatic cell selection with thaxtomin A as a positive selection agent was used to isolate variants of potato cv. Russet Burbank with strong to extreme resistance to common scab. Glasshouse and field trials identified 51 variants with significantly reduced disease incidence (frequency of infected tubers) and severity (tuber lesion coverage) compared with the parent cultivar. The most promising variants exhibited extreme disease resistance, rarely showing lesions, which were invariably superficial and shallower than those on the parent. Resistance traits were consistently expressed both in 10 glasshouse and two field trials at different locations, with varied inoculum and disease pressure. Disease-resistant variants differed in their response to thaxtomin A in tuber slice bioassays. Of 23 variants tested, 10 showed reduced thaxtomin A susceptibility, with the remaining 13 responding similar to that of the parent. Thus, toxin tolerance was not the only factor responsible for observed disease resistance; however, four of the five most disease-resistant variants had enhanced thaxtomin A tolerance, suggesting that this factor is important in the expression of strong disease resistance. Pathogenicity and toxin tolerance remained stable over a 6-year period, demonstrating that selected phenotypes were robust and genetic changes stable. The majority of disease-resistant variants had tuber yields equivalent to the parent cultivar in glasshouse trials. This suggests that selection for disease resistance was not associated with negative tuber attributes and that certain variants may have commercial merit, worthy of further agronomic testing.

Cross-induction of Systemic Induced Resistance between an Insect and a Fungal Pathogen in Austrian Pine over a Fertility Gradient

Evidence for cross-induction of systemic resistance or susceptibility in plant–fungus–herbivore interactions is mostly derived from herbaceous model systems and not perennial woody plants. Furthermore, the effects of environmental variables such as soil fertility on these tripartite interactions are generally unknown. This study examined cross-induction of systemic resistance in Pinus nigra (Austrian pine) to infection by Sphaeropsis sapinea (a fungal pathogen), or feeding by Neodiprion sertifer (European pine sawfly), by prior induction with either S. sapinea or N. sertifer, over a fertility gradient. In a replicated 3-year study, cross-induction of systemic induced resistance (SIR) was found to be both asymmetric within a single year and variable between years. Prior induction with insect defoliation induced SIR to subsequent fungal challenge in 2006 but not in 2005. In 2005, a fertility-independent negative systemic effect of the fungal infection on herbivore growth was detected while herbivore survival was affected by a significant interaction between induction treatment and fertility level in 2006. Prior infection by the fungus induced SIR against the same fungus in both years regardless of fertility levels. This is the first report of whole-plant SIR against a defoliating insect induced by a fungal pathogen and vice versa, under variable nutrient availability, in a conifer or any other tree.

Are gas exchange responses to resource limitation and defoliation linked to source:sink relationships?

Productivity of trees can be affected by limitations in resources such as water and nutrients, and herbivory. However, there is little understanding of their interactive effects on carbon uptake and growth. We hypothesized that: (1) in the absence of defoliation, photosynthetic rate and leaf respiration would be governed by limiting resource(s) and their impact on sink limitation; (2) photosynthetic responses to defoliation would be a consequence of changing source:sink relationships and increased availability of limiting resources; and (3) photosynthesis and leaf respiration would be adjusted in response to limiting resources and defoliation so that growth could be maintained. We tested these hypotheses by examining how leaf photosynthetic processes, respiration, carbohydrate concentrations and growth rates of Eucalyptus globulus were influenced by high or low water and nitrogen (N) availability, and/or defoliation. Photosynthesis of saplings grown with low water was primarily sink limited, whereas photosynthetic responses of saplings grown with low N were suggestive of source limitation. Defoliation resulted in source limitation. Net photosynthetic responses to defoliation were linked to the degree of resource availability, with the largest responses measured in treatments where saplings were ultimately source rather than sink limited. There was good evidence of acclimation to stress, enabling higher rates of C uptake than might otherwise have occurred.

