Leaf rust infection reduces herbivore-induced volatile emission in black poplar and attracts a generalist herbivore

Induction of volatile organic compounds in chrysanthemum plants following infection by Rhizoctonia solani

This study investigated the effects of Rhizoctonia solani J.G. Kühn infestation on the volatile organic compound (VOC) emissions and biochemical composition of ten cultivars of chrysanthemum (Chrysanthemum × morifolium /Ramat./ Hemsl.) to bring new insights for future disease management strategies and the development of resistant chrysanthemum cultivars. The chrysanthemum plants were propagated vegetatively and cultivated in a greenhouse under semi-controlled conditions. VOCs emitted by the plants were collected using a specialized system and analyzed by gas chromatography/mass spectrometry. Biochemical analyses of the leaves were performed, including the extraction and quantification of chlorophylls, carotenoids, and phenolic compounds. The emission of VOCs varied among the cultivars, with some cultivars producing a wider range of VOCs compared to others. The analysis of the VOC emissions from control plants revealed differences in both their quality and quantity among the tested cultivars. R. solani infection influenced the VOC emissions, with different cultivars exhibiting varying responses to the infection. Statistical analyses confirmed the significant effects of cultivar, collection time, and their interaction on the VOCs. Correlation analyses revealed positive relationships between certain pairs of VOCs. The results show significant differences in the biochemical composition among the cultivars, with variations in chlorophyll, carotenoids, and phenolic compounds content. Interestingly, R. solani soil and leaf infestation decreased the content of carotenoids in chrysanthemums. Plants subjected to soil infestation were characterized with the highest content of phenolics. This study unveils alterations in the volatile and biochemical responses of chrysanthemum plants to R. solani infestation, which can contribute to the development of strategies for disease management and the improvement of chrysanthemum cultivars with enhanced resistance to R. solani.

The same boat, different storm: stress volatile emissions in response to biotrophic fungal infections in primary and alternate hosts

Rust infection results in stress volatile emissions, but due to the complexity of host-pathogen interaction and variations in innate defense and capacity to induce defense, biochemical responses can vary among host species. Fungal-dependent modifications in volatile emissions have been well documented in numerous host species, but how emission responses vary among host species is poorly understood. Our recent experiments demonstrated that the obligate biotrophic crown rust fungus (P. coronata) differently activated primary and secondary metabolic pathways in its primary host Avena sativa and alternate host Rhamnus frangula. In A. sativa, emissions of methyl jasmonate, short-chained lipoxygenase products, long-chained saturated fatty acid derivatives, mono- and sesquiterpenes, carotenoid breakdown products, and benzenoids were initially elicited in an infection severity-dependent manner, but the emissions decreased under severe infection and photosynthesis was almost completely inhibited. In R. frangula, infection resulted in low-level induction of stress volatile emissions, but surprisingly, in enhanced constitutive isoprene emissions, and even severely-infected leaves maintained a certain photosynthesis rate. Thus, the same pathogen elicited a much stronger response in the primary than in the alternate host. We argue that future work should focus on resolving mechanisms of different fungal tolerance and resilience among primary and secondary hosts.

VOCs from fungi-infected apples attract and increase the oviposition of yellow peach moth Conogethes punctiferalis

BACKGROUND

Plant volatile organic compounds (VOCs) modified by plant-associated microbes can attract or repel the oviposition of herbivores. Here, we explored the effects of three different fungi on apples' VOCs and the cascading impacts on the oviposition preference of yellow peach moth [YPM, Conogethes punctiferalis (Guenée)].

RESULTS

Among Penicillium crustosum-infected apples (PCA), Rhizopus oryzae-infected apples (ROA), Colletotrichum gloeosporioides-infected apples (CGA) and healthy apples (HA), mated YPM females preferred to oviposit eggs on ROA and CGA, and showed significant attractiveness to VOCs from PCA, ROA, and CGA under laboratory conditions. The VOCs analyses showed that there were significant differences between fungi-infected apples (ROA, CGA) and control treatments (mechanically damaged apples (MDA), HA) in terms of the relative contents of 13 VOCs. The relative contents of ethyl 2-methylbutyrate, ethyl caprylate, estragole, ethyl hexanoate in ROA and CGA were higher than those in MDA. The relative content of isopropyl 2-methylbutyrate in ROA was significantly higher than those in HA and CGA. The relative contents of 2-methylbutyl acetate, butyl 2-methylbutyrate, hexyl 2-methylbutyrate, amyl hexanoate, hexyl hexanoate, (E, E)-α-farnesene in ROA and CGA were lower than those in HA. The relative content of hexyl acetate in ROA and CGA was significantly higher than that in MDA, but lower than that in HA. Additionally, 10 fungi-induced VOCs were detected in ROA and/or CGA. When 20 VOCs from ROA and/or CGA were tested as individuals or mixed blends in Y-tube olfactometer assays, mated YPM females preferred amyl 2-methylbutyrate, isoamyl 2-methylbutyrate, isopropyl 2-methylbutyrate, hexyl propionate (common VOCs in ROA, CGA, and HA), and heptacosane (a fungi-induced VOC in ROA), but no significant preferences were observed between individual compounds and mixed blends, except for hexyl propionate.

CONCLUSION

Different fungi infection increased the relative contents of common VOCs from healthy and fungi-infected apples, which ultimately resulted in the significant attractiveness for the oviposition of mated YPM females. This study clarified why fungi-infected apples were more attractive to YPMs than healthy apples and screened out the crucial VOCs for YPM oviposition. © 2023 Society of Chemical Industry.

