Differential allocation of constitutive and induced chemical defenses in pine tree juveniles: a test of the optimal defense theory

Insect herbivory immediately before the eclipse of the gymnosperms: The Dawangzhangzi plant assemblage of Northeastern China

The Early Cretaceous terrestrial revolution involved global shifts from gymnosperm- to angiosperm-dominated floras. However, responses of insect herbivores to these changes remain unexamined. We evaluated 2 176 highly sampled plant specimens representing 62 species/morphotypes from the 126 Ma Dawangzhangzi plant assemblage of Northeastern China. Our study consisted of horsetails, ferns, ginkgoaleans, czekanowskialeans, conifers, and an angiosperm. Their herbivory was evaluated by the functional feeding groups of hole feeding, margin feeding, and surface feeding (ectophytic feeders); piercer and suckers, and ovipositing insects (ectoendophytic feeders); mining, galling, and borings (endophytic feeders); and pathogens, collectively constituting 65 damage types (DTs). The plant assemblage was assessed for herbivory richness by DT richness, component community structure, and DT specialization on plant hosts; for herbivory intensity, it was evaluated for DT frequency, herbivorized surface area, and feeding event occurrences. Using feeding event occurrences, the data supported seven species/morphotypes as most intensely herbivorized: Liaoningocladus boii (76.6%), Czekanowskia sp. 1 (8.4%), Czekanowskia rigida (4.10%), Lindleycladus lanceolatus (3.5%), Ginkgoites sp. 2 (2.0%), Podozamites sp. 1 (1.1%), and Solenites sp. 1 (0.9%). The most herbivorized taxa were pinaleans (conifers), then czekanowskialeans, and lastly ginkgoaleans; the monodominant component community was the conifer Liaoningocladus boii. DT host specialization levels were low. The plant assemblage had an overall low 0.86% of foliage removed by herbivores, explained by physical and chemical antiherbivore defenses, and parasitoid attack. Although Paleozoic, gymnosperm-dominated assemblages had greater herbivory, component community structure of the three most herbivorized taxa are more similar to modern bracken fern and willow than modern gymnosperm taxa.

Analysis of the transcriptome of the needles and bark of Pinus radiata induced by bark stripping and methyl jasmonate

BACKGROUND

Plants are attacked by diverse insect and mammalian herbivores and respond with different physical and chemical defences. Transcriptional changes underlie these phenotypic changes. Simulated herbivory has been used to study the transcriptional and other early regulation events of these plant responses. In this study, constitutive and induced transcriptional responses to artificial bark stripping are compared in the needles and the bark of Pinus radiata to the responses from application of the plant stressor, methyl jasmonate. The time progression of the responses was assessed over a 4-week period.

RESULTS

Of the 6312 unique transcripts studied, 86.6% were differentially expressed between the needles and the bark prior to treatment. The most abundant constitutive transcripts were related to defence and photosynthesis and their expression did not differ between the needles and the bark. While no differential expression of transcripts were detected in the needles following bark stripping, in the bark this treatment caused an up-regulation and down-regulation of genes associated with primary and secondary metabolism. Methyl jasmonate treatment caused differential expression of transcripts in both the bark and the needles, with individual genes related to primary metabolism more responsive than those associated with secondary metabolism. The up-regulation of genes related to sugar break-down and the repression of genes related with photosynthesis, following both treatments was consistent with the strong down-regulation of sugars that has been observed in the same population. Relative to the control, the treatments caused a differential expression of genes involved in signalling, photosynthesis, carbohydrate and lipid metabolism as well as defence and water stress. However, non-overlapping transcripts were detected between the needles and the bark, between treatments and at different times of assessment. Methyl jasmonate induced more transcriptional responses in the bark than bark stripping, although the peak of expression following both treatments was detected 7 days post treatment application. The effects of bark stripping were localised, and no systemic changes were detected in the needles.

CONCLUSION

There are constitutive and induced differences in the needle and bark transcriptome of Pinus radiata. Some expression responses to bark stripping may differ from other biotic and abiotic stresses, which contributes to the understanding of plant molecular responses to diverse stresses. Whether the gene expression changes are heritable and how they differ between resistant and susceptible families identified in earlier studies needs further investigation.

