Plant exudates appeared long time ago in the history line of life in the early Devonian. They are typical saps produced by specialized cells. They can be naturally excreted outside or right after... Show morePlant exudates appeared long time ago in the history line of life in the early Devonian. They are typical saps produced by specialized cells. They can be naturally excreted outside or right after plants are damaged or infected. Through time, diverse cultures recognized them as valuable sources of natural medicines. Of various types of plant exudates, the most outstanding are gums, mucilage, essential oils, oleoresins, resins, phloem like saps, and latexes. Regardless of their commercial importance, in the aspect of biology, plant exudates are believed to be involved in plant defense mechanisms, but their roles still remain unclear. In this thesis, based on diverse experiments, many features of plant exudates were unveiled. Interestingly, even if plant exudates possess a broad chemical diversity, their specialized biological functions resulted in less variation of their metabolomes as compared with their bearing tissues. This lower variation indicates that plant exudates might partake of a general-broad defense against herbivores and pathogens in their bearing tissues. This defense involves both mechanical and chemical traits which might complement and modulate each other, eventually resulting in a chemo-mechanical defense layer. Of the diverse chemicals in plant exudates, terpenes are one of the most common metabolites in these saps. Show less
Plants produce an astonishing variety of secondary metabolites (SMs) which are thought to play vital roles in the fitness of plants through ecological interactions. The most characteristic features... Show morePlants produce an astonishing variety of secondary metabolites (SMs) which are thought to play vital roles in the fitness of plants through ecological interactions. The most characteristic features of SMs are their striking chemical diversity and inter- or intraspecific variation. Due to the large number, high structural diversity and multifunctionality of SMs, it is still an ongoing challenge to understand how this SM diversity comes about, and why such a large diversity is maintained in nature. In this thesis this question was studied using the pyrrolizidine alkaloids (PAs) of Jacobaea species as the study system from an evolutionary and biosynthetic perspective. PA variations were studied among and within Jacobaea species, and species-specific PA profiles were observed. In order to understand how PA diversity is related to species phylogeny, the evolutionary histories and phylogenetic signals of individual PAs were investigated under the phylogenetic context of Jacobaea species and no strong phylogenetic signals were found. To shed light on the mechanisms underlying PA diversity, a gene-to-metabolite approach targeting cytochrome P450 monooxygenases which play an important role in the evolution of chemical diversity was applied to study their involvement in PA biosynthesis and PA diversity. Show less
Jasmonates (JAs) are crucial plant signaling molecules that regulate defense responses against wounding, insects and necrotrophic pathogens. The biosynthesis of JAs is regulated by a positive... Show moreJasmonates (JAs) are crucial plant signaling molecules that regulate defense responses against wounding, insects and necrotrophic pathogens. The biosynthesis of JAs is regulated by a positive feedback loop. This thesis reveals the transcriptional regulatory mechanism behind this positive feedback loop in Arabidopsis. The studies show that the bHLH-domain transcription factors MYC2, MYC3 and MYC4 positively regulate most JAs biosynthesis genes directly and by controlling the expression of ORA47 gene, encoding a regulator of JAs biosynthesis. Show less
Although the introduction of invasive plant species in a given area causes economic and ecological problems, it still provides an ideal opportunity for ecologists to study evolutionary changes.... Show moreAlthough the introduction of invasive plant species in a given area causes economic and ecological problems, it still provides an ideal opportunity for ecologists to study evolutionary changes. According to the Evolution of Increased Competitive Ability hypothesis and Shifting Defense Hypothesis, the release from specialist herbivores after introduction is expected invasive plants to shift their costly defense against specialist herbivores to cheaper defense against local generalist herbivores. A net gain can be saved to increase plant growth and competitive ability. In this thesis I used Jacobaea vulgaris as a model plant species to study the evolutionary changes in plant anti-herbivore defenses and growth ability. I found that invasive Jacobaea vulgaris populations indeed have evolved better growth and competitive ability, higher chemical defense but decreased structural defense and regrowth ability compare to the native populations. In addition, we show that all studied traits of invasive populations from multiple geographically distinct regions changed toward the same expected direction. It indicates parallel evolution took place in these invasive regions since they all differed significantly in climatic conditions. Such parallel evolution is most likely due to the disappearance of selection pressures from specialist herbivores rather than the adaptation to local abiotic factors after invasion. Show less
