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In nature, plants continuously face the attack from insect herbivores. To defend themselves, plants have evolved a plethora of direct and indirect defence mechanisms that are present either constitutively or induced upon insect herbivory. In response to insect herbivory, plants activate phytohormonal signal-transduction pathways, transcriptional responses and biological processes. These changes in transcriptional and biological processes level influence the plant’s phenotype and consequently plant ecology during the current season or over seasons. To gain more insight into the dynamics of plant phenotype, elucidating transcriptomic responses to insect herbivory is critical. Various studies have investigated whole-genome transcriptional responses of plants against different insect herbivores, but only few have reported on responses to cell-content feeding thrips. Thrips (Thysanoptera) are minute cell-content feeding insects and are serious pests on many commercial and ornamental plants. Thrips species such as western flower thrips (WFT) and onion thrips are among the most devastating pests on e.g. sweet pepper and white cabbage plants, respectively. Therefore, using a high-density time-series approach, the focus of this thesis was to investigate whole-genome transcriptional responses of sweet pepper and white cabbage plants upon WFT and onion thrips feeding, respectively, and the underlying genetic mechanisms. In addition, I focussed on one particular gene family, the lipoxygenases and a gene in this family that is involved in thrips-induced crop resistance.
Chapter 2 was focused on the identification of the lipoxygenase gene family in pepper (Capsicum annuum). Lipoxygenases (LOXs) are non-heme, iron-containing dioxygenases involved in several developmental and defence-related plant processes, such as seed germination, fruit ripening, tuber development and JA-regulated plant defences. Through multiple in-silico analysis, a total of eight LOX genes were identified in pepper classifying four LOXs (CaLOX1, CaLOX3, CaLOX4 and CaLOX5) as 9-LOXs and four (CaLOX2, CaLOX6, CaLOX7 and CaLOX8) as 13-LOXs. Chapter 3 further narrowed down the LOX gene-family to one lipoxygenase (CaLOX2) gene through in-silico analysis and functionally characterized its involvement in jasmonate-dependent induced defence against WFT. Overall, this chapter shows involvement of CaLOX2 in JA-mediated plant resistance to thrips feeding. Chapter 4 elucidates the dynamics of transcriptional reprogramming of sweet pepper in response to WFT feeding. Approximately 8.6% (2060 up and 1002 downregulated) of the pepper genes were differentially expressed that categorized into 23 clusters (16 upregulated and 7 downregulated), each possessing a unique temporal expression pattern. Upregulated gene clusters were overrepresented with defence-related biological processes, whereas downregulated gene clusters were overrepresented with developmental processes. The transcription factor families ERF, MYB, NAC, bHLH and WRKY emerged as pivotal regulators in response to WFT feeding. The data show a chronological order in the activation of hormonal (JA, ET) and secondary metabolite (phenylpropanoids, flavonoids and terpenoids) pathways. Eventually, the comparative analysis of the WFT-induced transcriptional responses of Arabidopsis and sweet pepper plants to WFT feeding shows a conservation in the induction of the JA-pathway in both plants, whereas the majority of transcriptional responses are plant-specific. Chapter 5 focused on 1) elucidating whole-genome transcriptional reprogramming of white cabbage plants in response to onion thrips feeding and 2) comparative transcriptomics to disentangle similarities and differences in transcriptional responses between WFT-induced Arabidopsis and sweet pepper as well as onion-thrips-induced white cabbage. Approximately 9.7 % of the white cabbage genes showed differential expression with 48 (32 upregulated and 16 downregulated) gene clusters with upregulated clusters associated with defence and downregulated clusters with development-related biological processes. Phytohormone-related processes (JA, ET and SA) and secondary metabolite (phenylpropanoids, flavonoids, green-leaf volatiles and indolic glucosinolates) biosynthesis genes were induced, whereas aliphatic glucosinolate biosynthetic genes were suppressed. Comparative analysis of the transcriptional responses of Arabidopsis and sweet pepper to WFT and of white cabbage to onion thrips showed 1) conservation of the JA biosynthesis and signalling pathways, 2) conservation of involvement of TF families, such as MYB, bHLH and WRKY in regulating responses, 3) that the majority of the transcriptional responses to thrips are system-specific, 4) that genes involved in indole glucosinolate biosynthesis are upregulated, whereas genes involved in aliphatic glucosinolate biosynthesis are downregulated in both brassicaceous plants Arabidopsis and white cabbage and 5) that the white-cabbage transcriptomic response to onion thrips is relatively rapid and complex compared to the WFT-induced Arabidopsis and sweet pepper transcriptomic responses. The data presented in this thesis contributes to understand the intensity and complexity of dynamic plant transcriptional responses to thrips feeding, and this is discussed in Chapter 6.
|Qualification||Doctor of Philosophy|
|Award date||20 Jun 2019|
|Place of Publication||Wageningen|
|Publication status||Published - 2019|