Immunological memory of mountain birches: effects of phenolics on performance of the autumnal moth depend on herbivory history of trees

Elm tree defences against a specialist herbivore are moderately primed by an infestation in the previous season

The studies of the long-term effects of insect infestations on plant anti-herbivore defences tend to focus on feeding-induced damage. Infestations by an entire insect generation, including egg depositions as well as the feeding insects, are often neglected. Whilst there is increasing evidence that the presence of insect eggs can intensify plants' anti-herbivore defences against hatching larvae in the short term, little is known about how insect infestations, including insect egg depositions, affect plant defences in the long term. We addressed this knowledge gap by investigating long-term effects of insect infestation on elm's (Ulmus minor Mill. cv. 'Dahlem') defences against subsequent infestation. In greenhouse experiments, elms were exposed to elm leaf beetle (ELB, Xanthogaleruca luteola) infestation (adults, eggs and larvae). Thereafter, the trees cast their leaves under simulated winter conditions and were re-infested with ELB after the regrowth of their leaves under simulated summer conditions. Elm leaf beetles performed moderately worse on previously infested elms with respect to several developmental parameters. The concentrations of the phenylpropanoids kaempferol and quercetin, which are involved in egg-mediated, short-term effects on elm defences, were slightly higher in the ELB-challenged leaves of previously infested trees than in the challenged leaves of naïve trees. The expression of several genes involved in the phenylpropanoid pathway, jasmonic acid signalling, and DNA and histone modifications appeared to be affected by ELB infestation; however, prior infestation did not alter the expression intensities of these genes. The concentrations of several phytohormones were similarly affected in the currently challenged leaves of previously infested trees and naïve trees. Our study shows that prior infestation of elms by a specialised insect leads to moderately improved defences against subsequent infestation in the following growing season. Prior infestation adds a long-term effect to the short-term enhancer effect that plants show in response to egg depositions when defending against hatching larvae.

Responses of Six Wheat Cultivars (Triticum aestivum) to Wheat Aphid (Sitobion avenae) Infestation

Simple Summary

Sitobion avenae Fabricius is an important wheat aphid species in China, causing significant losses to wheat production. Improving host-plant resistance is an effective and environmentally friendly method of aphid control. Sitobion avenae resistance and the total phenolic and flavonoid content accumulation of six wheat cultivars to S. avenae infestation were investigated to elucidate responses of six wheat varieties against S. avenae. Among the six tested wheat cultivars, Yongliang No.15 and Ganchun No.18 demonstrated high resistance to S. avenae. The correlation analysis revealed a positive relationship between total phenol and flavonoid content accumulation and developmental duration (DD), and a negative relationship between accumulation and weight gain (WG) and mean relative growth rate (MRGR). The correlation between flavonoid and biological parameters was statistically stronger than total phenol. Our findings could serve as a theoretical basis for further research into the resistance mechanism of wheat varieties to S. avenae.

Abstract

Resistant variety screening is widely recommended for the management of Sitobion avenae. The purpose of this study was to assess responses of six wheat varieties (lines) to S. avenae. The aphid quantity ratio (AQR) was used to assess S. avenae resistance. Pearson’s correlation coefficient was used to perform a correlation analysis between AQR, biological parameters, and the accumulation of total phenolic and flavonoid content. When compared to the other cultivars, the results showed that two cultivars, Yongliang No.15 and Ganchun No.18, had high resistance against S. avenae. The correlation analysis revealed a positive relationship between total phenol and flavonoid content accumulation and developmental duration (DD), and a negative relationship between accumulation and weight gain (WG) and mean relative growth rate (MRGR). The correlation between flavonoid and biological parameters was statistically stronger than the correlation between total phenol and biological parameters. This research provides critical cues for screening and improving aphid-resistant wheat varieties in the field and will aid in our understanding of the resistance mechanism of wheat varieties against S. avenae.