Additive genetic variation in Pinus radiata bark chemistry and the chemical traits associated with variation in mammalian bark stripping

Secondary metabolites are suggested as a major mechanism explaining genetic variation in herbivory levels in Pinus radiata. The potential to incorporate these chemical traits into breeding/deployment programmes partly depends on the presence of additive genetic variation for the relevant chemical traits. In this study, near-infrared spectroscopy was used to quantify the constitutive and induced levels of 54 compounds in the bark of trees from 74 P. radiata full-sib families. The trees sampled for chemistry were protected from browsing and induced levels were obtained by subjecting half of the trees to artificial bark stripping. The treatment effect on bark chemistry was assessed along with narrow-sense heritability, the significance of non-additive genetic effects and the additive genetic correlations of compounds with bark stripping by mammalian herbivores that was observed in unprotected replicates of the field trial. The results indicated: (i) significant additive genetic variation, with low-moderate narrow-sense heritability estimates for most compounds; (ii) while significant induced effects were detected for some chemicals, no significant genetic variation in inducibility was detected; and (iii) sugars, fatty acids and a diterpenoid positively genetically correlated while a sesquiterpenoid negatively genetically correlated with bark stripping by the mammalian herbivore, the Bennett's wallaby (Macropus rufogriseus). At the onset of browsing, a trade-off with height was detected for selecting higher amounts of this sesquiterpenoid. However, overall, results showed potential to incorporate chemical traits into breeding/deployment programmes. The quantitative genetic analyses of the near infrared predicted chemical traits produced associations with mammalian bark stripping that mostly conform with those obtained using standard wet chemistry.

Influence of virus-host interactions on plant response to abiotic stress

Environmental factors play a significant role in controlling growth, development and defense responses of plants. Changes in the abiotic environment not only significantly alter the physiological and molecular pathways in plants, but also result in attracting the insect pests that carry a payload of viruses. Invasion of plants by viruses triggers the RNA silencing based defense mechanism in plants. In counter defense the viruses have gained the ability to suppress the host RNA silencing activities. A new paradigm has emerged, with the recognition that plant viruses also have the intrinsic capacity to modulate host plant response to environmental cues, in an attempt to favour their own survival. Thus, plant-virus interactions provide an excellent system to understand the signals in crosstalk between biotic (virus) and abiotic stresses. In this review, we have summarized the basal plant defense responses to pathogen invasion while emphasizing on the role of RNA silencing as a front line of defense response to virus infection. The emerging knowledge indicates overlap between RNA silencing with the innate immune responses during antiviral defense. The suppressors of RNA silencing serve as Avr proteins, which can be recognized by the host R proteins. The defense signals also function in concert with the phytohormones to influence plant responses to abiotic stresses. The current evidence on the role of virus induced host tolerance to abiotic stresses is also discussed.

Chemical Traits that Predict Susceptibility of Pinus radiata to Marsupial Bark Stripping

Bark stripping by mammals is a major problem in managed conifer forests worldwide. In Australia, bark stripping in the exotic plantations of Pinus radiata is mainly caused by native marsupials and results in reduced survival, growth, and in extreme cases death of trees. Herbivory is influenced by a balance between primary metabolites that are sources of nutrition and secondary metabolites that act as defences. Identifying the compounds that influence herbivory may be a useful tool in the management of forest systems. This study aimed to detect and identify both constitutive and induced compounds that are associated with genetic differences in susceptibility of two-year-old P. radiata trees to bark stripping by marsupials. An untargeted profiling of 83 primary and secondary compounds of the needles and bark samples from 21 susceptible and 21 resistant families was undertaken. These were among the most and least damaged families, respectively, screened in a trial of 74 families that were exposed to natural field bark stripping by marsupials. Experimental plants were in the same field trial but protected from bark stripping and a subset were subjected to artificial bark stripping to examine induced and constitutive chemistry differences between resistant and susceptible families. Machine learning (random forest), partial least squares plus discriminant analysis (PLS-DA), and principal components analysis with discriminant analysis (PCA-DA), as well as univariate methods were used to identify the most important totals by compound group and individual compounds differentiating the resistant and susceptible families. In the bark, the constitutive amount of two sesquiterpenoids - bicyclogermacrene and an unknown sesquiterpenoid alcohol -were shown to be of higher levels in the resistant families, whereas the constitutive sugars, fructose, and glucose, as well individual phenolics, were higher in the more susceptible families. The chemistry of the needles was not useful in differentiating the resistant and susceptible families to marsupial bark stripping. After artificial bark stripping, the terpenes, sugars, and phenolics responded in both the resistant and susceptible families by increasing or reducing amounts, which leveled the differences in the amounts of the compounds between the different resistant and susceptible classes observed at the constitutive level. Overall, based on the families with extreme values for less and more susceptibility, differences in the amounts of secondary compounds were subtle and susceptibility due to sugars may outweigh defence as the cause of the genetic variation in bark stripping observed in this non-native tree herbivory system.

