Thomas R. Hamilton-Kemp

Volatile compounds induced by herbivory act as aggregation kairomones for the Japanese beetle (Popillia japonica Newman)

The Japanese beetle is a polyphagous insect that typically aggregates on preferred host plants in the field. We studied the response of Japanese beetles to artificial damage, fresh feeding damage, and overnight feeding damage to test the hypothesis that beetles are attracted to feeding-induced volatiles. Crabapple leaves that had been damaged overnight by Japanese beetles or fall webworms attracted significantly more Japanese beetles than did undamaged leaves. Artificially damaged leaves or leaves freshly damaged by Japanese beetles, however, were not significantly more attractive than undamaged leaves. Leaves that had been damaged overnight by Japanese beetles or fall webworms produced a complex mixture of aliphatic compounds, phenylpropanoid-derived compounds, and terpenoids. In comparison, artificially damaged leaves or leaves with fresh Japanese beetle feeding damage generated a less complex blend of volatiles, mainly consisting of green-leaf odors. Feeding-induced odors may facilitate host location and/or mate finding by the Japanese beetle.

Effects of some natural volatile compounds on the pathogenic fungiAlternaria alternata andBotrytis cinerea.

A bioassay system was developed to test the effects of volatile compounds on the growth of hyphae from germinating fungal spores. Volatiles from crushed tomato leaves inhibited hyphal growth of two fungal pathogens,Alternaria alternata andBotrytis cinerea. Aldehydes, including C6 and C9 compounds formed by the lipoxygenase enzyme pathway upon wounding leaves, inhibited growth of both fungal species. Terpene hydrocarbons, 2-carene and limonene, had no significant effect on hyphal growth. The quantities of volatile compounds in the vapor phase of the bioassay system were measured by direct headspace sampling and GC analysis.

Effects of some leaf-emitted volatile compounds on aphid population increase

A role of some volatile compounds produced by plant tissues may be as defensive molecules against various pests, including arthropods. Volatile six-carbon compounds derived in plant tissue from polyunsaturated fatty acids via lipoxygenase/hydroperoxide lyase reduced tobacco aphid fecundity at certain concentrations when added to headspace vapor to which aphids were exposed. Both C6 aldehydes and alcohols were effective, with the alcohols having greater activity. (Z)-3-Hexenyl acetate at levels in the headspace similar to those of the alcohols and aldehydes did not reduce aphid fecundity. A 6-hr exposure period to the C6 aldehydes and alcohols was needed for maximum effect on the aphids feeding on tobacco leaves. Analysis of the direct versus indirect effects of these compounds indicates that the volatile aldehydes had both direct effects on aphid fecundity and indirect effects due to induced changes in the leaves upon which the aphids were feeding, while only indirect effects were observed for the alcohols. Tomato leaves have the capacity to produce volatile compounds at levels that impact aphid population increase, with the volatiles produced from crushed leaves having a much larger effect. The C6 aldehydes and alcohols may be components of the fecundity reduction seen with tomato volatiles; however, volatile terpenes showed no effect. These results can be of significance for the genetic alteration of plants for improved aphid resistance.

Volatiles from flowers of Nicotiana sylvestris, N/ Otophora and malus × domestica : headspace components and day/night changes in their relative concentrations

Abstract Headspace components from the flowers of Nicotiana otophora and apple (Malus × domestica) were trapped on Tenax and identified by GC and GC-MS. Aromatic compounds from phenylpropanoid metabolism including benzyl alcohol were predominant as was shown earlier for Nicotiana sylvestris. Studies of diurnal emissions of identified volatiles from these species showed there was a marked increase (ca 10 fold) in aromatic compounds released from N. sylvestris inflorescences in situ at night compared to day. Marked diurnal changes in compounds from other biosynthetic pathways were not observed. Emissions of benzyl alcohol and other phenylpropanoid derived volatiles did not increase at night in inflorescences of N. otophora or flowering branches of apple, respectively. Aromatic compound emissions from N. sylvestris at 90 min intervals were determined over a 24 hr period using a purge and trap GC system for analyses. The aromatics were at relatively low levels after midday but subsequently increased to reach peak emissions around 2–3 a.m. followed by declines in these levels of emissions. Possible functions for the dark period enhanced emission of volatile aromatic compounds are discussed.