Making the Shift from Research to Commercial Orchards: A Case Study in Aphid-Peach Tree Interactions as Affected by Nitrogen and Water Supplies

Simple Summary

Commercial orchards are amongst the most intensively sprayed crops, and alternative methods have to be found to replace pesticides. Limiting water and nitrogen (N) supply has shown to be effective in reducing aphid infestations under controlled conditions. To evaluate how far these techniques could be transferred to orchards subject to production constraints, an experiment was performed in a commercial orchard planted with two varieties differing in precocity and vigour. Limiting supplies of both water and N to trees was shown to reduce the severity of aphid infestation (green peach aphid, mealy plum aphid, and leaf curl aphid), although reducing only water supply was less effective. At shoot level, the composition and development of the infested shoots were only slightly affected by treatment, thereby indicating that aphids colonize shoots of similar condition, whose numbers are modulated by nutrition treatments. These results were consistent with variety and year. Limiting water and N supplies contributes not only to the control of aphid infestations, but also reduces nitrate leaching and the use of water, the consumption of which will inevitably need to a decrease due to climate change. However, the efficiency of aphid control could be enhanced by complementing these practices by other techniques such as adapted pruning or changes to ground cover.

Abstract

Peach orchards are intensively sprayed crops, and alternative methods must be found to replace pesticides. We intend here to evaluate if limiting water and nitrogen (N) supply could be effective in controlling aphid infestation in commercial orchards. N and water supply were therefore either unrestricted or restricted by 30% only for water, or for both water and N, in 2018 and 2019 on trees of two contrasting varieties. Natural infestations (green peach aphid, mealy plum aphid, leaf curl aphid) were monitored regularly at tree and shoot level. Infested and control shoots were compared for their development during the infestation period, their apex concentrations of total N, amino acids, non-structural carbohydrates, and polyphenols at infestation peak. At tree level, limiting both water and N supplies decreased the proportion of infested shoots by 30%, and the number of trees hosting the most harmful specie by 20 to 50%. Limiting only N supplies had almost no effect on infestation severity. At shoot level, the apex N concentration of infested shoots was stable (around 3.2% dry weight) and was found to be independent of treatment, variety, and year. The remaining biochemical variables were not affected by infestation status but by variety and year. Shoot development was only slightly affected by treatment. Aphids colonized the most vigorous shoots, being those with longer apical ramifications in 2018 and higher growth rates in 2019, in comparison with the controls. The differences were, respectively, 40 and 55%. It was concluded that a double restriction in water and N could limit, but not control, aphid infestations in commercial orchards.

Cyclic voltammetry of volatile memristors in the Venus flytrap: short-term memory

Plants have sensory, short-term and long-term memory. Possible candidates for memory in plants are memristors; resistors with memory. Memristors have been found in seeds, plants, flowers and fruits. The electrostimulation of plants by bipolar periodic waves can induce electrical responses with fingerprints of volatile or non-volatile memristors. Here, we show that the electrostimulation of the Venus flytrap (Dionaea muscipula Ellis) by unipolar sinusoidal or triangular periodic electrical trains induces electrical responses in plants with fingerprints of volatile memristors. The discovery of volatile generic memristors in plants opens new directions in the modelling and understanding of electrical phenomena in the plant kingdom.

Functional Characterization of Chemosensory Protein AmalCSP5 From Apple Buprestid Beetle, Agrilus mali (Coleoptera: Buprestidae)

In the sensitive and complex chemo-sensation system of insects, chemosensory proteins (CSPs) can facilitate the transfer of chemical information and play important roles for variable behaviors of insects. We cloned the chemosensory protein AmalCSP5 from antennae of the apple buprestid beetle (Agrilus mali Matsumura), a serious invasive pest of wild apple trees. Expression profiling showed that AmalCSP5 was expressed in various tissues, suggesting its significance in multiple physiological activities and behaviors of A. mali. AmalCSP5 was preferentially expressed in female antennae and male abdomens. AmalCSP5 was able to bind a variety of test volatiles, especially alcohols and esters. AmalCSP5 exhibited good binding affinity for all five test secondary compounds (i.e., procyanidin, phlorizin, kaemferol, chlorogenic acid, and rutin), suggesting its preferential binding abilities to nonvolatile host plant secondary metabolites and critical roles in gustatory perception of nonvolatiles. Tyr27 and Ser69 of AmalCSP5 could form hydrogen bonds with hexyl benzoate and hexyl hexanoate, respectively. Procyanidin, the best ligand among all test compounds, could form hydrogen bonds with three amino acid residues (i.e., Arg7, Leu8, and Lys41) of AmalCSP5. Thus, high ligand binding affinity for AmalCSP5 seemed to be dependent mainly on the formation of hydrogen bonds. The putative key amino acid residues of AmalCSP5 can be used as molecular targets for designing and screening new attractants and repellents for A. mali. Our results provide insights into binding interactions of AmalCSP5 with volatile and nonvolatile ligands, and a firm basis for developing eco-friendly management strategies of A. mali.