A geraniol synthase regulates plant defense via alternative splicing in tea plants

Geraniol is an important contributor to the pleasant floral scent of tea products and one of the most abundant aroma compounds in tea plants; however, its biosynthesis and physiological function in response to stress in tea plants remain unclear. The proteins encoded by the full-length terpene synthase (CsTPS1) and its alternative splicing isoform (CsTPS1-AS) could catalyze the formation of geraniol when GPP was used as a substrate in vitro, whereas the expression of CsTPS1-AS was only significantly induced by Colletotrichum gloeosporioides and Neopestalotiopsis sp. infection. Silencing of CsTPS1 and CsTPS1-AS resulted in a significant decrease of geraniol content in tea plants. The geraniol content and disease resistance of tea plants were compared when CsTPS1 and CsTPS1-AS were silenced. Down-regulation of the expression of CsTPS1-AS reduced the accumulation of geraniol, and the silenced tea plants exhibited greater susceptibility to pathogen infection than control plants. However, there was no significant difference observed in the geraniol content and pathogen resistance between CsTPS1-silenced plants and control plants in the tea plants infected with two pathogens. Further analysis showed that silencing of CsTPS1-AS led to a decrease in the expression of the defense-related genes PR1 and PR2 and SA pathway-related genes in tea plants, which increased the susceptibility of tea plants to pathogens infections. Both in vitro and in vivo results indicated that CsTPS1 is involved in the regulation of geraniol formation and plant defense via alternative splicing in tea plants. The results of this study provide new insights into geraniol biosynthesis and highlight the role of monoterpene synthases in modulating plant disease resistance via alternative splicing.

Volatilome and proteome responses to Colletotrichum lindemuthianum infection in a moderately resistant and a susceptible bean genotype

We analyzed the changes in the volatilome, proteome, stomatal conductance, salicylic and jasmonic acid contents of a susceptible and a moderately resistant genotype of common bean, Phaseoulus vulgaris L., challenged with Colletotrichum lindemuthianum, the causal agent of fungal anthracnose. Our results indicate differences at both proteome and volatilome levels between the two genotypes, before and after the infection, and different defense strategies. The moderately resistant genotype hindered pathogen infection, invasion, and replication mainly by maintaining epidermal and cell wall structure. The susceptible genotype was not able to limit the early stages of pathogen infection. Rather, stomatal conductance increased in the infected susceptible genotype, and enhanced synthesis of Green Leaf Volatiles and salicylic acid was observed, together with a strong hypersensitive response. Proteomic investigation provided a general framework for physiological changes, whereas observed variations in the volatilome suggested that volatile organic compounds may principally represent stress markers rather than defensive compounds per se.

Arbuscular mycorrhizal fungus changes alfalfa (Medicago sativa) metabolites in response to leaf spot (Phoma medicaginis) infection, with subsequent effects on pea aphid (Acyrthosiphon pisum) behavior

Plant disease occurs simultaneously with insect attack. Arbuscular mycorrhizal fungi (AMF) modify plant biotic stress response. Arbuscular mycorrhizal fungi and pathogens may modify plant volatile organic compound (VOC) production and insect behavior. Nevertheless, such effects are rarely studied, particularly for mesocosms where component organisms interact with each other. Plant-mediated effects of leaf pathogen (Phoma medicaginis) infection on aphid (Acyrthosiphon pisum) infestation, and role of AMF (Rhizophagus intraradices) in modifying these interactions were elucidated in a glasshouse experiment. We evaluated alfalfa disease occurrence, photosynthesis, phytohormones, trypsin inhibitor (TI) and total phenol response to pathogen and aphid attack, with or without AMF, and aphid behavior towards VOCs from AMF inoculated and non-mycorrhizal alfalfa, with or without pathogen infection. AM fungus enhanced alfalfa resistance to pathogen and aphid infestation. Plant biomass, root : shoot ratio, net photosynthetic rate, transpiration rate, stomatal conductance, salicylic acid, and TI were significantly increased in AM-inoculated alfalfa. Arbuscular mycorrhizal fungi and pathogen significantly changed alfalfa VOCs. Aphids preferred VOCs of AM-inoculated and nonpathogen-infected to nonmycorrhizal and pathogen-infected alfalfa. We propose that AMF alter plant response to multiple biotic stresses in ways both beneficial and harmful to the plant host, providing a basis for strategies to manage pathogens and herbivore pests.

Inter-laboratory comparison of plant volatile analyses in the light of intra-specific chemodiversity

INTRODUCTION

Assessing intraspecific variation in plant volatile organic compounds (VOCs) involves pitfalls that may bias biological interpretation, particularly when several laboratories collaborate on joint projects. Comparative, inter-laboratory ring trials can inform on the reproducibility of such analyses.

OBJECTIVES

In a ring trial involving five laboratories, we investigated the reproducibility of VOC collections with polydimethylsiloxane (PDMS) and analyses by thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS). As model plant we used Tanacetum vulgare, which shows a remarkable diversity in terpenoids, forming so-called chemotypes. We performed our ring-trial with two chemotypes to examine the sources of technical variation in plant VOC measurements during pre-analytical, analytical, and post-analytical steps.

METHODS

Monoclonal root cuttings were generated in one laboratory and distributed to five laboratories, in which plants were grown under laboratory-specific conditions. VOCs were collected on PDMS tubes from all plants before and after a jasmonic acid (JA) treatment. Thereafter, each laboratory (donors) sent a subset of tubes to four of the other laboratories (recipients), which performed TD-GC-MS with their own established procedures.