Induction of Defense Responses in Pinus sylvestris Seedlings by Methyl Jasmonate and Response to Heterobasidion annosum and Lophodermium seditiosum Inoculation

The induction of defense responses in Pinus sylvestris L. seedlings by methyl jasmonate (MeJA) was investigated in three experiments. Two different MeJA application methods were tested, and induction of defense responses was assayed by seedling inoculation with Heterobasidion annosum (Fr.) Bref. and Lophodermium seditiosum Minter, Staley and Millar. In the first experiment, five-year-old P. sylvestris ramets of one clone were directly treated with MeJA, followed by inoculation with H. annosum. In the second experiment, open-pollinated Scots pine seedlings were treated with MeJA by direct spraying and vaporization, and inoculation with H. annosum was done using a slightly modified protocol. In the third experiment, open-pollinated Scots pine seedlings were treated with MeJA by vaporization and inoculated with L. seditiosum. Direct application of MeJA induced seedling mortality, and in some cases, decreased resistance to inoculation with H. annosum. Application of MeJA by vaporization was less stressful for seedlings, and resulted in increased resistance to both H. annosum and L. seditiosum. In addition, an unforeseen Neodiprion sertifer (Geoffroy) and Hylobius abietis L. infestation provided anecdotal evidence of the efficacy of MeJA in inducing resistance to insect pests as well. Further studies are required on the induction of resistance to additional diseases and pests. Induced resistance could be used as a possible protective mechanism for Scots pine seedlings prior to planting during reforestation of stands to increase vitality and survival.

Fructans Prime ROS Dynamics and Botrytis cinerea Resistance in Arabidopsis

Naturally derived molecules can be used as priming or defense stimulatory agents to protect against biotic stress. Fructans have gained strong interest due to their ability to induce resistance in a number of crop species. In this study, we set out to establish the role of fructan-induced immunity against the fungal pathogen Botrytis cinerea in Arabidopsis thaliana. We show that both inulin- and levan-type fructans from different sources can enhance Arabidopsis resistance against B. cinerea. We found that inulin from chicory roots and levan oligosaccharides from the exopolysaccharide-producing bacterium Halomonas smyrnensis primed the NADPH-oxidase-mediated reactive oxygen species (ROS) burst in response to the elicitors flg22, derived from the bacterial flagellum, and oligogalacturonides (OGs), derived from the host cell wall. Neither induced a direct ROS burst typical of elicitors. We also found a primed response after infection with B. cinerea for H2O2 accumulation and the activities of ascorbate peroxidase and catalase. Sucrose accumulated as a consequence of fructan priming, and glucose and sucrose levels increased in fructan-treated plants after infection with B. cinerea. This study shows that levan-type fructans, specifically from bacterial origin, can prime plant defenses and that both inulin and levan oligosaccharide-mediated priming is associated with changes in ROS dynamics and sugar metabolism. Establishing fructan-induced immunity in Arabidopsis is an important step to further study the underlying mechanisms since a broad range of biological resources are available for Arabidopsis.

No carbon limitation after lower crown loss in Pinus radiata

BACKGROUND AND AIMS

Biotic and abiotic stressors can cause different defoliation patterns within trees. Foliar pathogens of conifers commonly prefer older needles and infection with defoliation that progresses from the bottom crown to the top. The functional role of the lower crown of trees is a key question to address the impact of defoliation caused by foliar pathogens.

METHODS

A 2 year artificial defoliation experiment was performed using two genotypes of grafted Pinus radiata to investigate the effects of lower-crown defoliation on carbon (C) assimilation and allocation. Grafts received one of the following treatments in consecutive years: control-control, control-defoliated, defoliated-control and defoliated-defoliated.