Plants are intelligent, here's how

HYPOTHESES

The drive to survive is a biological universal. Intelligent behaviour is usually recognized when individual organisms including plants, in the face of fiercely competitive or adverse, real-world circumstances, change their behaviour to improve their probability of survival.

SCOPE

This article explains the potential relationship of intelligence to adaptability and emphasizes the need to recognize individual variation in intelligence showing it to be goal directed and thus being purposeful. Intelligent behaviour in single cells and microbes is frequently reported. Individual variation might be underpinned by a novel learning mechanism, described here in detail. The requirements for real-world circumstances are outlined, and the relationship to organic selection is indicated together with niche construction as a good example of intentional behaviour that should improve survival. Adaptability is important in crop development but the term may be complex incorporating numerous behavioural traits some of which are indicated.

CONCLUSION

There is real biological benefit to regarding plants as intelligent both from the fundamental issue of understanding plant life but also from providing a direction for fundamental future research and in crop breeding.

Phytochemical Shift from Condensed Tannins to Flavonoids in Transgenic Betula pendula Decreases Consumption and Growth but Improves Growth Efficiency of Epirrita autumnata Larvae

Despite active research, antiherbivore activity of specific plant phenolics remains largely unresolved. We constructed silver birch (Betula pendula) lines with modified phenolic metabolism to study the effects of foliar flavonoids and condensed tannins on consumption and growth of larvae of a generalist herbivore, the autumnal moth (Epirrita autumnata). We conducted a feeding experiment using birch lines in which expression of dihydroflavonol reductase (DFR), anthocyanidin synthase (ANS) or anthocyanidin reductase (ANR) had been decreased by RNA interference. Modification-specific effects on plant phenolics, nutrients and phenotype, and on larval consumption and growth were analyzed using uni- and multivariate methods. Inhibiting DFR expression increased the concentration of flavonoids at the expense of condensed tannins, and silencing DFR and ANR decreased leaf and plant size. E. autumnata larvae consumed on average 82% less of DFRi plants than of unmodified controls, suggesting that flavonoids or glandular trichomes deter larval feeding. However, larval growth efficiency was highest on low-tannin DFRi plants, indicating that condensed tannins (or their monomers) are physiologically more harmful than non-tannin flavonoids for E. autumnata larvae. Our results show that genetic manipulation of the flavonoid pathway in plants can effectively be used to produce altered phenolic profiles required for elucidating the roles of low-molecular weight phenolics and condensed tannins in plant–herbivore relationships, and suggest that phenolic secondary metabolites participate in regulation of plant growth.

Insect community structure covaries with host plant chemistry but is not affected by prior herbivory

By feeding on plant tissue, insect herbivores can change several characteristics of their hosts. These changes have the potential to alter the quality of the plant for other herbivore species, potentially altering the structure of the community of species attacking the plant at a later point in time. We tested whether herbivory early in the season changes host plant performance, polyphenol chemistry, and the community structure of sessile herbivores later in the season. We experimentally manipulated densities of early-season moth caterpillars on a set of young oak trees and measured tree growth, reproduction, leaf chemistry, and the abundance and community composition of leafmining and galling species later in the season. The experimental manipulations of early-season herbivores did not affect late-season leaf chemistry or tree performance. Early-season herbivores had a weak negative effect on the abundance of gallers and a positive, tree-dependent effect on the overall diversity of late-season sessile herbivores. The chemical composition of leaves covaried with the species composition of the late-season leafmining and galling community. Both the chemical composition of the host tree and the late-season insect community structure were strongly affected by the growth location of the tree. Our results suggest that plant-mediated indirect effects between herbivores might play a limited role in this system, whereas the underlying variation in plant chemistry is an important factor structuring the associated insect community. Our results emphasize that factors other than prior herbivory can be important determinants of plant chemistry and the community composition of herbivores.

