Measurement of volatile plant compounds in field ambient air by thermal desorption–gas chromatography–mass spectrometry

New trends in methyl salicylate sensing and their implications in agriculture

Methyl salicylate (MeSal) is an organic compound present in plants during stress events and is therefore a key marker for early plant disease detection. It has usually been detected by conventional methods that require bulky and costly equipment, such as gas chromatography or mass spectrometry. Currently, however, chemical sensors provide an alternative for MeSal monitoring, showing good performance for its determination in the vapour or liquid phase. The most promising concepts used in MeSal determination include sensors based on electrochemical and conductometric principles, although other technologies based on mass-sensitive, microwave, or spectrophotometric principles also show promise. The receptor elements or sensitive materials are shown to be part of the key elements in these sensing technologies. A literature survey identified a significant contribution of bioreceptors, including enzymes, odourant-binding proteins or peptides, as well as receptors based on polymers or inorganic materials in MeSal determination. This work reviews these concepts and materials and discusses their future prospects and limitations for application in plant health monitoring.

Variation in the ratio of compounds in a plant volatile blend during transmission by wind

For plant volatiles to mediate interactions in tritrophic systems, they must convey accurate and reliable information to insects. However, it is unknown whether the ratio of compounds in plant volatile blends remains stable during wind transmission. In this study, volatiles released from an odor source were collected at different points in a wind tunnel and analyzed. The variation in the amounts of volatiles collected at different points formed a rough cone shape. The amounts of volatiles collected tended to decrease with increasing distance from the odor source. Principal component analyses showed that the volatile profiles were dissimilar among different collection points. The profiles of volatiles collected nearest the odor source were the most similar to the released odor. Higher wind speed resulted in a clearer spatial distribution of volatile compounds. Thus, variations in the ratios of compounds in odor plumes exist even during transport over short distances.

Sex- and tissue-specific transcriptome analyses and expression profiling of olfactory-related genes in Ceracris nigricornis Walker (Orthoptera: Acrididae)

BACKGROUND

The sophisticated insect olfactory system plays an important role in recognizing external odors and enabling insects to adapt to environment. Foraging, host seeking, mating, ovipositing and other forms of chemical communication are based on olfaction, which requires the participation of multiple olfactory genes. The exclusive evolutionary trend of the olfactory system in Orthoptera insects is an excellent model for studying olfactory evolution, but limited olfaction research is available for these species. The olfactory-related genes of Ceracris nigricornis Walker (Orthoptera: Acrididae), a severe pest of bamboos, have not yet been reported.

RESULTS

We sequenced and analyzed the transcriptomes from different tissues of C. nigricornis and obtained 223.76 Gb clean data that were assembled into 43,603 unigenes with an N50 length of 2235 bp. Among the transcripts, 66.79% of unigenes were annotated. Based on annotation and tBLASTn results, 112 candidate olfactory-related genes were identified for the first time, including 20 odorant-binding proteins (OBPs), 10 chemosensory-binding proteins (CSPs), 71 odorant receptors (ORs), eight ionotropic receptors (IRs) and three sensory neuron membrane proteins (SNMPs). The fragments per kilobase per million mapped fragments (FPKM) values showed that most olfactory-related differentially expressed genes (DEGs) were enriched in the antennae, and these results were confirmed by detecting the expression of olfactory-related genes with quantitative real-time PCR (qRT-PCR). Among these antennae-enriched genes, some were sex-biased, indicating their different roles in the olfactory system of C. nigricornis.

CONCLUSIONS

This study provides the first comprehensive list and expression profiles of olfactory-related genes in C. nigricornis and a foundation for functional studies of these olfactory-related genes at the molecular level.

Smells from the desert: Microbial volatiles that affect plant growth and development of native and non-native plant species

The plant microbiota can affect host fitness via the emission of microbial volatile organic compounds (mVOCs) that influence growth and development. However, evidence of these molecules and their effects in plants from arid ecosystems is limited. We screened the mVOCs produced by 40 core and representative members of the microbiome of agaves and cacti in their interaction with Arabidopsis thaliana and Nicotiana benthamiana. We used SPME-GC-MS to characterize the chemical diversity of mVOCs and tested the effects of selected compounds on growth and development of model and host plants. Our study revealed that approximately 90% of the bacterial strains promoted plant growth both in A. thaliana and N. benthamiana. Bacterial VOCs were mainly composed of esters, alcohols, and S-containing compounds with 25% of them not previously characterized. Remarkably, ethyl isovalerate, isoamyl acetate, 3-methyl-1-butanol, benzyl alcohol, 2-phenylethyl alcohol, and 3-(methylthio)-1-propanol, and some of their mixtures, displayed beneficial effects in A. thaliana and also improved growth and development of Agave tequilana and Agave salmiana in just 60 days. Volatiles produced by bacteria isolated from agaves and cacti are promising molecules for the sustainable production of crops in arid and semi-arid regions.