Survival of Escherichia coli O157:H7 on Strawberry Fruit and Reduction of the Pathogen Population by Chemical Agents

Survival of Escherichia coli O157:H7 was studied on strawberry, a fruit that is not usually washed during production, harvest, or postharvest handling. Two strains of the bacteria were tested separately on the fruit surface or injected into the fruit. Both strains of E. coli O157:H7 survived externally and internally at 23 degrees C for 24 h and at 10, 5, and -20 degrees C for 3 days. The largest reduction in bacterial population occurred at -20 degrees C and on the fruit surface during refrigeration. In all experiments, the bacteria inside the fruit either survived as well as or better than bacteria on the surface, and ATCC 43895 frequently exhibited greater survival than did ATCC 35150. Two strains of E. coli also survived at 23 degrees C on the surface and particularly inside strawberry fruit. Chemical agents in aqueous solution comprising NaOCl (100 and 200 ppm), Tween 80 (100 and 200 ppm), acetic acid (2 and 5%), Na3PO4 (2 and 5%), and H2O2 (1 and 3%) were studied for their effects on reduction of surface-inoculated (10(8) CFU/ml) E. coli O157:H7 populations on strawberry fruit. Dipping the inoculated fruit in water alone reduced the pathogen population about 0.8 log unit. None of the compounds with the exception of H2O2 exhibited more than a 2-log CFU/g reduction of the bacteria on the fruit surface. Three percent H202, the most effective chemical treatment, reduced the bacterial population on strawberries by about 2.2 log CFU/g.

Effect of Volatile Methyl Jasmonate on the Oxylipin Pathway in Tobacco, Cucumber, and Arabidopsis

The effect of atmospheric methyl jasmonate on the oxylipin pathway was investigated in leaves of tobacco (Nicotiana tabacum L.), cucumber (Cucumis sativa L.), and Arabidopsis thaliana (L.). Differential sensitivities of test plants to methyl jasmonate were observed. Thus, different concentrations of methyl jasmonate were required for induction of changes in the oxylipin pathway. Arabidopsis was the least and cucumber the most sensitive to methyl jasmonate. Methyl jasmonate induced the accumulation of lipoxygenase protein and a corresponding increase in extractable lipoxygenase activity. Atmospheric methyl jasmonate additionally induced hydroperoxide lyase activity and the enhanced production of several volatile six-carbon products. It is interesting that lipid hydroperoxidase activity, which is a measure of hydroperoxide lyase plus allene oxide synthase plus possibly other lipid hydroperoxide-metabolizing activities, was not changed by methyl jasmonate treatment. Methyl jasmonate selectively altered the activity of certain enzymes of the oxylipin pathway (lipoxygenase and hydroperoxide lyase) and increased the potential of leaves for greatly enhanced six-carbon-volatile production.

Production of the long-chain alcohols octanol, decanol, and dodecanol by Escherichia coli

As a follow-up to earlier studies on the emission of long-chain alcohols from broth cultures of Gram-negative enteric bacteria, E. coli was examined for the production of 1-octanol, 1-decanol, and 1-dodecanol. Ten strains of E. coli cultured in tryptic soy broth were assayed for volatile metabolites using solid-phase microextraction. Long-chain alcohols were produced by all strains with 1-decanol predominating with production ranging from 23.6 ng mL−1 to 148 ng mL−1. The production of long-chain alcohols followed the onset of the exponential growth phase of the broth culture. Doubling the concentration of glucose (5 g L−1) in the broth had no effect on the concentration of long-chain alcohols produced. Addition of octanoic, decanoic, or dodecanoic acids (as K+ salts) to the broth (100 mg L−1) markedly increased the production of the corresponding alcohols by E. coli, ranging from a 13-fold increase for decanol to a 51-fold increase for dodecanol. However, decanol remained the predominant alcohol detected in all assays. These neutral volatile alcohols may have application as vapor-phase indicators for certain classes of bacteria, particularly, Gram-negative enteric bacteria.

Comparison of long-chain alcohols and other volatile compounds emitted from food-borne and related Gram positive and Gram negative bacteria.