RESULTS

Chemotype-specific differences in VOC profiles were detected but with an overall high variation both across donor and recipient laboratories. JA-induced changes in VOC profiles were not reproducible. Laboratory-specific growth conditions led to phenotypic variation that affected the resulting VOC profiles.

CONCLUSION

Our ring trial shows that despite large efforts to standardise each VOC measurement step, the outcomes differed both qualitatively and quantitatively. Our results reveal sources of variation in plant VOC research and may help to avoid systematic errors in similar experiments.

Dynamic environmental interactions shaped by vegetative plant volatiles

Covering: up to November 2022Plants shape terrestrial ecosystems through physical and chemical interactions. Plant-derived volatile organic compounds in particular influence the behavior and performance of other organisms. In this review, we discuss how vegetative plant volatiles derived from leaves, stems and roots are produced and released into the environment, how their production and release is modified by abiotic and biotic factors, and how they influence other organisms. Vegetative plant volatiles are derived from different biosynthesis and degradation pathways and are released via distinct routes. Both biosynthesis and release are regulated by other organisms as well as abiotic factors. In turn, vegetative plant volatiles modify the physiology and the behavior of a wide range of organisms, from microbes to mammals. Several concepts and frameworks can help to explain and predict the evolution and ecology of vegetative plant volatile emission patterns of specific pathways: multifunctionality of specialized metabolites, chemical communication displays and the information arms race, and volatile physiochemistry. We discuss how these frameworks can be leveraged to understand the evolution and expression patterns of vegetative plant volatiles. The multifaceted roles of vegetative plant volatiles provide fertile grounds to understand ecosystem dynamics and harness their power for sustainable agriculture.

Bioactive volatile compounds from Penicillium digitatum-infected apples: Oviposition attractants for yellow peach moth Conogethes punctiferalis (Lepidoptera: Crambidae)

Introduction

Plant-associated microbes critically shape the dynamics of plant-and insect-associated communities. In previous studies, we reported that the yellow peach moth Conogethes punctiferalis (YPM) preferred to Penicillium digitatum-infected apples (PDA) for oviposition. However, the underlying mechanisms remains unclear.

Methods

In the present study, the behavioral and physiological experiments were conducted to determine how P. digitatum affects the oviposition selection of mated YPM females via altering host plant volatile organic compounds (VOCs).

Results

Mated YPM females were attracted to and laid more eggs on PDA than on non-infected apples (NIA), mechanically damaged apples (MDA), and P. digitatum in potato dextrose agar medium (PPD) in the oviposition selection experiments. Four-arm olfactometer assays further confirmed that odors in PDA were responsible for the attractiveness of mated YPM females. Further analyses showed that 38 VOCs were collected and identified from all treatments by GC-MS, with five specific VOCs (methyl 2-methylbutyrate, styrene, methyl caproate, butyl caprylate, and n-tetradecane) emitting from PDA. A principal component analysis (PCA) based on the absolute contents of 38 VOCs revealed a clear separation of PDA from NIA, MDA, and PPD. Moreover, when P. digitatum-induced specific VOCs were added to apples in individual or synthetic blends, there was a significantly higher percentage of mated YPM females to apples with individual or synthetic blends consisting of methyl 2-methylbutyrate, butyl caprylate, or n-tetradecane in Y-tube olfactometer experiments, suggesting that these three specific VOCs acted as predominant olfactory signals for mated YPM females to PDA.

Discussion

Taken together, the microbe P. digitatum was an important driver of the interactions between YPMs and host plants by altering plant volatiles. These findings may form the basis for developing attractant baits for field trapping YPMs in the future.

Sugarcane multitrophic interactions: Integrating belowground and aboveground organisms

Sugarcane is a crop of major importance used mainly for sugar and biofuel production, and many additional applications of its byproducts are being developed. Sugarcane cultivation is plagued by many insect pests and pathogens that reduce sugarcane yields overall. Recently emerging studies have shown complex multitrophic interactions in cultivated areas, such as the induction of sugarcane defense-related proteins by insect herbivory that function against fungal pathogens that commonly appear after mechanical damage. Fungi and viruses infecting sugarcane also modulate insect behavior, for example, by causing changes in volatile compounds responsible for insect attraction or repelling natural vector enemies via a mechanism that increases pathogen dissemination from infected plants to healthy ones. Interestingly, the fungus Fusarium verticillioides is capable of being vertically transmitted to insect offspring, ensuring its persistence in the field. Understanding multitrophic complexes is important to develop better strategies for controlling pathosystems affecting sugarcane and other important crops and highlights the importance of not only studying binary interactions but also adding as many variables as possible to effectively translate laboratory research to real-life conditions.

Penicillium fungi mediate behavioral responses of the yellow peach moth, Conogethes punctiferalis (Guenée) to apple fruits via altering the emissions of host plant VOCs

Plant-associated microbes have been reported as important but overlooked drivers of plant-herbivorous insect interactions. Influence of plant-associated microbes on plant-insect interactions is diverse, including beneficial, detrimental, and neutral. Here, we determined the effects of three Penicillium fungi, including Penicillium citrinum, Penicillium sumatrense, and Penicillium digitatum, on the oviposition selection and behavior of the yellow peach moth (YPM), Conogethes punctiferalis (Guenée). Compared with fungi noninfected apples (NIA), mechanically damaged apples (MDA), and P. citrinum in potato dextrose agar medium (PC), the oviposition selection and four-arm olfactometer experiments both showed that mated YPM females preferred to P. citrinum-infected apples (PCA). For P. sumatrense or P. digitatum, we also found that mated YPM females preferred to P. sumatrense-infected apples (PSA) or P. digitatum-infected apples (PDA), respectively. Among three Penicillium fungi-infected apples, the selection rates including oviposition and olfactometer behavior of mated YPM females on PDA were both higher than those on PSA and PCA. Further analyses of host plant volatile organic compounds (VOCs) by GC-MS showed that the absolute contents of ethyl hexanoate and (Z, E)-α-farnesene in PCA, PSA, and PDA were all higher than those in NIA, and a total of 16 novel VOCs were detected in fungi-infected apples (PCA, PSA, and PDA), indicating that fungi infection changed the components and proportions of apple VOCs. Taken together, three Penicillium fungi play significant roles in mediating the host selection of YPMs via altering the emissions of VOCs. These findings will be beneficial for developing formulations for field trapping of YPMs in the future.

