Volatile compounds act as a language that plants use for their communication and interaction with the surrounding environment. To date, a total of 1700 volatile compounds have been isolated from more than 90 plant families. These volatiles, released from leaves, flowers, and fruits into the atmosphere and from roots into the soil, defend plants against herbivores and pathogens or provide a reproductive advantage by attracting pollinators and seed dispersers. Plant volatiles constitute about 1% of plant secondary metabolites and are mainly represented by terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and amino acid derivatives. In this review we focus on the functions of plant volatiles, their biosynthesis and regulation, and the metabolic engineering of the volatile spectrum, which results in plant defense improvement and changes of scent and aroma properties of flowers and fruits.

Plant Volatiles: Recent Advances and Future Perspectives

Eco-physiological phases of insect diapause.

Meeting the energetic demands of insect diapause: nutrient storage and utilization.

Through a combined metabolomics and transcriptomics approach we analyzed the events that took place during the first 5 d of infesting intact tomato (Lycopersicon esculentum) plants with spider mites (Tetranychus urticae). Although the spider mites had caused little visible damage to the leaves after 1 d, they had already induced direct defense responses. For example, proteinase inhibitor activity had doubled and the transcription of genes involved in jasmonate-, salicylate-, and ethylene-regulated defenses had been activated. On day four, proteinase inhibitor activity and particularly transcript levels of salicylate-regulated genes were still maintained. In addition, genes involved in phospholipid metabolism were up-regulated on day one and those in the secondary metabolism on day four. Although transcriptional up-regulation of the enzymes involved in the biosynthesis of monoterpenes and diterpenes already occurred on day one, a significant increase in the emission of volatile terpenoids was delayed until day four. This increase in volatile production coincided with the increased olfactory preference of predatory mites (Phytoseiulus persimilis) for infested plants. Our results indicate that tomato activates its indirect defenses (volatile production) to complement the direct defense response against spider mites.

Differential Timing of Spider Mite-Induced Direct and Indirect Defenses in Tomato Plants

The tomato (Lycopersicon esculentum) mutant def-1, which is deficient in induced jasmonic acid (JA) accumulation upon wounding or herbivory, was used to study the role of JA in the direct and indirect defense responses to phytophagous mites (Tetranychus urticae). In contrast to earlier reports, spider mites laid as many eggs and caused as much damage on def-1 as on wild-type plants, even though def-1 lacked induction of proteinase inhibitor activity. However, the hatching-rate of eggs on def-1 was significantly higher, suggesting that JA-dependent direct defenses enhanced egg mortality or increased the time needed for embryonic development. As to gene expression, def-1 had lower levels of JA-related transcripts but higher levels of salicylic acid (SA) related transcripts after 1 d of spider mite infestation. Furthermore, the indirect defense response was absent in def-1, since the five typical spider mite-induced tomato-volatiles (methyl salicylate [MeSA], 4,8,12-trimethyltrideca-1,3,7,11-tetraene [TMTT], linalool, trans-nerolidol, and trans-β-ocimene) were not induced and the predatory mite Phytoseiulus persimilis did not discriminate between infested and uninfested def-1 tomatoes as it did with wild-type tomatoes. Similarly, the expression of the MeSA biosynthetic gene salicylic acid methyltransferase (SAMT) was induced by spider mites in wild type but not in def-1. Exogenous application of JA to def-1 induced the accumulation of SAMT and putative geranylgeranyl diphosphate synthase transcripts and restored MeSA- and TMTT-emission upon herbivory. JA is therefore necessary to induce the enzymatic conversion of SA into MeSA. We conclude that JA is essential for establishing the spider mite-induced indirect defense response in tomato.

Jasmonic Acid Is a Key Regulator of Spider Mite-Induced Volatile Terpenoid and Methyl Salicylate Emission in Tomato

When the chemical cues co-occurring with prey vary in time and space, foraging predators profit from an ability to repeatedly associate chemical cues with the presence of their prey. We demonstrate the ability of a predatory arthropod (the plant-inhabiting mite, Phytoseiulus persimilis) to learn the association of a positive stimulus (herbivorous prey, Tetranychus urticae) or a negative stimulus (hunger) with a chemical cue (herbivore-induced plant volatiles or green leaf volatiles). It has been suggested that the rate at which the integration of information becomes manifest as a change in behaviour, differs between categories of natural enemies (parasitoids versus insect predators: specialist versus generalist predators). We argue that these differences do not necessarily reflect differential learning ability, but rather relate to the ecologically relevant time scale at which the biotic environment changes.