Numerous reports have been published on the antimicrobial activity of synthetic volatile long chain alcohols, such as 1‐decanol and 1‐dodecanol, against bacteria and fungi. The objective of the present study was to survey microorganisms for emission patterns of naturally occurring long chain alcohols and other volatile components to determine if these compounds are associated with certain groups of bacteria. Cultures were grown in trypticase soy broth overnight and volatile compounds were trapped on a porous polymer and identified by mass spectrometry. Subsequently, volatile compounds were collected from 26 strains of food associated bacteria using solid‐phase microextraction and analyzed by gas chromatography. Alcohols comprising 1‐octanol, 1‐decanol, and 1‐dodecanol occurred as products from enteric Gram negative bacteria, which included Citrobacter, Enterobacter, Klebsiella, Salmonella, and Shigella. However, the long chain alcohols were not detected as products from the nonenteric Gram negative species studied which included Acinetobacter, Pseudomonas, and Shewanella. Among Gram positive bacteria, including Bacillus, Enterococcus, Lactococcus, Leuconostoc, Listeria, Staphylococcus, and Streptococcus, the only long chain alcohol detected was 1‐decanol and, if present, it occurred in relatively small amounts. Other classes of compounds emitted by bacteria included methylketones and sulfides. The methylketones were found as products from Gram positive and Gram negative bacteria, whereas the sulfides were closely associated with Gram positive bacteria. In summary, the emission patterns of volatile compounds from bacteria showed many trends including the association of long chain alcohols with enteric Gram negative bacteria. The results provide a basis for future in vivo studies to determine if volatile compounds such as natural long chain alcohols function in the ecology of food‐borne Gram negative bacterial pathogens.

Glycosidically bound volatile components of Nicotiana sylvestris and N. Suaveolens flowers

Abstract Glycosidically bound volatile components were extracted from flowers of Nicotiana sylvestris and N. suaveolens and isolated using liquid column chromatography with an Amberlite XAD-2 resin. These glycosides, which were putative precursors of fragrance compounds, were hydrolysed enzymatically and a number of the volatiles released were subsequently identified by GC-MS including compounds not previously identified in the floral headspace of these species. Average yields of volatiles released from glycosides were ca 230 and 1050 μg g −1 for N. sylvestris and N. suaveolens flowers, respectively. Compounds in the glycosidically bound fraction were all phenylpropanoid-derived volatiles (e.g. benzyl alcohol, benzaldehyde, E -cinnamyl alcohol, benzyl salicylate) with the exception of the monoterpene α-terpineol. No pronounced diurnal changes in the levels of glycosidically bound volatile compounds could be noted from flowers of either species. However, pronounced differences were obtained in the concentrations of glycosidically bound volatiles at different floral maturity stages.

Biosynthesis of trans-2-Hexenal in Response to Wounding in Strawberry Fruit

Wounded strawberry fruit produces a diverse group of volatile compounds including aldehydes, alcohols, and esters derived from the lipoxygenase (LOX) and hydroperoxide lyase (HPL) pathways. Because the wound volatiles may play an important role in plant-fungal interaction, the goal of this study was to develop a greater understanding about the biosynthesis of the major wound volatile, trans-2-hexenal (t-2-H), produced by strawberry fruit upon wounding. To that end, composition and quantity of total and free fatty acids of control and wounded strawberry fruit were analyzed. In addition, activities of the key enzymes, LOX and HPL, and production of C6 aldehydes were determined. Intact strawberry fruit did not produce detectable t-2-H which is derived from alpha-linolenic acid (18:3). However, in response to wounding by bruising, strawberry fruit emitted t-2-H and its precursor cis-3-hexenal (c-3-H). The level of total lipid 18:3 in the fruit increased 2-fold in response to wounding, whereas free 18:3 declined slightly ( approximately 30%). At 10 min following wounding, fruit exhibited a 25% increase in LOX activity, which leads to the production of 13-hydroperoxyoctadecatrienoic acid (13-HPOT) from 18:3. The activity of HPL, which catalyzes formation of cis-3-hexenal from 13-HPOT, increased 2-fold by 10 min after wounding. Thus, during a 15 min period after wounding, free 18:3 substrate availability and the activity of two key enzymes, LOX and HPL, changed in a manner consistent with increased c-3-H and t-2-H biosynthesis.