"Feature Detection" vs. "Predictive Coding" Models of Plant Behavior

In this article we consider the possibility that plants exhibit anticipatory behavior, a mark of intelligence. If plants are able to anticipate and respond accordingly to varying states of their surroundings, as opposed to merely responding online to environmental contingencies, then such capacity may be in principle testable, and subject to empirical scrutiny. Our main thesis is that adaptive behavior can only take place by way of a mechanism that predicts the environmental sources of sensory stimulation. We propose to test for anticipation in plants experimentally by contrasting two empirical hypotheses: “feature detection” and “predictive coding.” We spell out what these contrasting hypotheses consist of by way of illustration from the animal literature, and consider how to transfer the rationale involved to the plant literature.

Leaf surface lipophilic compounds as one of the factors of silver birch chemical defense against larvae of gypsy moth

Plant chemical defense against herbivores is a complex process which involves a number of secondary compounds. It is known that the concentration of leaf surface lipophilic compounds (SLCs), particularly those of flavonoid aglycones are increased with the defoliation treatment of silver birch Betula pendula. In this study we investigated how the alteration of SLCs concentration in the food affects the fitness and innate immunity of the gypsy moth Lymantria dispar. We found that a low SLCs concentrations in consumed leaves led to a rapid larval development and increased females’ pupae weight (= fecundity) compared to larvae fed with leaves with high SLCs content. Inversely, increasing the compounds concentration in an artificial diet produced the reverse effects: decreases in both larval weight and larval survival. Low SLCs concentrations in tree leaves differently affected larval innate immunity parameters. For both sexes, total hemocytes count in the hemolymph increased, while the activity of plasma phenoloxidase decreased when larvae consume leaves with reduced content of SLCs. Our results clearly demonstrate that the concentration of SLCs in silver birch leaves affects not only gypsy moth fitness but also their innate immune status which might alter the potential resistance of insects against infections and/or parasitoids.

Genetic and phenotypic relationships between immune defense, melanism and life-history traits at different temperatures and sexes in Tenebrio molitor

Insect cuticle melanism is linked to a number of life-history traits, and a positive relationship is hypothesized between melanism and the strength of immune defense. In this study, the phenotypic and genetic relationships between cuticular melanization, innate immune defense, individual development time and body size were studied in the mealworm beetle (Tenebrio molitor) using three different temperatures with a half-sib breeding design. Both innate immune defense and cuticle darkness were higher in females than males, and a positive correlation between the traits was found at the lowest temperature. The effect of temperature on all the measured traits was strong, with encapsulation ability and development time decreasing and cuticle darkness increasing with a rise in temperature, and body size showing a curved response. The analysis showed a highly integrated system sensitive to environmental change involving physiological, morphological and life-history traits.

Plant induced defenses depend more on plant age than previous history of damage: implications for plant-herbivore interactions

Herbivore-induced plant responses can significantly change as a function of plant developmental stage and previous history of damage. Yet, empirical tests that assess the combined role of multiple damage events and age-dependent constraints on the ability of plants to induce defenses within and among tissues are scarce. This question is of particular interest for annual and/or short-lived perennial plant species, whose responses to single or multiple damage events over a growing season are likely to interact with ontogenetic constraints in affecting a plant's ability to respond to herbivory. Using Plantago lanceolata and one of its specialist herbivores, Junonia coenia, we examined the effect of plant ontogeny (juvenile vs. mature developmental stages) and history of damage (single and multiple damage events early and/or late in the season) on plant responses to leaf damage. Plant responses to herbivory were assessed as induced chemical defenses (iridoid glycosides) and compensatory regrowth, in both above- and below-ground tissues. We found that constitutive concentration of iridoid glycosides markedly increased as plants matured, but plant ability to induce chemical defenses was limited to juvenile, but not mature, plant stages. In addition, induced defenses observed 7 d following herbivory in juvenile plants disappeared 5 wk after the first herbivory event, and mature plants that varied considerably in the frequency and intensity of damage received over 5 wk, did not differ significantly in their levels of chemical defenses. Also, only small changes in compensatory regrowth were detected. Finally, we did not observe changes in below-ground tissues' defenses or biomass a week following 50% removal of leaf tissues at either age class or history of damage. Together, these results suggest that in P. lanceolata and perhaps other systems, ontogenetic trajectories in plant growth and defenses leading to strong age-dependent induced responses may prevail over herbivore-induced indirect interactions.