How predatory mites learn to cope with variability in volatile plant signals in the environment of their herbivorous prey.

1. Diapause is generally believed to entail costs that manifest themselves as decreased survival, rate of development, and/or reproduction after diapause completion. The present study is a first step in investigating such diapause costs in the spider mite Tetranychus urticae.



2. The relationship was assessed between diapause duration and the post-diapause life-history traits of longevity and egg production, under six photoperiods at 19 °C.



3. The relationship between diapause duration and longevity was negative under five out of six treatments; in three cases this negative relationship was significant.



4. Most evident were the negative correlations between diapause duration and rate of oviposition, peak rate of oviposition, and total egg production. These phenotypic correlations suggest that there may be a trade-off between diapause duration and post-diapause reproduction.

Trade-off between diapause and other life-history traits in the spider mite Tetranychus urticae

Diapause in arthropods is a physiological state of dormancy that is generally thought to promote survival during harsh seasons and dispersal, but it may also serve to avoid predation in space and time. Here, we show that predation-related odours induce diapause in female adult spider mites. We argue that this response allows them to move into an area where they are free of enemies, yet forced to survive without food. Spider mites are specialised leaf feeders, but—in late summer—they experience severe predation on leaves. Hence, they face a dilemma: to stay on the leaf and risk being eaten or to move away from the leaf and risk death from starvation and thirst. Female two-spotted spider mites solve this dilemma by dramatically changing their physiology when exposed to predation-associated cues. This allows them to disperse away from leaves and to survive in winter refuges in the bark of trees or in the soil. We conclude that the mere presence of predation-associated cues causes some herbivorous mites to seek refuge, thereby retarding the growth rate of the population as a whole: a trait-mediated indirect effect that may have consequences for the stability of predator–prey systems and for ecosystem structure.

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"Sleeping with the enemy"--predator-induced diapause in a mite.

. Eight strains of the spider mite Tetranychus urticae, originating from different localities in western and central Europe, with latitudes ranging from 40.5 to 60oN, displayed marked differences in the period of chilling at 4oC required for diapause termination under a diapause-maintaining short-day photoperiodic regime at 19oC, to which the mites were transferred after the cold period. The higher the latitude from which the strains originated the longer was the period of chilling required for diapause termination, suggesting the presence of a gradient in diapause intensity, diapause being deeper the more northern the origin of the strains.



Two strains originating from higher altitudes appeared to have a much deeper diapause than expected from their latitudinal origin. In addition, these two mountain strains showed mutual differences in diapause intensity, notwithstanding the fact that they originated from similar latitudes and altitudes; local climatic conditions probably act as strong selective forces with regard to diapause depth.



All strains appeared to be sensitive to photoperiod during the period of diapause development. Diapause was quickly completed by a long-day photoperiod (LD 17:7 h), but was maintained by a short-day photoperiod (LD 10:14h). However, even under the latter regime sensitivity to photoperiod gradually diminished and eventually disappeared, thus leading to ‘spontaneous’ termination of diapause. The length of the period of diapause development, as measured by the sensitivity to photoperiod of diapausing mites, varied between strains; it was shorter in the southern strains and longer in the northern strains. The results indicate great variation in diapause intensity between strains, which is probably genetically determined and may have adaptive significance for this widespread species.



When young females which had just entered diapause were kept for ever longer periods of time under the diapause inducing short-day regime at which they had been reared, before being transferred to the cold room, the duration of the period of chilling required for diapause termination was found to decrease proportionally in all three strains tested. These results suggest that intensification of diapause does not occur in T. urticae; diapause intensity seems to be highest at the beginning of diapause and to diminish gradually during diapause development.

Geographic variation of diapause intensity in the spider mite Tetranychus urticae

In the spider mite Tetranychus urticae, both diapause induction (which takes place during the larval and nymphal stages) and diapause maintenance (in the adult female) are under photoperiodic contr...

Annemarie Kroon

Papers

How predatory mites learn to cope with variability in volatile plant signals in the environment of their herbivorous prey.

Trade-off between diapause and other life-history traits in the spider mite Tetranychus urticae

"Sleeping with the enemy"--predator-induced diapause in a mite.

Geographic variation of diapause intensity in the spider mite Tetranychus urticae

The Same Photoperiodic Clock May Control Induction and Maintenance of Diapause in the Spider Mite Tetranchus urticae