RESULTS

No upregulation of photosynthesis either biochemically or through stomatal control was observed in response to defoliation. The root:shoot ratio and leaf mass were not affected by any treatment, suggesting prioritization of crown regrowth following defoliation. In genotype B, defoliation appeared to impose C shortage and caused reduced above-ground growth and sugar storage in roots, while in genotype A, neither growth nor storage was altered. Root C storage in genotype B decreased only transiently and recovered over the second growing season.

CONCLUSIONS

In genotype A, the contribution of the lower crown to the whole-tree C uptake appears to be negligible, presumably conferring resilience to foliar pathogens affecting the lower crown. Our results suggest that there is no C limitation after lower-crown defoliation in P. radiata grafts. Further, our findings imply genotype-specific defoliation tolerance in P. radiata.

Fourier-transform infrared (FT-IR) spectroscopy analysis discriminates asymptomatic and symptomatic Norway spruce trees

Conifer trees, including Norway spruce, are threatened by fungi of the Heterobasidion annosum species complex, which severely affect timber quality and cause economic losses to forest owners. The timely detection of infected trees is complicated, as the pathogen resides within the heartwood and sapwood of infected trees. The presence of the disease and the extent of the wood decay often becomes evident only after tree felling. Fourier-transform infrared (FT-IR) spectroscopy is a potential method for non-destructive sample analysis that may be useful for identifying infected trees in this pathosystem. We performed FT-IR analysis of 18 phloem, 18 xylem, and 18 needle samples from asymptomatic and symptomatic Norway spruce trees. FT-IR spectra from 1066 - 912 cm^-1 could be used to distinguish phloem, xylem, and needle tissue extracts. FT-IR spectra collected from xylem and needle extracts could also be used to discriminate between asymptomatic and symptomatic trees using spectral bands from 1657 - 994 cm^-1 and 1104 - 994 cm^-1, respectively. A partial least squares regression model predicted the concentration of condensed tannins, a defense-related compound, in phloem of asymptomatic and symptomatic trees. This work is the first to show that FT-IR spectroscopy can be used for the identification of Norway spruce trees naturally infected with Heterobasidion spp.

Oak genotype and phenolic compounds differently affect the performance of two insect herbivores with contrasting diet breadth

Research on plant-herbivore interactions has long recognized that plant genetic variation plays a central role in driving insect abundance and herbivory, as well as in determining plant defense. However, how plant genes influence herbivore feeding performances, and which plant defensive traits mediate these effects, remain poorly understood. Here we investigated the feeding performances of two insect leaf chewers with contrasting diet breadth (the generalist Lymantria dispar L. and the specialist Thaumetopoea processionea L.) on different genotypes of pedunculate oak (Quercus robur L.) and tested the role of leaf phenolics. We used leaves from four clones of 30 Q. robur full-sibs grown in a common garden to estimate the performance of both herbivores in laboratory feeding trials and to quantify the concentration of constitutive chemical defences (phenolic compounds). We found that tree genetics influenced leaf consumption by T. processionea but not by L. dispar. However genetic variation among trees did not explain growth rate variation in either herbivore nor in leaf phenolics. Interestingly, all phenolic compounds displayed a positive relationship with L. dispar growth rate, and leaf consumption by both herbivores displayed a positive relationship with the concentrations of condensed tannins, suggesting that highly defended leaves could induce a compensatory feeding response. While genetic variation in oaks did not explain herbivore growth rate, we found positive genetic correlations between the two herbivores for leaf consumption and digestion. Overall, we found that oak genotype and phenolic compounds partly and independently contribute to variability in herbivore performance. We challenged the current view of plant-insect interaction and provided little support to the idea that the effect of plant genotype on associated organisms is driven by plant defences. Together, our results point to the existence of genetically determined resistance traits in oaks whose effects differ between herbivores and motivate further research on mechanisms governing oak-herbivore interactions.