Effect of Drought and Methyl Jasmonate Treatment on Primary and Secondary Isoprenoid Metabolites Derived from the MEP Pathway in the White Spruce Picea glauca

White spruce (Picea glauca) emits monoterpenes that function as defensive signals and weapons after herbivore attack. We assessed the effects of drought and methyl jasmonate (MeJA) treatment, used as a proxy for herbivory, on monoterpenes and other isoprenoids in P. glauca. The emission of monoterpenes was significantly increased after MeJA treatment compared to the control, but drought suppressed the MeJA-induced increase. The composition of the emitted blend was altered strongly by stress, with drought increasing the proportion of oxygenated compounds and MeJA increasing the proportion of induced compounds such as linalool and (E)-β-ocimene. In contrast, no treatment had any significant effect on the levels of stored monoterpenes and diterpenes. Among other MEP pathway-derived isoprenoids, MeJA treatment decreased chlorophyll levels by 40%, but had no effect on carotenoids, while drought stress had no impact on either of these pigment classes. Of the three described spruce genes encoding 1-deoxy-D-xylulose-5-phosphate synthase (DXS) catalyzing the first step of the MEP pathway, the expression of only one, DXS2B, was affected by our treatments, being increased by MeJA and decreased by drought. These findings show the sensitivity of monoterpene emission to biotic and abiotic stress regimes, and the mediation of the response by DXS genes.

Both Constitutive and Infection-Responsive Secondary Metabolites Linked to Resistance against Austropuccinia psidii (Myrtle Rust) in Melaleuca quinquenervia

Austropuccinia psidii is a fungal plant pathogen that infects species within the Myrtaceae, causing the disease myrtle rust. Myrtle rust is causing declines in populations within natural and managed ecosystems and is expected to result in species extinctions. Despite this, variation in response to A. psidii exist within some species, from complete susceptibility to resistance that prevents or limits infection by the pathogen. Untargeted metabolomics using Ultra Performance Liquid Chromatography with Ion Mobility followed by analysis using MetaboAnalyst 3.0, was used to explore the chemical defence profiles of resistant, hypersensitive and susceptible phenotypes within Melaleuca quinquenervia during the early stages of A. psidii infection. We were able to identify three separate pools of secondary metabolites: (i) metabolites classified structurally as flavonoids that were naturally higher in the leaves of resistant individuals prior to infection, (ii) organoheterocyclic and carbohydrate-related metabolites that varied with the level of host resistance post-infection, and (iii) metabolites from the terpenoid pathways that were responsive to disease progression regardless of resistance phenotype suggesting that these play a minimal role in disease resistance during the early stages of colonization of this species. Based on the classes of these secondary metabolites, our results provide an improved understanding of key pathways that could be linked more generally to rust resistance with particular application within Melaleuca.

Fungi as mediators linking organisms and ecosystems

Fungi form a major and diverse component of most ecosystems on Earth. They are both micro and macroorganisms with high and varying functional diversity as well as great variation in dispersal modes. With our growing knowledge of microbial biogeography, it has become increasingly clear that fungal assembly patterns and processes differ from other microorganisms such as bacteria, but also from macroorganisms such as plants. The success of fungi as organisms and their influence on the environment lies in their ability to span multiple dimensions of time, space, and biological interactions, that is not rivalled by other organism groups. There is also growing evidence that fungi mediate links between different organisms and ecosystems, with the potential to affect the macroecology and evolution of those organisms. This suggests that fungal interactions are an ecological driving force, interconnecting different levels of biological and ecological organisation of their hosts, competitors, and antagonists with the environment and ecosystem functioning. Here we review these emerging lines of evidence by focusing on the dynamics of fungal interactions with other organism groups across various ecosystems. We conclude that the mediating role of fungi through their complex and dynamic ecological interactions underlie their importance and ubiquity across Earth's ecosystems.

Volatile emission and biosynthesis in endophytic fungi colonizing black poplar leaves

Plant volatiles play a major role in plant-insect interactions as defense compounds or attractants for insect herbivores. Recent studies have shown that endophytic fungi are also able to produce volatiles and this raises the question of whether these fungal volatiles influence plant-insect interactions. Here, we qualitatively investigated the volatiles released from 13 endophytic fungal species isolated from leaves of mature black poplar (Populus nigra) trees. The volatile blends of these endophytes grown on agar medium consist of typical fungal compounds, including aliphatic alcohols, ketones and esters, the aromatic alcohol 2-phenylethanol and various sesquiterpenes. Some of the compounds were previously reported as constituents of the poplar volatile blend. For one endophyte, a species of Cladosporium, we isolated and characterized two sesquiterpene synthases that can produce a number of mono- and sesquiterpenes like (E)-β-ocimene and (E)-β-caryophyllene, compounds that are dominant components of the herbivore-induced volatile bouquet of black poplar trees. As several of the fungus-derived volatiles like 2-phenylethanol, 3-methyl-1-butanol and the sesquiterpene (E)-β-caryophyllene, are known to play a role in direct and indirect plant defense, the emission of volatiles from endophytic microbial species should be considered in future studies investigating tree-insect interactions.