Jasmonates (JAs) are plant signaling molecules that play important roles in defense against insects and necrotrophic pathogens. The receptor-repressor-transcription factor module COI1-JAZ-MYC is a... Show moreJasmonates (JAs) are plant signaling molecules that play important roles in defense against insects and necrotrophic pathogens. The receptor-repressor-transcription factor module COI1-JAZ-MYC is a key component in JAs signaling in Arabidopsis thaliana. JAs stimulate their own biosynthesis by inducing the expression of genes encoding the biosynthetic enzymes. The research aim was to study the molecular mechanisms controlling this positive feedback loop. The results show that the AP2/ERF-domain transcription factor ORA47 is the key regulator. Overexpression of ORA47 resulted in increased expression of all JAs biosynthesis genes tested and in elevated levels of several JAs, including jasmonic acid (JA) and the main bioactive compound JA-Isoleucine. The expression of the ORA47 gene and the activity of the ORA47 protein are stimulated by JA. The JA-responsive expression of the ORA47 gene is controlled by the transcript ion factors MYC2, MYC3 and MYC4 which interact with a G-box in the ORA47 promoter. The expression of ORA47 target genes is negatively affected by members of the JAZ repressor family, suggesting that they regulate ORA47 activity. However ORA47 does not directly interact with JAZ repressors in yeast two-hybrid assays. Therefore it is hypothesized that ORA47 is regulated via interaction with an adaptor protein that recruits certain members of the JAZ family. Members of the BTB-TAZ (BT) protein family were found to interact with ORA47 and with certain JAZ proteins. However analysis of quadruple bt mutants failed to detect an effect on the expression of JAs biosynthesis genes, indicating that BT proteins are not the hypothetical adaptor proteins. Show less
Plants produce a vast array of secondary metabolites (SMs). Some hypotheses were put forward to explain the diversity of structurally related SMs within the framework of plant defense against... Show morePlants produce a vast array of secondary metabolites (SMs). Some hypotheses were put forward to explain the diversity of structurally related SMs within the framework of plant defense against herbivores:1) SMs could be Selectively Neutral; 2) SM diversity is a result of the __Arms Race__ between plants and the herbivores; 3) Plants benefit from the SM diversity because of the Synergistic Effects among the SMs; 4) The SM diversity may be a response to the Selection from Multiple Herbivores. In this thesis, a hybrid family originated from an artificial cross between Jacobaea aquatica and Jacobaea vulgaris was used as a model system to study the selective forces from insect herbivores on pyrrolizidine alkaloids (PAs) evolution. The Jacobaea hybrid family showed great variation in PA variation and herbivore resistance. By means of three bioassays, one with a specialist (Tyria jacobaea) and two with generalist insect herbivores (Frankliniella occidentalis and Liriomyza trifolii), I could show that Jacobaea hybrid genotypes differed in the resistance to these herbivores and these differences were related to PA variation in the plants. Several PAs were involved in the resistance to insect herbivores. These results support the three hypotheses except Selectively Neutral Theory. Show less
Jasmonic acid (JA) is a plant hormone that plays an important role in defense against wounding, insects and microbial pathogens. In defense responses against microbial pathogens JA acts... Show moreJasmonic acid (JA) is a plant hormone that plays an important role in defense against wounding, insects and microbial pathogens. In defense responses against microbial pathogens JA acts synergistically with the stress hormone ethylene. ORA59, belonging to the plant-specific class of AP2-domain transcription factors, is the main regulator of JA/ethylene-responsive defense gene expression in the model plant species Arabidopsis thaliana. The aim of the research described in this thesis was to study how the activity of ORA59 is regulated by JA. Studies on the promoter of the defense gene PDF1.2 showed that ORA59 regulates gene expression by interaction with GCCGCC motifs. Studies of ORA59 at the protein level indicated that JA controls ORA59 activity by promoting the nuclear localization and stabilization of the protein. Based on the findings it was postulated that there is an F-box protein that targets ORA59 for degradation, and a repressor protein that sequesters ORA59 in the cytoplasm. A protein interacting with ORA59 was identified by yeast two-hybrid screening. Further studies showed that this protein acts as a repressor of ORA59 by retaining it in the cytoplasm, which leads to fine-tuning of basal resistance against pathogens. Show less