Type VI glandular trichome density and their derived volatiles are differently induced by jasmonic acid in developing and fully developed tomato leaves: Implications for thrips resistance

Variation in the induction of plant defenses along the plant canopy can determine distribution and colonization of arthropod herbivores within the plant. In tomato, type VI glandular trichomes, which are epidermal defensive structures, and their derived volatiles are induced by the phytohormone jasmonic acid (JA). How JA-mediated induction of these trichome-associated chemical defenses depends on the leaf developmental stage and correlates with resistance against herbivory is unknown. We showed that application of JA reduced thrips-associated damage, however the amplitude of this response was reduced in the fully developed leaves compared to those still developing. Although JA increased type-VI trichome densities in all leaf developmental stages, as well as JA-inducible defensive proteins, these increases were stronger in developing leaves. Remarkably, the concentration of trichome-derived volatiles was induced by JA to a larger degree in developing leaves than in fully developed leaves. In fully developed leaves, the increase in trichome-derived volatiles was explained by an enhanced production per trichome, while in developing leaves this was mainly caused by increases in type-VI trichome densities. Together, we showed that JA-mediated induction of trichome density and chemistry depends on leaf development stage, and it might explain the degree of thrips-associated leaf damage in tomato.

Spatial differences in (Z)-3-hexen-1-ol production preferentially reduces Spodoptera litura larva attack on the young leaves of Nicotiana benthamiana

Plants synthesize specialized metabolites which possess extremely important ecological functions including direct defense, indirect defense, and signaling. The optimal defense theory (ODT) proposes that defensive metabolites are preferentially allocated to the tissues with high fitness value or in locations that are easily injured. In our present study, using the model plant Nicotiana benthamiana, we found that direct defense of N. benthamiana against Spodoptera litura (Fabricius) larvae showed spatial differences in the sites producing defensive chemicals. The upper leaves possessed significantly stronger direct defense ability than the middle and lower leaves. Interestingly, the strong defense ability of the upper leaves was not due to occurrences of well-known defensive metabolites such as nicotine and chlorogenic acid. After damage, the middle and lower leaves emitted higher amounts of (Z)-3-hexen-1-ol than the upper leaves, which could both attract larvae and significantly increase the amount of middle and lower leaf eaten by the larvae. The spatial difference in (Z)-3-hexen-1-ol emission may be due to spatial differences in expression of lipoxygenase (NbLOX2), which is responsible for the formation and emission of (Z)-3-hexen-1-ol. This study provided new insight into ODT, showing that plants effectively protect easily injured tissues through reduction in concentration of herbivore-feeding stimulant in the tissues.

Rapid increase in growth and productivity can aid invasions by a non-native tree

Research on biological invasions has produced detailed theories describing range expansions of introduced populations. However, current knowledge of evolutionary factors associated with invasive range expansions, especially those related to rapid evolution of long-lived organisms, is still rudimentary. Here, we used a system of six 40-year-old invasive pine populations that originated from replicated introduction events to study evolution in productivity, growth, and chemical defence traits. We tested the hypotheses that invasive populations were undergoing rapid phenotypic change as populations spread, that populations exhibit trade-offs between evolution in growth and chemical defences, and that rates of rapid evolution in plant growth and productivity effect rates of invasion. Although all invasions started from replicated pools of genetic material and equal propagule pressure, we found divergence in mean values for the six invasive populations in the six traits measured. Not only were there between-population variations but also invasive populations were also rapidly changing along each invasive population expansion. Two populations displayed greater leaf areas (LAs) and smaller specific LAs (SLAs) during range expansion. Four populations had faster growth rates at the leading edge of the invasion front in comparison with plants at the rear edge. In terms of total plant defences, non-volatile resin increased in plants along one invasion gradient and decreased in a second, total needle phenolics increased in plants along one invasion gradient and total wood phenolics increased in plants along the one invasion gradient and decreased in a second. We found no trade-offs between investments in growth and chemical defence. Also, faster rates of change in growth rate and LA were positively associated with greater dispersal distances of invasive populations, suggesting rapid evolution may increase invasiveness. Understanding the roles of both natural and human-mediated ecological and evolutionary processes in population-level dynamics is key to understanding the ability of non-native species to invade.

