Plant odour plumes as mediators of plant-insect interactions.

Insect olfactory orientation along odour plumes has been studied intensively with respect to pheromonal communication, whereas little knowledge is available on how plant odour plumes (POPs) affect olfactory searching by an insect for its host plants. The primary objective of this review is to examine the role of POPs in the attraction of insects. First, we consider parameters of an odour source and the environment which determine the size, shape and structure of an odour plume, and we apply that knowledge to POPs. Second, we compare characteristics of insect pheromonal plumes and POPs. We propose a 'POP concept' for the olfactory orientation of insects to plants. We suggest that: (i) an insect recognises a POP by means of plant volatile components that are encountered in concentrations higher than a threshold detection limit and that occur in a qualitative and quantitative blend indicating a resource; (ii) perception of the fine structure of a POP enables an insect to distinguish a POP from an unspecific odorous background and other interfering plumes; and (iii) an insect can follow several POPs to their sources, and may leave the track of one POP and switch to another one if this conveys a signal with higher reliability or indicates a more suitable resource. The POP concept proposed here may be a useful tool for research in olfactory-mediated plant-insect interactions.

Can insect egg deposition 'warn' a plant of future feeding damage by herbivorous larvae?

Plant anti-herbivore defence is inducible by both insect feeding and egg deposition. However, little is known about the ability of insect eggs to induce defences directed not against the eggs themselves, but against larvae that subsequently hatch from the eggs. We studied how oviposition (OP) by the sawfly Diprion pini on Pinus sylvestris foliage affects the plant's defensive potential against sawfly larvae. Larvae that initiated their development on P. sylvestris twigs on which they hatched from eggs gained less weight and suffered higher mortality than those fed on egg-free twigs. The poor performance of these larvae also affected the next herbivore generation since fecundity of resulting females was lower than that of females which spent their larval development on egg-free pine. Transcript levels of P. sylvestris sesquiterpene synthases (PsTPS1, PsTPS2) were increased by D. pini OP, reached their highest levels just before larval hatching, and decreased when larvae started to feed. However, concentrations of terpenoid and phenolic metabolites presumed to act as feeding deterrents or toxins for herbivores did not change significantly after OP and feeding. Nevertheless, our performance data suggest that insect egg deposition may act to 'warn' a plant of upcoming feeding damage by larvae.

Species-specific responses of pine sesquiterpene synthases to sawfly oviposition.

Pinus sylvestris (Scots pine) is known to respond to eggs laid by the sawfly Diprion pini on its needles by releasing a blend of terpenoids, including the sesquiterpene (E)-beta-farnesene. These compounds attract a wasp, Closterocerus ruforum, which parasitizes sawfly eggs. D. pini oviposition also enhances the transcription of two sesquiterpene synthases, an (E)-beta-caryophyllene/alpha-humulene synthase (PsTPS1) and a 1(10),5-germacradiene-4-ol synthase (PsTPS2). To gain a better understanding of the function of these sesquiterpenes in promoting insect egg parasitism, we compared the outcome of D. pini oviposition on P. sylvestris with interactions between other pine and sawfly species: Neodiprion sertifer eggs on P. sylvestris, Gilpinia pallida eggs on P. sylvestris, D. pini eggs on Pinus nigra. The first of these attracts the parasitoid C. ruforum, while the latter two do not. As determined by quantitative real-time PCR, both PsTPS1 and PsTPS2 transcripts increased significantly only for those species combinations where the odor of egg-laden pine needles was attractive to C. ruforum. Moreover, enhanced transcription of these genes was found only at those time periods when odor was attractive, i.e. 3days after oviposition. Thus, the PsTPS1 and PsTPS2 genes are good markers for parasitoid attraction. We also characterized a sesquiterpene synthase from P. sylvestris (PsTPS5) which produces (E)-beta-farnesene, the compound previously determined to be responsible for C. ruforum attraction. However, transcript levels of PsTPS5 were not enhanced by oviposition of sawfly species that cause C. ruforum attraction. More research on this experimental system is required to determine the role of oviposition-induced sesquiterpenes in attracting egg parasitoids and the role of sesquiterpene synthases in regulating sesquiterpene formation.

Relevance of the endosymbiosis of Blochmannia floridanus and carpenter ants at different stages of the life cycle of the host.

Expression of several genes possibly involved in the symbiotic relationship between the obligate intracellular endosymbiont Blochmannia floridanus and its ant host Camponotus floridanus was investigated at different developmental stages of the host by real-time quantitative PCR. These included a set of genes related to nitrogen metabolism (ureC, ureF, glnA, and speB) as well as genes involved in the synthesis of the aromatic amino acid tyrosine (tyrA, aspC, and hisC). The overall transcriptional activity of Blochmannia was found to be quite low during early developmental stages and to increase steadily with host age. However, a concerted peak of gene expression related to nitrogen recycling could be detected around the entire process of pupation, while expression of biosynthesis pathways for aromatic amino acids was elevated only during a short phase in pupation. These data suggest an important role of certain metabolic functions for the symbiotic interactions of the bacteria and an individual host organism in early phases of development. General relevance of Blochmannia for its ant host was tested in fostering experiments with worker groups of Camponotus floridanus, and their success in raising pupae from first-instar larvae was used as a fitness measure. Groups treated with antibiotics had a significantly reduced success in raising the brood in comparison to untreated control groups, indicating that the symbiosis is relevant for the development of the entire colony.