Don Cipollini

NOVEL WEAPONS: INVASIVE PLANT SUPPRESSES FUNGAL MUTUALISTS IN AMERICA BUT NOT IN ITS NATIVE EUROPE

Why some invasive plant species transmogrify from weak competitors at home to strong competitors abroad remains one of the most elusive questions in ecology. Some evidence suggests that disproportionately high densities of some invaders are due to the release of biochemicals that are novel, and therefore harmful, to naive organisms in their new range. So far, such evidence has been restricted to the direct phytotoxic effects of plants on other plants. Here we found that one of North Americas most aggressive invaders of undisturbed forest understories, Alliaria petiolata (garlic mustard) and a plant that inhibits mycorrhizal fungal mutualists of North American native plants, has far stronger inhibitory effects on mycorrhizas in invaded North American soils than on mycorrhizas in European soils where A. petiolata is native. This antifungal effect appears to be due to specific flavonoid fractions in A. petiolata extracts. Furthermore, we found that suppression of North American mycorrhizal fungi by A. petiolata corresponds with severe inhibition of North American plant species that rely on these fungi, whereas congeneric European plants are weakly affected. These results indicate that phytochemicals, benign to resistant mycorrhizal symbionts in the home range, may be lethal to naïve native mutualists in the introduced range and indirectly suppress the plants that rely on them.

Salicylic acid inhibits jasmonic acid-induced resistance of Arabidopsis thaliana to Spodoptera exigua

The role of salicylic acid (SA) in plant responses to pathogens has been well documented, but its direct and indirect effects on plant responses to insects are not so well understood. We examined the effects of SA, alone and in combination with jasmonic acid (JA), on the performance of the generalist herbivore, Spodoptera exigua, in wild‐type and mutant Arabidopsis thaliana genotypes that varied genetically in their ability to mount SA‐ and JA‐mediated defence responses. In one experiment, growth of S. exigua larvae was highest on the Wassilewskija wild‐type, intermediate on the Columbia wild‐type and the JA‐deficient fad mutant, and lowest on the nim1‐1 and jar1‐mutants, which are defective in the SA and JA pathways, respectively. Activity of guaiacol peroxidase, polyphenoloxidase, n‐acetylglucosaminidase, and trypsin inhibitor varied by genotype but did not correlate with insect performance. SA treatment increased growth of S. exigua larvae by ∼35% over all genotypes, but had no discernable effect on activities of the four defence proteins. In a second experiment, growth of S. exigua was highest across treatments on the cep1 mutant, a constitutive expressor of high SA levels and systemic acquired resistance, and lowest on the fad mutant, which is JA‐deficient. JA treatment generally increased activity of all four defence proteins, increased total glucosinolate levels and reduced insect growth by ∼25% over all genotypes. SA generally inhibited expression of JA‐induced resistance to S. exigua when both hormones were applied simultaneously. Across genotypes and treatments, larval mass was negatively correlated with the activity of trypsin inhibitor and polyphenoloxidase and with total glucosinolate levels, and insect damage was negatively correlated with the activity of polyphenoloxidase. SA had little effect on the induction of defence protein activity by JA. However, SA attenuated the induction of glucosinolates by JA and therefore may explain better the interactive effects of SA and JA on insect performance. This study illustrates that direct and indirect cross‐effects of SA on resistance to S. exigua can occur in A. thaliana. Effects of SA may be mediated through effects on plant defence chemistry or other aspects of the suitability of foliage for insect feeding and growth.