Defensive Traits in Young Pine Trees Cluster into Two Divergent Syndromes Related to Early Growth Rate

The combination of defensive traits leads to the evolution of 'plant defense syndromes' which should provide better protection against herbivores than individual traits on their own. Defense syndromes can be generally driven by plant phylogeny and/or biotic and abiotic factors. However, we lack a solid understanding of (i) the relative importance of shared evolution vs. convergence due to similar ecological conditions and (ii) the role of induced defense strategies in shaping defense syndromes. We investigate the relative roles of evolutionary and ecological factors shaping the deployment of pine defense syndromes including multiple constitutive and induced chemical defense traits. We performed a greenhouse experiment with seedlings of eighteen species of Pinaceae family, and measured plant growth rate, constitutive chemical defenses and their inducibility. Plant growth rate, but not phylogenetic relatedness, determined the deployment of two divergent syndromes. Slow-growing pine species living in harsh environments where tissue replacement is costly allocated more to constitutive defenses (energetically more costly to produce than induced). In contrast, fast-growing species living in resource-rich habitats had greater inducibility of their defenses, consistent with the theory of constitutive-induced defense trade-offs. This study contributes to a better understanding of evolutionary and ecological factors driving the deployment of defense syndromes.

Susceptibility to the pinewood nematode (PWN) of four pine species involved in potential range expansion across Europe

The pine wilt disease (PWD), caused by the pinewood nematode (PWN) Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle, is one of the most serious threats to pine forests worldwide. Here we studied several components of susceptibility to PWN infection in a model group of pine species widely distributed in Europe (Pinus pinaster Ait., P. pinea L., P. sylvestris L. and P. radiata D. Don), specifically concerning anatomical and chemical traits putatively related to nematode resistance, whole-plant nematode population after experimental inoculation, and several biochemical and physiological traits indicative of plant performance, damage and defensive responses 60 days post inoculation (dpi) in 3-year-old plants. Pinus pinaster was the most susceptible species to PWN colonization, with a 13-fold increase in nematode population size following inoculation, showing up to 35-fold more nematodes than the other species. Pinus pinea was the most resistant species, with an extremely reduced nematode population 60 dpi. Axial resin canals were significantly wider in P. pinaster than in the other species, which may have facilitated nematode dispersal through the stem and contributed to its high susceptibility; nevertheless, this trait does not seem to fully determinate the susceptible character of a species, as P. sylvestris showed similar nematode migration rates to P. pinaster but narrower axial resin canals. Nematode inoculation significantly affected stem water content and polyphenolic concentration, and leaf chlorophyll and lipid peroxidation in all species. In general, P. pinaster and P. sylvestris showed similar chemical responses after infection, whereas P. radiata, which co-exists with the PWN in its native range, showed some degree of tolerance to the nematode. This work provides evidence that the complex interactions between B. xylophilus and its hosts are species-specific, with P. pinaster showing a strong susceptibility to the pathogen, P. pinea being the most tolerant species, and P. sylvestris and P. radiata having a moderate susceptibility, apparently through distinct coping mechanisms.

Molecular characterization of two isoforms of a farnesyl pyrophosphate synthase gene in wheat and their roles in sesquiterpene synthesis and inducible defence against aphid infestation

Aphids are important pests of wheat (Triticum aestivum) that affect crop production globally. Herbivore-induced emission of sesquiterpenes can repel pests, and farnesyl pyrophosphate synthase (FPS) is a key enzyme involved in sesquiterpene biosynthesis. However, fps orthologues in wheat and their functional roles in sesquiterpene synthesis and defence against aphid infestation are unknown. Here, two fps isoforms, Tafps1 and Tafps2, were identified in wheat. Quantitative real-time polymerase chain reaction (qRT-PCR) and in vitro catalytic activity analyses were conducted to investigate expression patterns and activity. Heterologous expression of these isoforms in Arabidopsis thaliana, virus-induced gene silencing (VIGS) in wheat and aphid behavioural assays were performed to understand the functional roles of these two isoforms. We demonstrated that Tafps1 and Tafps2 played different roles in induced responses to aphid infestation and in sesquiterpene synthesis. Heterologous expression in A. thaliana resulted in repulsion of the peach aphid (Myzus persicae). Wheat plants with these two isoforms transiently silenced were significantly attractive to grain aphid (Sitobion avenae). Our results provide new insights into induced defence against aphid herbivory in wheat, in particular, the different roles of the two Tafps isoforms in both sesquiterpene biosynthesis and defence against aphid infestation.