Photosynthetic responses of field-grown Pinus radiata trees to artificial and aphid-induced defoliation

The phloem-feeding aphid Essigella californica represents a potential threat to the productivity of Pinus radiata plantations in south-eastern Australia. Five- and nine-year-old field trials were used to characterize the effects of artificial and natural aphid-induced (E. californica) defoliation, respectively, on shoot photosynthesis and growth. Photosynthetic capacity (A(max)) was significantly greater following a 25% (D25) (13.8 µmol m(-2) s(-1)) and a 50% (D50) (15.9 µmol m(-2) s(-1)) single-event upper-crown artificial defoliation, 3 weeks after defoliation than in undefoliated control trees (12.9 µmol m(-2) s(-1)). This response was consistently observed for up to 11 weeks after the defoliation event; by Week 16, there was no difference in A(max) between control and defoliated trees. In the D50 treatment, this increased A(max) was not sufficient to fully compensate for the foliage loss as evidenced by the reduced diameter increment (by 15%) in defoliated trees 36 weeks after defoliation. In contrast, diameter increment of trees in the D25 treatment was unaffected by defoliation. The A(max) of trees experiencing upper-crown defoliation by natural and repeated E. californica infestations varied, depending on host genotype. Despite clear differences in defoliation levels between resistant and susceptible genotypes (17 vs. 35% of tree crown defoliated, respectively), growth of susceptible genotypes was not significantly different from that of resistant genotypes. The observed increases in A(max) in the lower crown of the canopy following attack suggested that susceptible genotypes were able to partly compensate for the loss of foliage by compensatory photosynthesis. The capacity of P. radiata to regulate photosynthesis in response to natural aphid-induced defoliation provides evidence that the impact of E. californica attack on stem growth will be less than expected, at least for up to 35% defoliation.

Novel Detection of Formylated Phloroglucinol Compounds (FPCs) in the Wound Wood of Eucalyptus globulus and E. nitens

This study characterized the chemical responses of Eucalyptus globulus and Eucalyptus nitens to artificial inoculation with a basidiomycete decay fungus. Nine-year-old trees responded to mechanical wounding or inoculation with the decay fungus by producing new wound wood characterized by the presence of dark extractives 17 months after wounding. Analysis of crude wound wood extracts by HPLC coupled to negative ion electrospray mass spectrometry revealed the presence of a complex mixture of many unidentified formylated phloroglucinol compounds (FPCs), in addition to a diverse range of other polyphenolic compounds (hydrolyzable tannins, proanthocyanidins, flavanone glycoside, stilbene glycosides). Prior to this study, FPCs have only been reported from leaves and buds of Eucalyptus spp. Unequivocal evidence for the presence of macrocarpal A and B, and sideroxylonal A and B in the crude extracts was obtained, as well as evidence for a wide range of as yet unreported FPCs. Subsequent preliminary in vitro fungal and bacterial bioassays did not support an antimicrobial role for FPCs in host-pathogen interactions in eucalypts.

Whole-plant versus leaf-level regulation of photosynthetic responses after partial defoliation in Eucalyptus globulus saplings

Increases in photosynthetic capacity (A1500) after defoliation have been attributed to changes in leaf-level biochemistry, water, and/or nutrient status. The hypothesis that transient photosynthetic responses to partial defoliation are regulated by whole-plant (e.g. source–sink relationships or changes in hydraulic conductance) rather than leaf-level mechanisms is tested here. Temporal variation in leaf-level gas exchange, chemistry, whole-plant soil-to-leaf hydraulic conductance (KP), and aboveground biomass partitioning were determined to evaluate mechanisms responsible for increases in A1500 of Eucalyptus globulus L. potted saplings. A1500 increased in response to debudding (B), partial defoliation (D), and combined B&D treatments by up to 36% at 5 weeks after treatment. Changes in leaf-level factors partly explained increases in A1500 of B and B&D treatments but not for D treatment. By week 5, saplings in B, B&D, and D treatments had similar leaf-specific KP to control trees by maintaining lower midday water potentials and higher transpiration rate per leaf area. Whole-plant source:sink ratios correlated strongly with A1500. Further, unlike KP, temporal changes in source:sink ratios tracked well with those observed for A1500. The results indicate that increases in A1500 after partial defoliation treatments were largely driven by an increased demand for assimilate by developing sinks rather than improvements in whole-plant water relations and changes in leaf-level factors. Three carbohydrates, galactional, stachyose, and, to a lesser extent, raffinose, correlated strongly with photosynthetic capacity, indicating that these sugars may function as signalling molecules in the regulation of longer term defoliation-induced gas exchange responses.