Boron fertilization enhances the induced defense of silver birch

Boron (B) deficiency is a common micronutrient deficiency that has been reported to affect the phenolic metabolism of plants. Thus, it may play a role in defense against herbivorous animals. However, the role of B in a plant's resistance to herbivores has not received any particular attention from researchers. In this study, we tested the effects of B nutrition 1) on the biochemical and mechanical defenses of birches and the growth of seedlings, and 2) the resistance of seedlings to larvae of the autumnal moth, Epirrita autumnata. Boron fertilization improved the resistance of birch, which was shown as reduced pupal weight of the herbivore. However, B fertilized trees suffered from heavier defoliation than unfertilized ones due to compensation feeding of larvae. The growth of the seedlings was diminished, and several biochemical changes occurred in leaves of herbivore seedlings, and B also played a role in these changes. Polyphenoloxidases (PPOs) and peroxidases (PODs) and their substrates, chlorogenic acids, were induced by herbivory in B fertilized seedlings but not in unfertilized seedlings. The lower pupal weights and increased consumption of the herbivores were probably linked to the plants' phenoloxidase-mediated production of reactive quinones, which decrease the nutritive value. Herbivory upon new stems led to an increase in the number of resin glands that provide defense against mammalian herbivores. Herbivory also had a substantially negative effect on B concentration in leaves of B fertilized seedlings. We postulate that B nutrition of trees may play a significant role in the induced defense of birches.

Phenolic compounds in ectomycorrhizal interaction of lignin modified silver birch

BACKGROUND

The monolignol biosynthetic pathway interconnects with the biosynthesis of other secondary phenolic metabolites, such as cinnamic acid derivatives, flavonoids and condensed tannins. The objective of this study is to evaluate whether genetic modification of the monolignol pathway in silver birch (Betula pendula Roth.) would alter the metabolism of these phenolic compounds and how such alterations, if exist, would affect the ectomycorrhizal symbiosis.

RESULTS

Silver birch lines expressing quaking aspen (Populus tremuloides L.) caffeate/5-hydroxyferulate O-methyltransferase (PtCOMT) under the 35S cauliflower mosaic virus (CaMV) promoter showed a reduction in the relative expression of a putative silver birch COMT (BpCOMT) gene and, consequently, a decrease in the lignin syringyl/guaiacyl composition ratio. Alterations were also detected in concentrations of certain phenolic compounds. All PtCOMT silver birch lines produced normal ectomycorrhizas with the ectomycorrhizal fungus Paxillus involutus (Batsch: Fr.), and the formation of symbiosis enhanced the growth of the transgenic plants.

CONCLUSION

The down-regulation of BpCOMT in the 35S-PtCOMT lines caused a reduction in the syringyl/guaiacyl ratio of lignin, but no significant effect was seen in the composition or quantity of phenolic compounds that would have been caused by the expression of PtCOMT under the 35S or UbB1 promoter. Moreover, the detected alterations in the composition of lignin and secondary phenolic compounds had no effect on the interaction between silver birch and P. involutus.

A comparative approach to the principal mechanisms of different memory systems

The term "memory" applies not only to the preservation of information in neuronal and immune systems but also to phenomena observed for example in plants, single cells, and RNA viruses. We here compare the different forms of information storage with respect to possible common features. The latter may be characterized by (1) selection of pre-existing information, (2) activation of memory systems often including transcriptional, and translational, as well as epigenetic and genetic mechanisms, (3) subsequent consolidation of the activated state in a latent form (standby mode), and (4) reactivation of the latent state of memory systems when the organism is exposed to the same (or conditioned) signal or to previous selective constraints. These features apparently also exist in the "evolutionary memory," i.e., in evolving populations which have highly variable mutant spectra.

What is plant behaviour?

The nature of plant behaviour is discussed, and it is concluded that it is best described as what plants do. The possibility that plant behaviour is simply signal-induced phenotypic plasticity is outlined, and some limitations of this assumption are considered. Natural environments present many challenges to growing plants, and the consequent signalling that plants perceive is becoming extremely complex. Plant behaviour is active, purposeful and intentional, and examples are discussed. Much plant behaviour, concerned with stress and herbivory, is also based on an assessment of the future likelihood of further damaging episodes and is therefore predictive. Plant behaviour involves the acquisition and processing of information. Informational terminology provides a suitable way of incorporating the concepts of learning, memory and intelligence into plant behaviour, capabilities that plants are rarely credited with. Finally, trade-offs, cost-benefit assessments and decision making are common plant behavioural attributes. It is suggested that intelligent assessments that involve the whole plant are essential to optimize these adaptive capabilities.