Positive effects of plant genotypic and species diversity on anti-herbivore defenses in a tropical tree species

Despite increasing evidence that plant intra- and inter-specific diversity increases primary productivity, and that such effect may in turn cascade up to influence herbivores, there is little information about plant diversity effects on plant anti-herbivore defenses, the relative importance of different sources of plant diversity, and the mechanisms for such effects. For example, increased plant growth at high diversity may lead to reduced investment in defenses via growth-defense trade-offs. Alternatively, positive effects of plant diversity on plant growth may lead to increased herbivore abundance which in turn leads to a greater investment in plant defenses. The magnitude of trait variation underlying diversity effects is usually greater among species than among genotypes within a given species, so plant species diversity effects on resource use by producers as well as on higher trophic levels should be stronger than genotypic diversity effects. Here we compared the relative importance of plant genotypic and species diversity on anti-herbivore defenses and whether such effects are mediated indirectly via diversity effects on plant growth and/or herbivore damage. To this end, we performed a large-scale field experiment where we manipulated genotypic diversity of big-leaf mahogany (Swietenia macrophylla) and tree species diversity, and measured effects on mahogany growth, damage by the stem-boring specialist caterpillar Hypsipyla grandella, and defensive traits (polyphenolics and condensed tannins in stem and leaves). We found that both forms of plant diversity had positive effects on stem (but not leaf) defenses. However, neither source of diversity influenced mahogany growth, and diversity effects on defenses were not mediated by either growth-defense trade-offs or changes in stem-borer damage. Although the mechanism(s) of diversity effects on plant defenses are yet to be determined, our study is one of the few to test for and show producer diversity effects on plant chemical defenses.

Integrated transcriptomics and metabolomics decipher differences in the resistance of pedunculate oak to the herbivore Tortrix viridana L

BACKGROUND

The interaction between insect pests and their host plants is a never-ending race of evolutionary adaption. Plants have developed an armament against insect herbivore attacks, and attackers continuously learn how to address it. Using a combined transcriptomic and metabolomic approach, we investigated the molecular and biochemical differences between Quercus robur L. trees that resisted (defined as resistant oak type) or were susceptible (defined as susceptible oak type) to infestation by the major oak pest, Tortrix viridana L.

RESULTS

Next generation RNA sequencing revealed hundreds of genes that exhibited constitutive and/or inducible differential expression in the resistant oak compared to the susceptible oak. Distinct differences were found in the transcript levels and the metabolic content with regard to tannins, flavonoids, and terpenoids, which are compounds involved in the defence against insect pests. The results of our transcriptomic and metabolomic analyses are in agreement with those of a previous study in which we showed that female moths prefer susceptible oaks due to their specific profile of herbivore-induced volatiles. These data therefore define two oak genotypes that clearly differ on the transcriptomic and metabolomic levels, as reflected by their specific defensive compound profiles.

CONCLUSIONS

We conclude that the resistant oak type seem to prefer a strategy of constitutive defence responses in contrast to more induced defence responses of the susceptible oaks triggered by feeding. These results pave the way for the development of biomarkers for an early determination of potentially green oak leaf roller-resistant genotypes in natural pedunculate oak populations in Europe.

Inducibility of chemical defences by two chewing insect herbivores in pine trees is specific to targeted plant tissue, particular herbivore and defensive trait

There is increasing evidence that plants can react to biotic aggressions with highly specific responses. However, few studies have attempted to jointly investigate whether the induction of plant defences is specific to a targeted plant tissue, plant species, herbivore identity, and defensive trait. Here we studied those factors contributing to the specificity of induced defensive responses in two economically important pine species against two chewing insect pest herbivores. Juvenile trees of Pinus pinaster and P. radiata were exposed to herbivory by two major pest threats, the large pine weevil Hylobius abietis (a bark-feeder) and the pine processionary caterpillar Thaumetopoea pityocampa (a folivore). We quantified in two tissues (stem and needles) the constitutive (control plants) and herbivore-induced concentrations of total polyphenolics, volatile and non-volatile resin, as well as the profile of mono- and sesquiterpenes. Stem chewing by the pine weevil increased concentrations of non-volatile resin, volatile monoterpenes, and (marginally) polyphenolics in stem tissues. Weevil feeding also increased the concentration of non-volatile resin and decreased polyphenolics in the needle tissues. Folivory by the caterpillar had no major effects on needle defensive chemistry, but a strong increase in the concentration of polyphenolics in the stem. Interestingly, we found similar patterns for all these above-reported effects in both pine species. These results offer convincing evidence that induced defences are highly specific and may vary depending on the targeted plant tissue, the insect herbivore causing the damage and the considered defensive compound.