Rehydration Process in Rustyback Fern (Asplenium ceterach L.): Profiling of Volatile Organic Compounds

Simple Summary

Severe environmental changes, such as drought, can delay growth, the development of plants, and induce injury to their tissues. However, a group of land plant species, called resurrection or desiccation-tolerant plants, is able to lose 95% of their cellular water and still remain viable for long periods, resuming full metabolic activity upon rehydration. Recovery from near-complete water loss is complex and requires the coordination of physical and chemical processes in the resurrection plants. Under stress conditions plants also synthesize and release a wide variety of volatile organic compounds with diverse biological and ecological functions. The rehydration process in resurrection rustyback fern (Asplenium ceterach) resulted in complete plant recovery within 72 h, accompanied by high emission of volatiles, mainly belonging to the group of fatty acid derivatives. These findings could have significant implications from biotechnological and ecological perspectives since the rustyback fern has been recently recognized as a valuable source of bioactive compounds.

Abstract

When exposed to stressful conditions, plants produce numerous volatile organic compounds (VOCs) that have different biological and environmental functions. VOCs emitted during the rehydration process by the fronds of desiccation tolerant fern Asplenium ceterach L. were investigated. Headspace GC–MS analysis revealed that the volatiles profile of rustyback fern is mainly composed of fatty acid derivatives: isomeric heptadienals (over 25%) and decadienals (over 20%), other linear aldehydes, alcohols, and related compounds. Aerial parts of the rustyback fern do not contain monoterpene-type, sesquiterpene-type, and diterpene-type hydrocarbons or corresponding terpenoids. Online detection of VOCs using proton-transfer reaction mass spectrometry (PTR–MS) showed a significant increase in emission intensity of dominant volatiles during the first hours of the rehydration process. Twelve hours after re-watering, emission of detected volatiles had returned to the basal levels that corresponded to hydrated plants. During the early phase of rehydration malondialdehyde (MDA) content in fronds, as an indicator of membrane damage, decreased rapidly which implies that lipoxygenase activity is not stimulated during the recovery process of rustyback fern.

A peroxisomal β-oxidative pathway contributes to the formation of C6-C1 aromatic volatiles in poplar

Benzenoids (C6-C1 aromatic compounds) play important roles in plant defense and are often produced upon herbivory. Black cottonwood (Populus trichocarpa) produces a variety of volatile and nonvolatile benzenoids involved in various defense responses. However, their biosynthesis in poplar is mainly unresolved. We showed feeding of the poplar leaf beetle (Chrysomela populi) on P. trichocarpa leaves led to increased emission of the benzenoid volatiles benzaldehyde, benzylalcohol, and benzyl benzoate. The accumulation of salicinoids, a group of nonvolatile phenolic defense glycosides composed in part of benzenoid units, was hardly affected by beetle herbivory. In planta labeling experiments revealed that volatile and nonvolatile poplar benzenoids are produced from cinnamic acid (C6-C3). The biosynthesis of C6-C1 aromatic compounds from cinnamic acid has been described in petunia (Petunia hybrida) flowers where the pathway includes a peroxisomal-localized chain shortening sequence, involving cinnamate-CoA ligase (CNL), cinnamoyl-CoA hydratase/dehydrogenase (CHD), and 3-ketoacyl-CoA thiolase (KAT). Sequence and phylogenetic analysis enabled the identification of small CNL, CHD, and KAT gene families in P. trichocarpa. Heterologous expression of the candidate genes in Escherichia coli and characterization of purified proteins in vitro revealed enzymatic activities similar to those described in petunia flowers. RNA interference-mediated knockdown of the CNL subfamily in gray poplar (Populus x canescens) resulted in decreased emission of C6-C1 aromatic volatiles upon herbivory, while constitutively accumulating salicinoids were not affected. This indicates the peroxisomal β-oxidative pathway participates in the formation of volatile benzenoids. The chain shortening steps for salicinoids, however, likely employ an alternative pathway.

Social networking in crop plants: Wired and wireless cross-plant communications

The plant-associated microbial community (microbiome) has an important role in plant-plant communications. Plants decipher their complex habitat situations by sensing the environmental stimuli and molecular patterns and associated with microbes, herbivores and dangers. Perception of these cues generates inter/intracellular signals that induce modifications of plant metabolism and physiology. Signals can also be transferred between plants via different mechanisms, which we classify as wired- and wireless communications. Wired communications involve direct signal transfers between plants mediated by mycorrhizal hyphae and parasitic plant stems. Wireless communications involve plant volatile emissions and root exudates elicited by microbes/insects, which enable inter-plant signalling without physical contact. These producer-plant signals induce microbiome adaptation in receiver plants via facilitative or competitive mechanisms. Receiver plants eavesdrop to anticipate responses to improve fitness against stresses. An emerging body of information in plant-plant communication can be leveraged to improve integrated crop management under field conditions.