Plant behavioural ecology: dynamic plasticity in secondary metabolites

Behaviour is in part the ability to respond rapidly and reversibly in response to environmental stimuli during the lifetime of an individual. Plants and animals both exhibit behaviour, but plant behaviour is most often examined in the context of morphologically plastic growth. Rapid and reversible secondary metabolite production and release is also a key mechanism by which plants behave. Here, we review plant biochemical plasticity as plant behaviour, and explicitly focus on evidence for responses that display rapid induction, reversibility and ecological relevance. Rapid induction and attenuation of plant secondary metabolites occur as chemically mediated root foraging, plant defence, allelochemistry and to regulate mutualistic relationships. We describe a wealth of information on the induction of various plant biochemical responses to environmental stimuli but found a limited body of literature on the reversibility of induced biochemical responses. Understanding the full cycle of dynamic plasticity in secondary metabolites is an important niche for future research. Biochemical behaviours extend beyond the plant kingdom; however, they clearly illustrate the capacity for plants to behave in ways that closely mirror the classic definitions and research approaches applied to behaviour in animals.

Hydrolyzable tannins as "quantitative defenses": limited impact against Lymantria dispar caterpillars on hybrid poplar

The high levels of tannins in many tree leaves are believed to cause decreased insect performance, but few controlled studies have been done. This study tested the hypothesis that higher foliar tannin levels produce higher concentrations of semiquinone radicals (from tannin oxidation) in caterpillar midguts, and that elevated levels of radicals are associated with increased oxidative stress in midgut tissues and decreased larval performance. The tannin-free leaves of hybrid poplar (Populus tremulaxP. alba) were treated with hydrolyzable tannins, producing concentrations of 0%, 7.5% or 15% dry weight, and fed to Lymantria dispar caterpillars. As expected, larvae that ingested control leaves contained no measurable semiquinone radicals in the midgut, those that ingested 7.5% hydrolyzable tannin contained low levels of semiquinone radicals, and those that ingested 15% tannin contained greatly increased levels of semiquinone radicals. Ingested hydrolyzable tannins were also partially hydrolyzed in the midgut. However, increased levels of semiquinone radicals in the midgut were not associated with oxidative stress in midgut tissues. Instead, it appears that tannin consumption was associated with increased metabolic costs, as measured by the decreased efficiency of conversion of digested matter to body mass (ECD). Decreased ECD, in turn, decreased the overall efficiency of conversion of ingested matter to body mass (ECI). Contrary to our hypothesis, L. dispar larvae were able to maintain similar growth rates across all tannin treatment levels, in part, because of compensatory feeding. We conclude that hydrolyzable tannins act as "quantitative defenses" in the sense that high levels appear to be necessary to increase levels of semiquinone radicals in the midguts of caterpillars. However, these putative resistance factors are not sufficient to decrease the performance of tannin-tolerant caterpillars such as L. dispar.

Electrical memory in Venus flytrap

Electrical signaling, memory and rapid closure of the carnivorous plant Dionaea muscipula Ellis (Venus flytrap) have been attracting the attention of researchers since the XIX century. The electrical stimulus between a midrib and a lobe closes the Venus flytrap upper leaf in 0.3 s without mechanical stimulation of trigger hairs. Here we developed a new method for direct measurements of the exact electrical charge utilized by the D. muscipula Ellis to facilitate the trap closing and investigated electrical short memory in the Venus flytrap. As soon as the 8 microC charge for a small trap or a 9 microC charge for a large trap is transmitted between a lobe and midrib from the external capacitor, the trap starts to close at room temperature. At temperatures 28-36 degrees C a smaller electrical charge of 4.1 microC is required to close the trap of the D. muscipula. The cumulative character of electrical stimuli points to the existence of short-term electrical memory in the Venus flytrap. We also found sensory memory in the Venus flytrap. When one sustained mechanical stimulus was applied to only one trigger hair, the trap closed in a few seconds.