Additive genetic variation in resistance traits of an exotic pine species: little evidence for constraints on evolution of resistance against native herbivores

The apparent failure of invasions by alien pines in Europe has been explained by the co-occurrence of native pine congeners supporting herbivores that might easily recognize the new plants as hosts. Previous studies have reported that exotic pines show reduced tolerance and capacity to induce resistance to those native herbivores. We hypothesize that limited genetic variation in resistance to native herbivores and the existence of evolutionary trade-offs between growth and resistance could represent additional potential constraints on the evolution of invasiveness of exotic pines outside their natural range. In this paper, we examined genetic variation for constitutive and induced chemical defences (measured as non-volatile resin in the stem and total phenolics in the needles) and resistance to two major native generalist herbivores of pines in cafeteria bioassays (the phloem-feeder Hylobius abietis and the defoliator Thaumetopoea pityocampa) using half-sib families drawn from a sample of the population of Pinus radiata introduced to Spain in the mid-19th century. We found (i) significant genetic variation, with moderate-to-high narrow-sense heritabilities for both the production of constitutive non-volatile resin and induced total phenolics, and for constitutive resistance against T. pityocampa in bioassays, (ii) no evolutionary trade-offs between plant resistance and growth traits or between the production of different quantitative chemical defences and (iii) a positive genetic correlation between constitutive resistance to the two studied herbivores. Overall, results of our study indicate that the exotic pine P. radiata has limited genetic constraints on the evolution of resistance against herbivores in its introduced range, suggesting that, at least in terms of interactions with these enemies, this pine species has potential to become invasive in the future.

Bottom-up effects of host-plant species diversity and top-down effects of ants interactively increase plant performance

While plant diversity is well known to increase primary productivity, whether these bottom-up effects are enhanced by reciprocal top-down effects from the third trophic level is unknown. We studied whether pine tree species diversity, aphid-tending ants and their interaction determined plant performance and arthropod community structure. Plant diversity had a positive effect on aphids, but only in the presence of mutualistic ants, leading to a threefold greater number of both groups in the tri-specific cultures than in monocultures. Plant diversity increased ant abundance not only by increasing aphid number, but also by increasing ant recruitment per aphid. The positive effect of diversity on ants in turn cascaded down to increase plant performance; diversity increased plant growth (but not biomass), and this effect was stronger in the presence of ants. Consequently, bottom-up effects of diversity within the same genus and guild of plants, and top-down effects from the third trophic level (predatory ants), interactively increased plant performance.

Relative importance of genetic, ontogenetic, induction, and seasonal variation in producing a multivariate defense phenotype in a foundation tree species

Plant adaptations for defense against herbivory vary both among species and among genotypes. Moreover, numerous forms of within-plant variation in defense, including ontogeny, induction, and seasonal gradients, allow plants to avoid expending resources on defense when herbivores are absent. We used an 18-year-old cottonwood common garden composed of Populus fremontii, Populus angustifolia, and their naturally occurring F(1) hybrids (collectively referred to as "cross types") to quantify and compare the relative influences of three hierarchical levels of variation (between cross types, among genotypes, and within individual genotypes) on univariate and multivariate phytochemical defense traits. Within genotypes, we evaluated ontogeny, induction (following cottonwood leaf beetle herbivory), and seasonal variation. We compared the effect sizes of each of these sources of variation on the plant defense phenotype. Three major patterns emerged. First, we observed significant differences in concentrations of defense phytochemicals among cross types, and/or among genotypes within cross types. Second, we found significant genetic variation for within-plant differences in phytochemical defenses: (a) based on ontogeny, levels of constitutive phenolic glycosides were nearly three times greater in the mature zone than in the juvenile zone within one cottonwood cross type, but did not significantly differ within another cross type; (b) induced levels of condensed tannins increased up to 65 % following herbivore damage within one cottonwood cross type, but were not significantly altered in another cross type; and (c) concentrations of condensed tannins tended to increase across the season, but did not do so across all cross types. Third, our estimates of effect size demonstrate that the magnitude of within-plant variation in a phytochemical defense can rival the magnitude of differences in defense among genotypes and/or cross types. We conclude that, in cottonwood and likely other plant species, multiple forms of within-individual variation have the potential to substantially influence ecological and evolutionary processes.