Costs of induced responses in plants

Abstract Induced responses to herbivores in plants are thought to be a form of adaptive phenotypic plasticity, whereby plants save metabolic costs by expressing defenses only when they are necessary. Although costs of being inducible have never been examined, there have been many studies of costs of induced responses, with approaches ranging from the manipulation of induction with herbivores or wounding, and more recently, with chemical elicitors. While some early studies reported no significant costs of induced responses, evidence for significant costs of induced responses in both wild and agricultural species has recently increased in the literature. Recent studies have made it clear that benefits of induced responses in the field in the presence of herbivores can counterbalance costs of induced responses seen in the absence of herbivores. Moreover, as has been shown for constitutive resistance, costs of induced responses may vary with environmental conditions, including the presence of competitors and altered resource availability. Ecological costs of induced responses may include increased susceptibility to untargeted herbivores, either through the altered attraction of specific herbivores or due to defense pathway cross-talk, but the actual fitness consequences of such ecological costs have been little studied. Mutant and transgenic plants altered in induced responses are increasingly being identified or produced, as are specific elicitors of direct and indirect defenses. Their use, coupled with the increasing ability to analyze global gene expression in plants, will advance studies on the specificity, magnitude, and mechanisms of costs of induced responses. Induzierte Antworten von Pflanzen auf Angriffe durch Herbivore werden als eine Form adaptiver phanotypischer Plastizitat betrachtet, wobei Pflanzen metabolische Kosten sparen, wenn sie Abwehr nur dann mobilisieren, wenn sie auch wirklich benotigt wird. Wahrend Kosten der Induzierbarkeit bisher nie untersucht wurden, gibt es viele Studien zu Kosten von induzierten Antworten. In diesen Studien wird Induktion durch Herbivore, mechanische Verwundung, oder durch chemische Elicitoren manipuliert. Wahrend einige erste Studien keine signifikanten Kosten der induzierten pflanzlichen Antworten fanden, ergaben neuere Untersuchungen Anhaltspunkte fur deutliche Kosten der Induktion sowohl bei Wild- wie auch bei Kulturpflanzen. Die neueren Studien zeigen deutlich, dass der Nutzen, der aus induzierten Antworten in Gegenwart von Herbivoren gezogen werden kann, die Kosten ausbalanciert, die in Abwesenheit von Herbivoren investiert werden. Ebenso wie konstitutive Resistenz kann auch die induzierte Antwort mit den verschiedenen Umweltfaktoren wie z.B. Konkurrenz und Nahrungsverfugbarkeit variieren. Die okologischen Kosten induzierter Antworten konnen auch zu einer erhohten Anfalligkeit gegenuber solchen spezialisierten Herbivoren fuhren, die durch die induzierte Antwort sogar angelockt werden. Weitere, allerdings bisher wenig untersuchte okologische Kosten konnten dadurch entstehen, dass Wechselwirkungen zwischen induzierten Abwehr-Biogenesewegen auftreten. Pflanzenmutanten oder transgene Pflanzen mit veranderter induzierter Antwort werden mehr und mehr identifiziert oder produziert. Auch der Kenntnisstand uber Elicitoren der direkten und indirekten Verteidigung erweitert sich zunehmend. Wenn genetisch veranderte Pflanzen genutzt werden und die Kenntnisse uber die Elicitoren ebenso eingesetzt werden wie die Fahigkeit zur Analyse der Genexpression in Pflanzen, dann konnen die Untersuchungen zur Spezifitat, zum Ausmas, und den Mechanismen der Kosten von induzierten Antworten der Pflanzen auf Herbivorenbefall Fortschritte machen.

Leaf and root extracts of the invasive shrub, Lonicera maackii, inhibit seed germination of three herbs with no autotoxic effects

In addition to effects mediated by resource competition, some invasive plants may impact surrounding vegetation by secreting compounds that are directly inhibitory to growth. Lonicera maackii, an invasive Asian shrub of forests and open areas in eastern and midwestern North America, has devastating effects on understory vegetation, some of which persist even after this shrub is removed. In this study, we explored the potential of aqueous extracts of the leaves and roots of this plant to inhibit seed germination of Impatiens capensis, Alliaria petiolata, Arabidopsis thaliana, and L. maackii in Petri dish bioassays. Both L. maackii root and leaf extracts significantly decreased germination in the three herb species. This inhibitory effect generally increased with increasing extract concentration and was more pronounced with application of leaf extract than root extract. However, when the same extracts were applied to seeds of L. maackii itself, germination was delayed in some cases, but was not significantly reduced by the end of the experiment. Germination of L. maackii seeds even reached significantly higher levels in some extract treatments than in no-extract controls. This implies that L. maackii can successfully inhibit the germination of other plants with few autotoxic effects and may even promote the germination of its own seeds.

