Quantitative Effects of Cyanogenesis on an Adapted Herbivore

Cyanogenesis, a Plant Defence Strategy against Herbivores

Plants and phytophagous arthropods have coevolved in a long battle for survival. Plants respond to phytophagous feeders by producing a battery of antiherbivore chemical defences, while herbivores try to adapt to their hosts by attenuating the toxic effect of the defence compounds. Cyanogenic glucosides are a widespread group of defence chemicals that come from cyanogenic plants. Among the non-cyanogenic ones, the Brassicaceae family has evolved an alternative cyanogenic pathway to produce cyanohydrin as a way to expand defences. When a plant tissue is disrupted by an herbivore attack, cyanogenic substrates are brought into contact with degrading enzymes that cause the release of toxic hydrogen cyanide and derived carbonyl compounds. In this review, we focus our attention on the plant metabolic pathways linked to cyanogenesis to generate cyanide. It also highlights the role of cyanogenesis as a key defence mechanism of plants to fight against herbivore arthropods, and we discuss the potential of cyanogenesis-derived molecules as alternative strategies for pest control.

Ontogenetic changes in the targets of natural selection in three plant defenses

The evolution of plant defenses has traditionally been studied at single plant ontogenetic stages, overlooking the fact that natural selection acts continuously on organisms along their development, and that the adaptive value of phenotypes can change along ontogeny. We exposed 20 replicated genotypes of Turnera velutina to field conditions to evaluate whether the targets of natural selection on different defenses and their adaptative value change across plant development. We found that low chemical defense was favored in seedlings, which seems to be explained by the assimilation efficiency and the ability of the specialist herbivore to sequester cyanogenic glycosides. Whereas trichome density was unfavored in juvenile plants, it increased relative plant fitness in reproductive plants. At this stage we also found a positive correlative gradient between cyanogenic potential and sugar content in extrafloral nectar. We visualize this complex multi-trait combination as an ontogenetic defensive strategy. The inclusion of whole-plant ontogeny as a key source of variation in plant defense revealed that the targets and intensity of selection change along the development of plants, indicating that the influence of natural selection cannot be inferred without the assessment of ontogenetic strategies in the expression of multiple defenses.

Role of cyanogenic glycosides in the seeds of wild lima bean, Phaseolus lunatus: defense, plant nutrition or both?

Cyanogenic glycosides present in the seeds of wild lima bean plants are associated with seedling defense but do not affect seed germination and seedling growth. Wild lima bean plants contain cyanogenic glycosides (CNGs) that are known to defend the plant against leaf herbivores. However, seed feeders appear to be unaffected despite the high levels of CNGs in the seeds. We investigated a possible role of CNGs in seeds as nitrogen storage compounds that influence plant growth, as well as seedling resistance to herbivores. Using seeds from four different wild lima bean natural populations that are known to vary in CNG levels, we tested two non-mutually exclusive hypotheses: (1) seeds with higher levels of CNGs produce seedlings that are more resistant against generalist herbivores and, (2) seeds with higher levels of CNGs germinate faster and produce plants that exhibit better growth. Levels of CNGs in the seeds were negatively correlated with germination rates and not correlated with seedling growth. However, levels of CNGs increased significantly soon after germination and seeds with the highest CNG levels produced seedlings with higher CNG levels in cotyledons. Moreover, the growth rate of the generalist herbivore Spodoptera littoralis was lower in cotyledons with high-CNG levels. We conclude that CNGs in lima bean seeds do not play a role in seed germination and seedling growth, but are associated with seedling defense. Our results provide insight into the potential dual function of plant secondary metabolites as defense compounds and storage molecules for growth and development.

Biofilm compartmentalisation of the rumen microbiome: modification of fermentation and degradation of dietary toxins

Many deleterious chemicals in plant materials ingested by ruminants produce clinical effects, varying from losses of production efficiency through to death. Many of the effects are insidious, often going unrecognised by animal managers. When secondary plant compounds enter the rumen, they may undergo modification by rumen microbes, which often removes the deleterious compounds, but in specific instances, the deleterious effect may be enhanced. Improved understanding of rumen ecology, particularly concerning the biofilm mode of microbial fermentation, has led to major advances in our understanding of fermentation. In the present review, the potential impact of the physical structuring of the rumen microbiome is discussed in relation to how several economically important secondary plant compounds and other toxins are metabolised by the rumen microbiome and how their toxic effects may be remedied by providing inert particles with a large surface area to weight ratio in the diet. These particles provide additional surfaces for attachment of rumen microorganisms that help alleviate toxicity problems associated with deleterious compounds, including fluoroacetate, mimosine, mycotoxins, cyanoglycosides and hydrogen cyanide. The review first summarises the basic science of biofilm formation and describes the properties of biofilms and their roles in the rumen. It then addresses how biofilms on inert solids and fermentable particulates may assist in detoxification of potentially toxic compounds. A hypothesis that explains how nitrate poisoning may occur as a result of compartmentalisation of nitrate and nitrite reduction in the rumen is included.