Insect herbivory facilitates the establishment of an invasive plant pathogen

Plants can be severely affected by insect herbivores and phytopathogenic fungi, but interactions between these plant antagonists are poorly understood. We analysed the impact of feeding damage by the abundant herbivore Orchestes fagi on infection rates of beech (Fagus sylvatica) leaves with Petrakia liobae, an invasive plant pathogenic fungus. The fungus was not detected in hibernating beetles, indicating that O. fagi does not serve as vector for P. liobae, at least not between growing seasons. Abundance of the fungus in beech leaves increased with feeding damage of the beetle and this relationship was stronger for sun-exposed than for shaded leaves. A laboratory experiment revealed sun-exposed leaves to have thicker cell walls and to be more resistant to pathogen infection than shaded leaves. Mechanical damage significantly increased frequency and size of necroses in the sun, but not in shade leaves. Our findings indicate that feeding damage of adult beetles provides entry ports for fungal colonization by removal of physical barriers and thus promotes infection success by pathogenic fungi. Feeding activity by larvae probably provides additional nutrient sources or eases access to substrates for the necrotrophic fungus. Our study exemplifies that invasive pathogens may benefit from herbivore activity, which may challenge forest health in light of climate change.

Evolution of isoprenyl diphosphate synthase-like terpene synthases in fungi

Terpene synthases (TPSs) and trans-isoprenyl diphosphate synthases (IDSs) are among the core enzymes for creating the enormous diversity of terpenoids. Despite having no sequence homology, TPSs and IDSs share a conserved “α terpenoid synthase fold” and a trinuclear metal cluster for catalysis, implying a common ancestry with TPSs hypothesized to evolve from IDSs anciently. Here we report on the identification and functional characterization of novel IDS-like TPSs (ILTPSs) in fungi that evolved from IDS relatively recently, indicating recurrent evolution of TPSs from IDSs. Through large-scale bioinformatic analyses of fungal IDSs, putative ILTPSs that belong to the geranylgeranyl diphosphate synthase (GGDPS) family of IDSs were identified in three species of Melampsora. Among the GGDPS family of the two Melampsora species experimentally characterized, one enzyme was verified to be bona fide GGDPS and all others were demonstrated to function as TPSs. Melampsora ILTPSs displayed kinetic parameters similar to those of classic TPSs. Key residues underlying the determination of GGDPS versus ILTPS activity and functional divergence of ILTPSs were identified. Phylogenetic analysis implies a recent origination of these ILTPSs from a GGDPS progenitor in fungi, after the split of Melampsora from other genera within the class of Pucciniomycetes. For the poplar leaf rust fungus Melampsora larici-populina, the transcripts of its ILTPS genes were detected in infected poplar leaves, suggesting possible involvement of these recently evolved ILTPS genes in the infection process. This study reveals the recurrent evolution of TPSs from IDSs since their ancient occurrence and points to the possibility of a wide distribution of ILTPS genes in three domains of life.

Herbivory meets fungivory: insect herbivores feed on plant pathogenic fungi for their own benefit

Plants are regularly colonised by fungi and bacteria, but plant-inhabiting microbes are rarely considered in studies on plant-herbivore interactions. Here we show that young gypsy moth (Lymantria dispar) caterpillars prefer to feed on black poplar (Populus nigra) foliage infected by the rust fungus Melampsora larici-populina instead of uninfected control foliage, and selectively consume fungal spores. This consumption, also observed in a related lepidopteran species, is stimulated by the sugar alcohol mannitol, found in much higher concentration in fungal tissue and infected leaves than uninfected plant foliage. Gypsy moth larvae developed more rapidly on rust-infected leaves, which cannot be attributed to mannitol but rather to greater levels of total nitrogen, essential amino acids and B vitamins in fungal tissue and fungus-infected leaves. Herbivore consumption of fungi and other microbes may be much more widespread than commonly believed with important consequences for the ecology and evolution of plant-herbivore interactions.

Aboveground phytochemical responses to belowground herbivory in poplar trees and the consequence for leaf herbivore preference

Belowground (BG) herbivory can influence aboveground (AG) herbivore performance and food preference via changes in plant chemistry. Most evidence for this phenomenon derives from studies in herbaceous plants but studies in woody plants are scarce. Here we investigated whether and how BG herbivory on black poplar (Populus nigra) trees by Melolontha melolontha larvae influences the feeding preference of Lymantria dispar (gypsy moth) caterpillars. In a food choice assay, caterpillars preferred to feed on leaves from trees that had experienced attack by BG herbivores. Therefore, we investigated the effect of BG herbivory on the phytochemical composition of P. nigra trees alone and in combination with AG feeding by L. dispar caterpillars. BG herbivory did not increase systemic AG tree defences like volatile organic compounds, protease inhibitors and salicinoids. Jasmonates and salicylic acid were also not induced by BG herbivory in leaves but abscisic acid concentrations drastically increased together with proline and few other amino acids. Leaf coating experiments with amino acids suggest that proline might be responsible for the caterpillar feeding preference via presumptive phagostimulatory properties. This study shows that BG herbivory in poplar can modify the feeding preference of AG herbivores via phytochemical changes as a consequence of root-to-shoot signaling.

Roles of plant volatiles in defence against microbial pathogens and microbial exploitation of volatiles

Plants emit a large variety of volatile organic compounds during infection by pathogenic microbes, including terpenes, aromatics, nitrogen-containing compounds, and fatty acid derivatives, as well as the volatile plant hormones, methyl jasmonate, and methyl salicylate. Given the general antimicrobial activity of plant volatiles and the timing of emission following infection, these compounds have often been assumed to function in defence against pathogens without much solid evidence. In this review, we critically evaluate current knowledge on the toxicity of volatiles to fungi, bacteria, and viruses and their role in plant resistance as well as how they act to induce systemic resistance in uninfected parts of the plant and in neighbouring plants. We also discuss how microbes can detoxify plant volatiles and exploit them as nutrients, attractants for insect vectors, and inducers of volatile emissions, which stimulate immune responses that make plants more susceptible to infection. Although much more is known about plant volatile-herbivore interactions, knowledge of volatile-microbe interactions is growing and it may eventually be possible to harness plant volatiles to reduce disease in agriculture and forestry. Future research in this field can be facilitated by making use of the analytical and molecular tools generated by the prolific research on plant-herbivore interactions.

