Semester

Summer

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Division of Plant and Soil Sciences

Committee Chair

Michael Gutensohn

Committee Co-Chair

Yong-Lak Park

Committee Member

Yong-Lak Park

Committee Member

Kevin Daly

Committee Member

Vagner Benedito

Committee Member

Matthew Jenks

Abstract

The potato aphid (Macrosiphum euphorbiae Thomas) poses a serious problem in the commercial production of horticultural crops including tomato, since it causes damage by stylet feeding and the transmission of viruses for which it serves as the vector. Application of conventional pesticides being the fundamental tactic in the control of the aphid is increasingly considered insufficient and problematic due to emerging pest resistances and biosafety issues, highlighting the continuing need to develop new efficient and sustainable approaches. Alternatively, glandular trichomes of plants are well-known epidermal hairy tissues producing various secondary metabolites that involve in plant-insect interactions, while the terpene compounds produced from type VI glandular trichomes were frequently referred to as antixenotic or antibiotic resistant against both piercing-sucking and chewing-biting herbivores. Recent studies of terpene production in glandular trichomes of tomato, found on leaves and stems, demonstrated significant differences between cultivated (Solanum lycopersicum L.) and wild tomatoes (Solanum habrochaites S.Knapp & D.M.Spooner), as well as quantitative and qualitative variation of sesquiterpenes among the wild tomato accessions. Thus, the current research aimed to explore (1) if certain sesquiterpene chemotypes from the wild accessions support resistance against the potato aphid by affecting its pre- and postlanding behaviors during their interaction, and (2) would the resistance in cultivated tomato be improved if defensive sesquiterpenes were produced in a different tissue along the aphid stylet pathway by metabolic engineering.

To determine the effects of glandular trichome derived sesquiterpenes produced from wild tomato, five chemotypes (I-V) from a collection of S. habrochaites accessions were re-confirmed by gas chromatography-mass spectrometry (GC-MS). The performance (longevity and fecundity) of wingless aphids on tomato accessions, their feeding behaviors on an artificial diet, and the choice behaviors of winged aphids in an open Y-track olfactometer were analyzed. The results suggested that chemotype IV and V accessions which respectively produce mixtures of caryophyllene/α-humulene and santalene/bergamotene significantly reduced aphid longevity and fecundity in clip-cages while they were significantly repellent to winged aphids in the olfactometer. The trichome extracts from the two groups significantly affected aphid survivorship, gel saliva investment, and honeydew production in artificial diets, and significantly reduced the attraction of winged aphids to cultivated tomato. The same effects on feeding and choice behaviors were also observed by using pure caryophyllene/α-humulene as well as the trichome extract from one introgression line LA3935 which has santalene/bergamotene isomers predominantly produced in glandular trichomes.

Cultivated tomato lines generally produce a large quantity of TPS20-derived monoterpenes and low quantities of TPS9-derived δ-elemene and TPS12-derived sesquiterpenes. To explain the susceptibility of cultivated tomato plants against the potato aphid, the performance parameters (longevity and fecundity) of wingless aphids were analyzed on four tomato lines that quantitatively differ in their terpene production, i.e., two cultivated tomato lines (Alisa Craig and Castlemart) with normal terpene production and their trichome mutants (hairless and odorless-2) producing respectively lesser amounts of sesquiterpenes and only tiny amounts of all terpene compounds. A principal component analysis (PCA) indicated that the performance parameters were negatively correlated with the production of the TPS12-derived sesquiterpenes, while no strong relationships were formed between the performance parameters and the production of TPS20-derived monoterpenes, which were further confirmed in artificial diets showing that the increasing concentration of TPS20-derived monoterpenes has little effects on aphid survivorship and production of gel saliva and honeydew. Additionally, a specific concentration of the TPS20-derived monoterpenes was significantly attractive to winged aphids in the olfactometer. Thus, the analyses explained the susceptibility of the cultivated tomato by revealing the contrasting roles of glandular trichome-derived monoterpenes and sesquiterpenes: while TPS12-derived sesquiterpenes contribute to host plant resistance against the potato aphid, TPS20-derived monoterpenes appear to be exploited as a cue for host plant orientation by the species.

Since the two sesquiterpene mixtures produced in glandular trichomes of wild tomato accessions affect aphid feeding, two multicistronic expression constructs were developed to engineering their production in epidermal cells, as their epidermal-specific formation was presumed to reduce tissue penetration by aphids and subsequent performance. Both constructs contained sequences encoding a prenyl transferase (TPS), a respective terpene synthase (TPS), and an enhanced green fluorescent protein (GFP) as visible marker. All three coding sequences were linked by short nucleotide sequences encoding the foot-and-mouth disease virus 2A self-processing oligopeptide which allows their co-expression under the control of an epidermis-specific Arabidopsis CER5-promoter. Transient expression of both constructs by infiltrating odorless-2 leaves leads to the formation of the two sets of defensive sesquiterpenes, β-caryophyllene/α-humulene and santalene/bergamotene. The epidermis-specific transgene expression and terpene formation were verified by fluorescence microscopy and tissue fractionation with subsequent analysis of terpene profiles, respectively. In addition, the longevity and fecundity of the potato aphid feeding on infiltrated leaves were significantly reduced.

This study overall identified two groups of sesquiterpenes in glandular trichomes of S. habrochaites accessions that support antixenotic resistance against the potato aphid by negatively affecting their performance and choice behavior. The defensive traits engineered in epidermal cells from the wild tomato also improved the resistance in susceptible tomato plants, suggesting a novel and sustainable non-pesticide strategy for managing the aphid species. Further studies need to be conducted to test whether the defensive sesquiterpenes are also antibiotic resistant with effects on aphid physiology and biology, and to produce the sesquiterpenes in other tissues, such as companion cells, along aphid stylet pathway, to evaluate tomato plant resistance to the potato aphid.

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