Coevolution of Cyanogenic Bamboos and Bamboo Lemurs on Madagascar

Feeding strategies of specialist herbivores often originate from the coevolutionary arms race of plant defenses and counter-adaptations of herbivores. The interaction between bamboo lemurs and cyanogenic bamboos on Madagascar represents a unique system to study diffuse coevolutionary processes between mammalian herbivores and plant defenses. Bamboo lemurs have different degrees of dietary specialization while bamboos show different levels of chemical defense. In this study, we found variation in cyanogenic potential (HCNp) and nutritive characteristics among five sympatric bamboo species in the Ranomafana area, southeastern Madagascar. The HCNp ranged from 209±72 μmol cyanide*g-1 dwt in Cathariostachys madagascariensis to no cyanide in Bambusa madagascariensis. Among three sympatric bamboo lemur species, the greater bamboo lemur (Prolemur simus) has the narrowest food range as it almost exclusively feeds on the highly cyanogenic C. madagascariensis. Our data suggest that high HCNp is the derived state in bamboos. The ancestral state of lemurs is most likely "generalist" while the ancestral state of bamboo lemurs was determined as equivocal. Nevertheless, as recent bamboo lemurs comprise several "facultative specialists" and only one "obligate specialist" adaptive radiation due to increased flexibility is likely. We propose that escaping a strict food plant specialization enabled facultative specialist bamboo lemurs to inhabit diverse geographical areas.

Is protection against florivory consistent with the optimal defense hypothesis?

BACKGROUND

Plant defense traits require resources and energy that plants may otherwise use for growth and reproduction. In order to most efficiently protect plant tissues from herbivory, one widely accepted assumption of the optimal defense hypothesis states that plants protect tissues most relevant to fitness. Reproductive organs directly determining plant fitness, including flowers and immature fruit, as well as young, productive leaf tissue thus should be particularly well-defended. To test this hypothesis, we quantified the cyanogenic potential (HCNp)-a direct, chemical defense-systemically expressed in vegetative and reproductive organs in lima bean (Phaseolus lunatus), and we tested susceptibility of these organs in bioassays with a generalist insect herbivore, the Large Yellow Underwing (Noctuidae: Noctua pronuba). To determine the actual impact of either florivory (herbivory on flowers) or folivory on seed production as a measure of maternal fitness, we removed varying percentages of total flowers or young leaf tissue and quantified developing fruit, seeds, and seed viability.

RESULTS

We found extremely low HCNp in flowers (8.66 ± 2.19 μmol CN(-) g(-1) FW in young, white flowers, 6.23 ± 1.25 μmol CN(-) g(-1) FW in mature, yellow flowers) and in pods (ranging from 32.05 ± 7.08 to 0.09 ± 0.08 μmol CN(-) g(-1) FW in young to mature pods, respectively) whereas young leaves showed high levels of defense (67.35 ± 3.15 μmol CN(-) g(-1) FW). Correspondingly, herbivores consumed more flowers than any other tissue, which, when taken alone, appears to contradict the optimal defense hypothesis. However, experimentally removing flowers did not significantly impact fitness, while leaf tissue removal significantly reduced production of viable seeds.

CONCLUSIONS

Even though flowers were the least defended and most consumed, our results support the optimal defense hypothesis due to i) the lack of flower removal effects on fitness and ii) the high defense investment in young leaves, which have high consequences for fitness. These data highlight the importance of considering plant defense interactions from multiple angles; interpreting where empirical data fit within any plant defense hypothesis requires understanding the fitness consequences associated with the observed defense pattern.

Differential effects of type and quantity of leaf damage on growth, reproduction and defence of lima bean (Phaseolus lunatus L.)