Metabolic and physiological analyses reveal that Populus cathayana males adopt an energy-saving strategy to cope with phosphorus deficiency

Dioecious trees have evolved sex-specific adaptation strategies to cope with inorganic phosphorus (Pi) limitation. Yet, little is known about the effects of Pi limitation on plant metabolism, particularly in dioecious woody plants. To identify potential gender-specific metabolites appearing in response to Pi limitation in poplars, we studied the metabolic and ionomic responses in the roots and leaves of Populus cathayana Rehd males and females exposed to a 60-day period of Pi deficiency. Besides significant decreases in phosphorus contents in both Pi-deficient roots and leaves, the calcium level decreased significantly and the sulfur content increased significantly in Pi-deficient male roots, while the zinc and ferrum contents increased significantly in Pi-deficient female roots. Inorganic P deficiency caused a smaller change in the abscisic acid content, but a significant increase in the jasmonic acid content was detected in both leaves and roots. Salicylic acid significantly decreased under Pi deficiency in male leaves and female roots. Changes were found in phospholipids and phosphorylated metabolites (e.g., fructose-6-phosphate, glycerol-3-phosphate, glucose-6-phosphate, phosphoric acid and inositol-1-phosphate) in roots and leaves. Both P. cathayana males and females relied on inorganic pyrophosphate-dependent but not on Pi-dependent glycolysis under Pi-deficient conditions. Sex-specific metabolites in leaves were primarily in the category of primary metabolites (e.g., amino acids), while in roots primarily in the category of secondary metabolites (e.g., organic acids) and sugars. The metabolome analysis revealed that sexually different pathways occurred mainly in amino acid metabolism, and the tissue-related differences were in the shikimate pathway and glycolysis. We observed changes in carbon flow, reduced root biomass and increased amino acid contents in P. cathayana males but not in females, which indicated that males have adopted an energy-saving strategy to adapt to Pi deficiency. Thus, this study provides new insights into sex-specific metabolic responses to Pi deficiency.

Eyes on the future - evidence for trade-offs between growth, storage and defense in Norway spruce

Carbon (C) allocation plays a central role in tree responses to environmental changes. Yet, fundamental questions remain about how trees allocate C to different sinks, for example, growth vs storage and defense. In order to elucidate allocation priorities, we manipulated the whole-tree C balance by modifying atmospheric CO₂ concentrations [CO₂ ] to create two distinct gradients of declining C availability, and compared how C was allocated among fluxes (respiration and volatile monoterpenes) and biomass C pools (total biomass, nonstructural carbohydrates (NSC) and secondary metabolites (SM)) in well-watered Norway spruce (Picea abies) saplings. Continuous isotope labelling was used to trace the fate of newly-assimilated C. Reducing [CO₂ ] to 120 ppm caused an aboveground C compensation point (i.e. net C balance was zero) and resulted in decreases in growth and respiration. By contrast, soluble sugars and SM remained relatively constant in aboveground young organs and were partially maintained with a constant allocation of newly-assimilated C, even at expense of root death from C exhaustion. We conclude that spruce trees have a conservative allocation strategy under source limitation: growth and respiration can be downregulated to maintain 'operational' concentrations of NSC while investing newly-assimilated C into future survival by producing SM.

Plant volatile emission depends on the species composition of the neighboring plant community

BACKGROUND

Plants grow in multi-species communities rather than monocultures. Yet most studies on the emission of volatile organic compounds (VOCs) from plants in response to insect herbivore feeding focus on one plant species. Whether the presence and identity of neighboring plants or plant community attributes, such as plant species richness and plant species composition, affect the herbivore-induced VOC emission of a focal plant is poorly understood.

METHODS

We established experimental plant communities in pots in the greenhouse where the focal plant species, red clover (Trifolium pratense), was grown in monoculture, in a two species mixture together with Geranium pratense or Dactylis glomerata, or in a mixture of all three species. We measured VOC emission of the focal plant and the entire plant community, with and without herbivory of Spodoptera littoralis caterpillars caged on one red clover individual within the communities.

RESULTS

Herbivory increased VOC emission from red clover, and increasing plant species richness changed emissions of red clover and also from the entire plant community. Neighbor identity strongly affected red clover emission, with highest emission rates for plants growing together with D. glomerata.

CONCLUSION

The results from this study indicate that the blend of VOCs perceived by host searching insects can be affected by plant-plant interactions.

Extended phenotype in action. Two possible roles for silica needles in plants: not just injuring herbivores but also inserting pathogens into their tissues

Phytoliths are silica bodies of various shapes including in the shape of sharp needles formed by many land plants. Defense from herbivory is one of the several known functions of phytoliths, especially the mechanical defense by abrasion of the mouthparts of arthropods and the teeth of mammalian herbivores. Another, although somewhat lesser-known, anti-herbivory defensive mechanism of phytoliths is wounding by sharp silica needles. We discuss and illuminate an even much less known defensive mechanism by phytoliths, i.e., the ability of needle-like phytoliths to insert microscopic pathogens (bacteria, fungi, viruses) into herbivores' tissues. We do it by comparison and by showing analogy with the better-known insertion of microbial pathogens into the body of herbivores by thorns, spines, and prickles. This largely overlooked and understudied defensive mechanism is a special case of a double extended phenotype; plants' defense, and the multiplication and dispersal of microorganisms, and is thus a case of mutualism.