Induced accumulation of phenolics and sawfly performance in Scots pine in response to previous defoliation

Phenolic compounds often accumulate in foliar tissues of deciduous woody plants in response to previous insect defoliation, but similar responses have been observed infrequently in evergreen conifers. We studied the effects of defoliation on the foliar chemistry of Scots pine (Pinus sylvestris L.) and cocoon mass, and survival of the pine sawfly (Diprion pini L.). In two successive years, needles were excised early in the season leaving only the current-year shoot intact (defoliated trees); untreated entire shoots served as controls (control trees). A year after the second defoliation, pine sawfly larvae were transferred to the trees. Delayed induced resistance in Scots pine in response to defoliation was indicated by (1) reduced cocoon mass in defoliated trees and (2) increased concentrations of phenolics and soluble condensed tannins in the foliage of defoliated trees compared with controls. Myricetin-3-galactoside, which showed the strongest induced response (104% and 71% increase in current-year (C) and previous-year (C+1) needles) of the compounds analyzed, also entered the regression model explaining variation in sawfly performance. Other compounds that entered the model, e.g., (+)-catechin, showed weaker responses to defoliation than myricetin-3-galactoside. Hyperin, condensed tannins and quercitrin showed strong induced responses in C or C+1 needles, or both, but these compounds did not explain the variation in sawfly performance. Accumulation of phenolics is sometimes associated with the reduced foliage nitrogen (N) concentrations in deciduous trees, and our results suggest that this may also be the case in evergreen conifers. Based on the earlier findings that defoliation reduces needle N concentration and N deficiency results in the accumulation of the same phenolic compounds, i.e., myricetin and quercetin glycosides, and soluble condensed tannins, we suggest that the accumulation of phenolics in defoliated trees occurred in response to the reduced foliar N concentration.

Plant electrical memory

Electrical signaling, short-term memory and rapid closure of the carnivorous plant Dionaea muscipula Ellis (Venus flytrap) have been attracting the attention of researchers since the XIX century. We found that the electrical stimulus between a midrib and a lobe closes the Venus flytrap upper leaf without mechanical stimulation of trigger hairs. The closing time of Venus flytrap by electrical stimulation is the same as mechanically induced closing. Transmission of a single electrical charge between a lobe and the midrib causes closure of the trap and induces an electrical signal propagating between both lobes and midrib. The Venus flytrap can accumulate small subthreshold charges, and when the threshold value is reached, the trap closes. Repeated application of smaller charges demonstrates the summation of stimuli. The cumulative character of electrical stimuli points to the existence of short-term electrical memory in the Venus flytrap.

Direct effects of heavy metal pollution on the immune function of a geometrid moth, Epirrita autumnata

Previously, we found that Epirrita autumnata larvae, which were fed with leaves that came from a metal polluted area, had an enhanced immune function. To investigate further the cause of the enhanced immunity we examined experimentally the direct influence of the heavy metals, copper and nickel, on the immune function of the geometrid moth we artificially added the metals to the surface of birch leaves to examine their direct effects. As a measurement of the strength of immune function, we used encapsulation rate against a nylon monofilament. A moderate amount of Ni and Cu in the diet of moth larvae increased their encapsulation rate, but a large amount of Cu caused the immune function to decrease. The results indicate that Ni and Cu have direct effects on immunity. Moderate amounts of those metals increase immune function, but large amounts lead to inhibition of immune function.

Foliar oxidases as mediators of the rapidly induced resistance of mountain birch against Epirrita autumnata

Induced resistance of the mountain birch against its main defoliator Epirrita autumnata is a well-characterized phenomenon. The causal mechanism for this induced deterioration, however, has not been unequivocally explained, and no individual compound or group of traditional defensive compounds has been shown to explain the phenomenon. Phenolic compounds are the main secondary metabolites in mountain birch leaves, and the biological activity of phenolics usually depends on their oxidation. In this study, we found that the activity of polyphenoloxidases (PPOs), enzymes that oxidize o-diphenols to o-diquinones, was induced in trees with introduced larvae, and bioassays showed that both growth and consumption rates of larvae were reduced in damaged trees. PPO activity was negatively associated with both larval growth and consumption rates in trees with bagged larvae, but not in control trees. Our results suggest that the oxidation of phenolics by PPOs may be a causal explanation for the rapidly induced resistance of mountain birch against E. autumnata. This finding also helps to explain why correlations between insect performance and phenolics (without measuring indices explaining their oxidation) may not produce consistent results.