Folivores are major plant antagonists in most terrestrial ecosystems. However, the quantitative effects of leaf area loss on multiple interacting plant traits are still little understood. We sought to contribute to filling this lack of understanding by applying different types of leaf area removal (complete leaflets versus leaflet parts) and degrees of leaf damage (0, 33 and 66%) to lima bean (Phaseolus lunatus) plants. We quantified various growth and fitness parameters including above- and belowground biomass as well as the production of reproductive structures (fruits, seeds). In addition, we measured plant cyanogenic potential (HCNp; direct chemical defence) and production of extrafloral nectar (EFN; indirect defence). Leaf damage reduced above- and belowground biomass production in general, but neither variation in quantity nor type of damage resulted in different biomass. Similarly, the number of fruits and seeds was significantly reduced in all damaged plants without significant differences between treatment groups. Seed mass, however, was affected by both type and quantity of leaf damage. Leaf area loss had no impact on HCNp, whereas production of EFN decreased with increasing damage. While EFN production was quantitatively affected by leaf area removal, the type of damage had no effect. Our study provides a thorough analysis of the quantitative and qualitative effects of defoliation on multiple productivity-related and defensive plant traits and shows strong differences in plant response depending on trait. Quantifying such plant responses is vital to our understanding of the impact of herbivory on plant fitness and productivity in natural and agricultural ecosystems.

Salinity-mediated cyanogenesis in white clover (Trifolium repens) affects trophic interactions

BACKGROUND AND AIMS

Increasing soil salinity poses a major plant stress in agro-ecosystems worldwide. Surprisingly little is known about the quantitative effect of elevated salinity on secondary metabolism in many agricultural crops. Such salt-mediated changes in defence-associated compounds may significantly alter the quality of food and forage plants as well as their resistance against pests. In the present study, the effects of soil salinity on cyanogenesis in white clover (Trifolium repens), a forage crop of international importance, are analysed.

METHODS

Experimental clonal plants were exposed to five levels of soil salinity, and cyanogenic potential (HCNp, total amount of accumulated cyanide in a given plant tissue), β-glucosidase activity, soluble protein concentration and biomass production were quantified. The attractiveness of plant material grown under the different salt treatments was tested using cafeteria-style feeding trials with a generalist (grey garden slug, Deroceras reticulatum) and a specialist (clover leaf weevil, Hypera punctata) herbivore.

KEY RESULTS

Salt treatment resulted in an upregulation of HCNp, whereas β-glucosidase activity and soluble protein concentration showed no significant variation among treatments. Leaf area consumption of both herbivore species was negatively correlated with HCNp, indicating bottom-up effects of salinity-mediated changes in HCNp on plant consumers.

CONCLUSIONS

The results suggest that soil salinity leads to an upregulation of cyanogenesis in white clover, which results in enhanced resistance against two different natural herbivores. The potential implications for such salinity-mediated changes in plant defence for livestock grazing remain to be tested.

How useful are olfactometer experiments in chemical ecology research?

Olfactometer experiments, in which arthropods are given the choice between two or more odor sources to test behavioral preferences, are commonly used in chemical ecology research. Results of such often lead to conclusions on behavior in an ecologically relevant setting. However, it is widely unknown how well these experiments reflect actual behavior in nature. Recently, we used natural insect herbivores of wild lima bean plants to evaluate their behavior in Y-tube olfactometer experiments compared with feeding experiments. We demonstrated that depending on volatile concentration, insect sex significantly determined preference, and that independent of sex, the actual feeding choice of insects depended on defensive short-distance cues, which did not correlate with volatile cues emitted by the plants. Thus, our study shows that olfactory decisions do not reflect actual feeding choice and that olfactometer experiments may only provide a limited and simplified picture of actual decision making by insects.

Acclimation to elevated CO2 increases constitutive glucosinolate levels of Brassica plants and affects the performance of specialized herbivores from contrasting feeding guilds

Plants growing under elevated CO2 concentration may acclimate by modifying chemical traits. Most studies have focused on the effects of environmental change on plant growth and productivity. Potential effects on chemical traits involved in resistance, and the consequences of such effects on plant-insect interactions, have been largely neglected. Here, we evaluated the performance of two Brassica specialist herbivores from contrasting feeding guilds, the leaf-feeding Pieris brassicae and the phloem-feeding Brevicoryne brassicae, in response to potential CO2-mediated changes in primary and major secondary metabolites (glucosinolates) in Brassica oleracea. Plants were exposed to either ambient (400 ppm) or elevated (800 ppm) CO2 concentrations for 2, 6, or 10 weeks. Elevated CO2 did not affect primary metabolites, but significantly increased glucosinolate content. The performance of both herbivores was significantly reduced under elevated CO2 suggesting that CO2-mediated increases in constitutive defense chemistry could benefit plants. However, plants with up-regulated defenses could also be subjected to intensified herbivory by some specialized herbivores, due to a chemically-mediated phagostimulatory effect, as documented here for P. brassicae larvae. Our results highlight the importance of understanding acclimation and responses of plants to the predicted increases in atmospheric CO2 concentrations and the concomitant effects of these responses on the chemically-mediated interactions between plants and specialized herbivores.