Salicylic acid activates poplar defense against the biotrophic rust fungus Melampsora larici-populina via increased biosynthesis of catechin and proanthocyanidins

Poplar trees synthesize flavan-3-ols (catechin and proanthocyanidins) as a defense against foliar rust fungi, but the regulation of this defense response is poorly understood. Here, we investigated the role of hormones in regulating flavan-3-ol accumulation in poplar during rust infection. We profiled levels of defense hormones, signaling genes, and flavan-3-ol metabolites in black poplar leaves at different stages of rust infection. Hormone levels were manipulated by external sprays, genetic engineering, and drought to reveal their role in rust fungal defenses. Levels of salicylic acid (SA), jasmonic acid, and abscisic acid increased in rust-infected leaves and activated downstream signaling, with SA levels correlating closely with those of flavan-3-ols. Pretreatment with the SA analog benzothiadiazole increased flavan-3-ol accumulation by activating the MYB-bHLH-WD40 complex and reduced rust proliferation. Furthermore, transgenic poplar lines overproducing SA exhibited higher amounts of flavan-3-ols constitutively via the same transcriptional activation mechanism. These findings suggest a strong association among SA, flavan-3-ol biosynthesis, and rust resistance in poplars. Abscisic acid also promoted poplar defense against rust infection, but likely through stomatal immunity independent of flavan-3-ols. Jasmonic acid did not confer any apparent defense responses to the fungal pathogen. We conclude that SA activates flavan-3-ol biosynthesis in poplar against rust infection.

Terpene Synthase Genes in Quercus robur - Gene Characterization, Expression and Resulting Terpenes Due to Cockchafer Feeding

Root herbivory caused by larvae of the forest cockchafer (Melolontha hippocastani) enhances the impact of drought on trees, particularly in oak forest rejuvenations. In Germany, geographically distant oak stands show differences in infestation strength by the forest cockchafer. While in Southwestern Germany this insect causes severe damage, oak forests in northern Germany are rarely infested. It is known that root-released volatile organic compounds (VOCs) are perceived by soil herbivores, thus guiding the larvae toward the host roots. In this work, we exposed seedlings of two distant oak provenances to forest cockchafer larvae and studied their population genetic properties, their root-based VOC chemotypes, their attraction for larvae and terpene synthase gene expression. Based on nuclear and chloroplast marker analysis, we found both oak populations to be genetically highly variable while showing typical patterns of migration from different refugial regions. However, no clear association between genetic constitution of the different provenances and the abundance of cockchafer populations on site was observed. In contrast to observations in the field, bioassays revealed a preference of the larvae for the northeastern oak provenance. The behavior of larvae was most likely related to root-released volatile terpenes and benzenoids since their composition and quantity differed between oak populations. We assume repellent effects of these compounds because the populations attractive to insects showed low abundance of these compounds. Five different oak terpene synthase (TPS) genes were identified at the genomic level which can be responsible for biosynthesis of the released terpenes. TPS gene expression patterns in response to larval feeding revealed geographic variation rather than genotypic variation. Our results support the assumption that root-released VOC are influencing the perception of roots by herbivores.

Microbe-induced plant volatiles

Plants emit a plethora of volatile organic compounds in response to biotic and abiotic stresses. These compounds act as infochemicals for ecological communication in the phytobiome. This study reviews the role of microbe-induced plant volatiles (MIPVs) in plant-microbe interactions. MIPVs are affected by the taxonomic position of the microbe, the identity of the plant and the type of interaction. Plants also emit exclusive blends of volatiles in response to nonhost and host interactions, as well as to beneficial microbes and necrotrophic/biotrophic pathogens. These MIPVs directly inhibit pathogen growth and indirectly promote resistance/susceptibility to subsequent plant pathogen attack. Viruses and phloem-limiting bacteria modify plant volatiles to attract insect vectors. Susceptible plants can respond to MIPVs from resistant plants and become resistant. Recent advances in our understanding of the molecular mechanisms of MIPV synthesis in plants and how plant pathogen effectors manipulate their biosynthesis are discussed. This knowledge will help broaden our understanding of plant-microbe interactions and should facilitate the development of new emerging techniques for sustainable plant disease management.

Rust Infection of Black Poplar Trees Reduces Photosynthesis but Does Not Affect Isoprene Biosynthesis or Emission

Poplar (Populus spp.) trees are widely distributed and play an important role in ecological communities and in forestry. Moreover, by releasing high amounts of isoprene, these trees impact global atmospheric chemistry. One of the most devastating diseases for poplar is leaf rust, caused by fungi of the genus Melampsora. Despite the wide distribution of these biotrophic pathogens, very little is known about their effects on isoprene biosynthesis and emission. We therefore infected black poplar (P. nigra) trees with the rust fungus M. larici-populina and monitored isoprene emission and other physiological parameters over the course of infection to determine the underlying mechanisms. We found an immediate and persistent decrease in photosynthesis during infection, presumably caused by decreased stomatal conductance mediated by increased ABA levels. At the same time, isoprene emission remained stable during the time course of infection, consistent with the stability of its biosynthesis. There was no detectable change in the levels of intermediates or gene transcripts of the methylerythritol 4-phosphate (MEP) pathway in infected compared to control leaves. Rust infection thus does not affect isoprene emission, but may still influence the atmosphere via decreased fixation of CO₂.