The Alcohol and Epoxy Alcohol of Zingiberene, Produced in Trichomes of Wild Tomato, Are More Repellent to Spider Mites Than Zingiberene

Abstract

Allelochemicals that are present in trichome secretions of wild tomato species play a major role in mediating interactions with arthropods, often conferring a high level of resistance via antibiosis and antixenosis. Many accessions of the wild tomato relative, Solanum habrochaites (S.h), possess high levels of resistance to arthropods. The monocyclic sesquiterpene hydrocarbon, 7-epi-zingiberene, is a major defensive component found in trichome secretions of certain accessions of S.h. We have used LA2329, an S.h. accession, as a donor in a breeding program designed to introgress zingiberene into cultivated tomato. However, the composition of trichome secretions in our population of LA2329 is segregating, with some individuals producing mainly 7-epi-zingiberene in their secretions while others producing two additional, unidentified compounds in their trichome secretions. To investigate if these other compounds may also contribute to arthropod resistance, trichome secretions were collected from plants of S.h LA2329 grown under greenhouse conditions and then major compounds were isolated by silica gel column chromatography and tested for their ability to repel two spotted-spider mite (TSSM), Tetranychus urticae. Isolation and identification of allelochemicals were aided by use of gas chromatography/mass spectroscopy. The results revealed the presence of three predominate chromatographic peaks: 7-epi-zingiberene, 9-hydroxy zingiberene, and 9-hydroxy,10,11-epoxy-zingiberene. Results of testing isolated compounds for repellency to TSSM using bridge bioassays revealed that the repellent activities of 9-hydroxy zingiberene and 9-hydroxy,10,11-epoxy-zingiberene were each significantly higher than that for 7-epi-zingiberene. These results support the idea that the degree of repellency may differ among plant allelochemicals and also emphasize the potential value of introgressing the presence of 9-hydroxy zingiberene and 9-hydroxy,10,11-epoxy-zingiberene into cultivated tomato to enhance its arthropod resistance.

Keywords: 7-epi-zingiberene; 9-hydroxy-zingiberene; antixenosis; breeding; insect resistance; mite repellency; tomato; trichome.

Figure 1

Chromatogram (GC-FID) of n-hexane leaflet wash from: (1) the LA2329-B chemotype showing a single major component at retention time of 6.00 min; (2) the LA2329-A chemotype showing three major components at retention times of 6.00, 6.91, and 7.23 min; (3) LA2167 showing three major components at retention times of 6.00, 6.91, and 7.23 min; (4) PI127826 showing three major components at retention times of 6.00, 6.91, and 7.23 min.

Figure 2

Abundance of 7-epi-zingiberene, 9-hydroxy-zingiberene, and 9-hydroxy,10-11-epoxy-zingiberene in four plants, two chemotypes of LA2329 (LA2329-A and LA2329-B), LA2167 and PI127826. Abundance of a compound labeled by the same letter(s) are not significantly different (P < 0.05).

Figure 3

Electron ionization mass spectra of [1] 7-epi-zingiberene (C15H24), [2] 9-hydroxy-zingiberene (C15H24OH), and of [3] 9-hydroxy-10-11-epoxy-zingiberene (C15H24O2), present in trichome secretions of S. habrochaites accessions LA2329-A, LA2167, and PI127826. The names of three major components were obtained by comparing our results from GC-MS with those results published by Anonymous (2017) which was based on the analysis of trichome secretion components present on LA2167.

Figure 4

Proposed biosynthetic pathway for the zingiberene derivatives present in trichome secretion of S. habrochaites accession LA2167. zFPS: Z,Z-FPP synthase, ShZS: zingiberene synthase (also referred to herein as ShZIS), ShZPO, zingiberene polyoxidase. This figure is based on information in Anonymous (2017).