Distance and sex determine host plant choice by herbivorous beetles

BACKGROUND

Plants respond to herbivore damage with the release of volatile organic compounds (VOCs). This indirect defense can cause ecological costs when herbivores themselves use VOCs as cues to localize suitable host plants. Can VOCs reliably indicate food plant quality to herbivores?

METHODOLOGY

We determined the choice behavior of herbivorous beetles (Chrysomelidae: Gynandrobrotica guerreroensis and Cerotoma ruficornis) when facing lima bean plants (Fabaceae: Phaseolus lunatus) with different cyanogenic potential, which is an important constitutive direct defense. Expression of inducible indirect defenses was experimentally manipulated by jasmonic acid treatment at different concentrations. The long-distance responses of male and female beetles to the resulting induced plant volatiles were investigated in olfactometer and free-flight experiments and compared to the short-distance decisions of the same beetles in feeding trials.

CONCLUSION

Female beetles of both species were repelled by VOCs released from all induced plants independent of the level of induction. In contrast, male beetles were repelled by strongly induced plants, showed no significant differences in choice behavior towards moderately induced plants, but responded positively to VOCs released from little induced plants. Thus, beetle sex and plant VOCs had a significant effect on host searching behavior. By contrast, feeding behavior of both sexes was strongly determined by the cyanogenic potential of leaves, although females again responded more sensitively than males. Apparently, VOCs mainly provide information to these beetles that are not directly related to food quality. Being induced by herbivory and involved in indirect plant defense, such VOCs might indicate the presence of competitors and predators to herbivores. We conclude that plant quality as a food source and finding a potentially enemy-free space is more important for female than for male insect herbivores, whereas the presence of a slightly damaged plant can help males to localize putative mating partners.

Increased host investment in extrafloral nectar (EFN) improves the efficiency of a mutualistic defensive service

Extrafloral nectar (EFN) plays an important role as plant indirect defence through the attraction of defending ants. Like all rewards produced in the context of a mutualism, however, EFN is in danger of being exploited by non-ant consumers that do not defend the plant against herbivores. Here we asked whether plants, by investing more in EFN, can improve their indirect defence, or rather increase the risk of losing this investment to EFN thieves. We used the obligate plant-ant Acacia-Pseudomyrmex system and examined experimentally in the field during the dry and the rainy seasons how variations in EFN secretion are related to (i) ant activity, to (ii) the ant-mediated defence against herbivores and (iii) the exploitation of EFN by non-ant consumers. Extrafloral investment enhanced ant recruitment and was positively related to the ant mediated defence against herbivores. The ant-mediated protection from exploiters also increased in proportion to the nectar sugar concentration. Although the daily peak of EFN production coincided with the highest activity of EFN thieves, Pseudomyrmex ferrugineus ants protected this resource effectively from exploiters. Nevertheless, the defensive effects by ants differed among seasons. During the dry season, plants grew slower and secreted more EFN than in the rainy season, and thus, experienced a higher level of ant-mediated indirect defence. Our results show that an increased plant investment in an indirect defence trait can improve the resulting defensive service against both herbivores and exploiters. EFN secretion by obligate ant-plants represents a defensive trait for which the level of investment correlates positively with the beneficial effects obtained.

Local adaptation of aboveground herbivores towards plant phenotypes induced by soil biota

BACKGROUND

Soil biota may trigger strong physiological responses in plants and consequently induce distinct phenotypes. Plant phenotype, in turn, has a strong impact on herbivore performance. Here, we tested the hypothesis that aboveground herbivores are able to adapt to plant phenotypes induced by soil biota.

METHODOLOGY AND PRINCIPAL FINDINGS

We bred spider mites for 15 generations on snap beans with three different belowground biotic interactions: (i) no biota (to serve as control), (ii) arbuscular mycorrhizal fungi and (ii) root-feeding nematodes. Subsequently, we conducted a reciprocal selection experiment using these spider mites, which had been kept on the differently treated plants. Belowground treatments induced changes in plant biomass, nutrient composition and water content. No direct chemical defence through cyanogenesis was detected in any of the plant groups. Growth rates of spider mites were higher on the ecotypes on which they were bred for 15 generations, although the statistical significance disappeared for mites from the nematode treatment when corrected for all multiple comparisons.

CONCLUSION/SIGNIFICANCE

These results demonstrate that belowground biota may indeed impose selection on the aboveground insect herbivores mediated by the host plant. The observed adaptation was driven by variable quantitative changes of the different separately studied life history traits (i.e. fecundity, longevity, sex-ratio, time to maturity).