Design of an Attractant for Empoasca onukii (Hemiptera: Cicadellidae) Based on the Volatile Components of Fresh Tea Leaves

The tea leafhopper, Empoasca onukii Matsuda, is a serious pest of the tea plant. E. onukii prefers to inhabit vigorously growing tender tea leaves. The host selection of E. onukii adults may be associated with plant volatile compounds (VOCs). We sought to identify potentially attractive VOCs from tea leaves at three different ages and test the behavioral responses of E. onukii adults to synthetic VOC blends in the laboratory and field to aid in developing an E. onukii adult attractant. In darkness, the fresh or mature tea leaves of less than 1-mo old could attract more leafhoppers than the mature branches (MB) that had many older leaves (leaf age >1 mo). Volatile analysis showed that the VOC composition of the fresh leaves was the same as that of the mature leaves, but linalool and indole were not at detectable levels in VOCs from the MB. Moreover, the mass ratio differed for each common volatile in the three types of tea leaves. When under competition with volatiles from the MB, the leafhoppers showed no significant tropism to each single volatile but could be attracted by the synthetic volatile blend imitating the fresh leaves. With the removal of some volatile components, the effective synthetic volatile blend was mixed with (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate, and linalool at a mass ratio of 0.6:23:12.6. These three volatiles may be the key components for the host selection of E. onukii adults and could be used as an attractant in tea gardens.

Does Background Odor in Tea Gardens Mask Attractants? Screening and Application of Attractants for Empoasca onukii Matsuda

Plant volatiles help herbivores to locate their hosts, and therefore, they could be used to help develop pesticide-free pest management strategies. To develop an attractant for tea leafhopper (Empoasca onukii), we screened nine tea plant volatile compounds for their attractiveness using Y-tube olfactometer assays. Results indicated that tea leafhoppers significantly preferred ocimene, limonene, (Z)-3-hexenol, and (Z)-3-hexenyl acetate over clean air. These compounds were combined in a blend which lost its attractiveness at concentrations below 10-2 g/ml in liquid paraffin. In field tests, the blend was attractive to leafhoppers only in autumn, but not in summer. Analyses of the tea field background odor showed that all four components of the blend were present at much higher concentrations in summer (0.05-0.001 ng/liter) than in autumn (~10- to 25-fold lower). In field Y-tube bioassays, compared with the tea field background odor, the blend was attractive at a concentration of 10-1 g/ml in liquid paraffin, but not at 10-2 g/ml. These results suggest that field background odor can disrupt the attractiveness of an attractant based on plant volatiles to herbivores.

Field background odour should be taken into account when formulating a pest attractant based on plant volatiles

Attractants for pest monitoring and controlling can be developed based on plant volatiles. Previously, we showed that tea leafhopper (Empoasca onukii) preferred grapevine, peach plant, and tea plant odours to clean air. In this research, we formulated three blends with similar attractiveness to leafhoppers as peach, grapevine, and tea plant volatiles; these blends were composed of (Z)-3-hexenyl acetate, (E)-ocimene, (E)-4,8-dimethyl-1,3,7-nonatriene, benzaldehyde, and ethyl benzoate. Based on these five compounds, we developed two attractants, formula-P and formula-G. The specific component relative to tea plant volatiles in formula-P was benzaldehyde, and that in formula-G was ethyl benzoate. These two compounds played a role in attracting leafhoppers. In laboratory assays, the two attractants were more attractive than tea plant volatiles to the leafhoppers, and had a similar level of attractiveness. However, the leafhoppers were not attracted to formula-P in the field. A high concentration of benzaldehyde was detected in the background odour of the tea plantations. In laboratory tests, benzaldehyde at the field concentration was attractive to leafhoppers. Our results indicate that the field background odour can interfere with a point-releasing attractant when their components overlap, and that a successful attractant must differ from the field background odour.