Plant Density and Nutrient Availability Constrain Constitutive and Wound-induced Expression of Trypsin Inhibitors in Brassica napus

We investigated the effects of plant density on plant size, leaf total soluble protein content, and constitutive and wound-induced levels of proteinaceous trypsin inhibitors in pot-grown Brassica napus seedlings in two greenhouse studies. We manipulated plant density by varying the number of intraspecific neighbors surrounding a target plant in the center of each pot. In general, constitutive and induced levels of trypsin inhibitors were significantly reduced by competition in a density-dependent manner, to the extent that induction was greatly reduced or abolished in target plants surrounded by six neighbors. To investigate whether the effects of plant density on inhibitor production were mediated by nutrient availability, we manipulated the concentration of a complete fertilizer applied to target plants surrounded by six neighbors in two greenhouse studies. In general, constitutive and wound-induced levels of inhibitors in plants surrounded by six neighbors were increased by nutrient addition in a dose-dependent manner, such that wound-induction was completely restored in competing plants under conditions of high nutrient availability. Leaf total soluble protein content, measured only in the second trial of each experiment, was not affected by any of the treatments. The effects of plant density, nutrient addition, and wounding on inhibitor levels in all experiments were independent of their effecs on above-ground plant size at the time of wounding. Overall, our results suggest that decreasing nutrient availability mediates the density-dependent reductions in inhibitor levels in B. napus seedings.

STRETCHING THE LIMITS OF PLASTICITY: CAN A PLANT DEFEND AGAINST BOTH COMPETITORS AND HERBIVORES?

Plants exhibit numerous adaptive responses to competitors and herbivores mediated by hormones, including induction of the shade avoidance syndrome in response to neighbor shade, and induction of chemical defenses in response to herbivore damage. Although largely studied in isolation, interactions between plastic responses to competitors and herbivores can occur and may be mediated by external and internal resource availability, direct interactions between hormones controlling plastic responses, or other pleiotropic effects. Such interactions may have positive or negative consequences for plant fitness and could serve as an important ecological and evolutionary constraint on the expression of these responses. In this article, I illustrate the potential for plastic responses in plants to physiologically interact, with particular reference to Arabidopsis thaliana. I also suggest avenues for future study utilizing characterized plant material, tools to manipulate the expression of plastic responses, and ways to monitor physiological and ecological consequences that are available using this and other plant species.

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.

Expression of Constitutive and Inducible Chemical Defenses in Native and Invasive Populations of Alliaria petiolata

The Evolution of Increased Competitive Ability (EICA) hypothesis posits that invasive plants in introduced habitats with reduced herbivore pressure will evolve reduced levels of costly resistance traits. In light of this hypothesis, we examined the constitutive and inducible expression of five chemical defense traits in Alliaria petiolata from four invasive North American and seven native European populations. When grown under common conditions, significant variation among populations within continents was found for trypsin inhibitors and peroxidase activity, and glucosinolates and trypsin inhibitors were significantly jasmonate-inducible across populations. Across populations, constitutive levels of glucosinolates and trypsin inhibitors were negatively correlated with their degree of induction, with three North American populations tending to have lower constitutive levels and higher inducibility of glucosinolates than the seven European populations. Alliarinoside and isovitexin 6″-O-β-d-glucopyranoside levels were both higher in North American plants than in European plants, but levels of these compounds were generally increased by jasmonate in European plants and decreased by the same treatment in North American plants. Aside from the tendency for invasive populations to have reduced constitutive glucosinolate levels coupled with increased inducibility, little support for the predictions of EICA was evident in the chemical defenses that we studied.

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.

Does competition magnify the fitness costs of induced responses in Arabidopsis thaliana ? A manipulative approach

Studies have increasingly shown that the constitutive or induced expression of resistance in plants is costly to fitness in the absence of enemy attack. If such costs are based on resource allocation tradeoffs, it has been hypothesized that resource limitation associated with plant competition increases the fitness costs of resistance. In two greenhouse studies, I examined the expression and costs of induced responses in pot-grown Arabidopsis thaliana grown alone or surrounded by six intraspecific neighbors. In the first study, I manipulated the expression of systemic acquired resistance (SAR) with the application of salicylic acid and monitored peroxidase activity and total seed production in treated plants. I used five lines of A. thaliana that varied in their competence to express SAR in this study. In the second study, I manipulated the expression of induced resistance (IR) with the application of jasmonic acid and monitored trypsin inhibitor activity and total seed production in treated plants. I used two lines of A. thaliana that varied in the competence to express IR in this study. Lines varied in their chemical and fitness responses to hormone treatment and competition in each study, but the application of wound-related hormones significantly increased levels of chemical defenses and reduced total seed production by an average of 15% overall in both studies. Competition reduced peroxidase activity by 6% in the first study, had no effect on trypsin inhibitor levels in the second study, and reduced total seed production by an average of 35% overall in both studies. However, there was no interaction between competition and hormone treatment on total seed production in either study. Induced responses were costly to fitness in A. thaliana, but competition had no effect on the magnitude of costs.