Intraspecific variation in herbivore community composition and transcriptional profiles in field-grown Brassica oleracea cultivars

Intraspecific differences in plant defence traits are often correlated with variation in transcriptional profiles and can affect the composition of herbivore communities on field-grown plants. However, most studies on transcriptional profiling of plant-herbivore interactions have been carried out under controlled conditions in the laboratory or greenhouse and only a few examine intraspecific transcriptional variation. Here, intraspecific variation in herbivore community composition and transcriptional profiles between two Brassica oleracea cultivars grown in the field is addressed. Early in the season, no differences in community composition were found for naturally occurring herbivores, whereas cultivars differed greatly in abundance, species richness, and herbivore community later in the season. Genome-wide transcriptomic analysis using an Arabidopsis thaliana oligonucleotide microarray showed clear differences for the expression levels of 26 genes between the two cultivars later in the season. Several defence-related genes showed higher levels of expression in the cultivar that harboured the lowest numbers of herbivores. Our study shows that herbivore community composition develops differentially throughout the season on the two B. oleracea cultivars grown in the field. The correlation between the differences in herbivore communities and differential expression of particular defence-related genes is discussed.

Analyzing plant defenses in nature

A broad range of chemical plant defenses against herbivores has been studied extensively under laboratory conditions. In many of these cases there is still little understanding of their relevance in nature. In natural systems, functional analyses of plant traits are often complicated by an extreme variability, which affects the interaction with higher trophic levels. Successful analyses require consideration of the numerous sources of variation that potentially affect the plant trait of interest. In our recent study on wild lima bean (Phaseolus lunatus L.) in South Mexico, we applied an integrative approach combining analyses for quantitative correlations of cyanogenic potential (HCNp; the maximum amount of cyanide that can be released from a given tissue) and herbivory in the field with subsequent feeding trials under controlled conditions. This approach allowed us to causally explain the consequences of quantitative variation of HCNp on herbivore-plant interactions in nature and highlights the importance of combining data obtained in natural systems with analyses under controlled conditions.

Analyzing plant defenses in nature

A broad range of chemical plant defenses against herbivores has been studied extensively under laboratory conditions. In many of these cases there is still little understanding of their relevance in nature. In natural systems, functional analyses of plant traits are often complicated by an extreme variability, which affects the interaction with higher trophic levels. Successful analyses require consideration of the numerous sources of variation that potentially affect the plant trait of interest. In our recent study on wild lima bean (Phaseolus lunatus L.) in South Mexico, we applied an integrative approach combining analyses for quantitative correlations of cyanogenic potential (HCNp; the maximum amount of cyanide that can be released from a given tissue) and herbivory in the field with subsequent feeding trials under controlled conditions. This approach allowed us to causally explain the consequences of quantitative variation of HCNp on herbivore-plant interactions in nature and highlights the importance of combining data obtained in natural systems with analyses under controlled conditions.

Quantitative variability of cyanogenesis in Cathariostachys madagascariensis-the main food plant of bamboo lemurs in Southeastern Madagascar

Giant bamboo (Cathariostachys madagascariensis) is a major food plant for three sympatric species of bamboo-eating lemurs (Hapalemur aureus, H. griseus, and Prolemur simus) in the rain forests of southeastern Madagascar. This plant species is strongly cyanogenic. However, quantitative data on cyanide concentration in C. madagascariensis are scarce. Previous studies reported 15 mg cyanide per 100 g fresh shoot material (corresponding to approx. 57 micromol cyanide per gram dry weight). However, we found mean concentrations (+/-SE) ranging from 139.3+/-19.32 in ground shoots to 217.7+/-16.80 micromol cyanide per gram dry weight in branch shoots. Thus, cyanogenesis of C. madagascariensis was up to four times higher than reported before. In contrast to the strongly cyanogenic shoots no cyanide could be detected in differently aged leaves of C. madagascariensis confirming earlier studies. Within individual shoots fine-scaled analysis revealed a characteristic ontogenetic pattern of cyanide accumulation. Highest concentrations were found in youngest parts near the apical meristem, whereas concentrations decreased in older shoot parts. Beyond the general intra-individual variability of cyanogenic features analyses indicated site-specific variability of both, the ontogenetic pattern of cyanide concentration as well as the total amount of cyanide accumulated in shoots. Additionally, analyses of soluble proteins-one important nutritive measure affecting food plant quality-demonstrated a converse quantitative relation of protein concentrations in leaves to cyanide concentration in shoots at the site-specific level. We, thus, suggest integrative analyses on quantitative variation of cyanogenesis together with nutritive plant parameters in future studies. This approach would allow obtaining more detailed insights into spatial variability of giant bamboo's overall browse quality and its impact on lemur herbivores.