Effect of Rhizobacteria on Induction of Volatile Organic Compounds and Consequences for Corn Herbivores and Tritrophic Interactions
Type of DegreePhD Dissertation
Entomology and Plant Pathology
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Herbivorous insects depend on plants for survival. When herbivores feed on a plant the immobile plant responds directly by mobilizing it defenses, or indirectly by attracting natural enemies of the herbivore. Although plant defense is mostly constitutive, induced defense can provide an advantage to the plant. Insect oral secretions, plant volatiles and microbes, such as rhizobacteria, are common elicitors of induced systemic defense in plants. Rhizobacteria are plant activators with broad applicability that includes promotion of soil health, growth of plant and health. However, little information is available on how rhizobacteria mediate crop plant-insect and tritrophic interactions. This study used Zea mays (corn), its important herbivores (Ostrinia nubilalis and Diabotrica virgifera virgifera) and D. v. virgifera natural enemy, as model systems to examine the effects rhizobacteria have on the induction of plant defense. The goal of this dissertation is to understand how rhizobacteria and plant-derived elicitor mediate plant-insect and tritrophic interactions. In chapter II, I investigated the role of rhizobacteria in mediation of oviposition of O. nubilalis in choice bioassays. Also, headspace volatile organic compounds (VOCs) from plants were analyzed by gas chromatography-mass spectrometry (GC-MS). Ostrinia nubilalis laid significantly fewer eggs on bacilli-treated plants compared to untreated plants. When a pair of bacilli-treated versus untreated plants were presented in two-choice oviposition experiments, significantly higher numbers of eggs were laid on untreated plants compared to bacilli-treated plants. Results showed that bacilli-treated plants emitted fewer VOCs than untreated plants, which in part, explain the relatively fewer numbers of eggs on bacilli-treated plants. These results indicate that selected bacilli treatments can alter corn plant volatiles with important ramifications for plant-insect interactions. In chapter III, I tested the hypothesis that application of bacilli as seed treatments to corn affects host-selection and feeding behavior of second instar larvae of D. v. virgifera. Corn, Bacillus pumilus strain INR-7 and D. v. virgifera second instar were used as a model system in horizontal olfactometer assays. Treatments for this study were Bacillus pumilus strain INR-7, two bacilli mixtures (Blend-8 or Blend-9) or untreated (control). Each bacteria treatment (plant) was compared with the untreated group (plant) in feeding preference studies. The result showed that D. v. virgifera larvae preferred untreated plants (76%) compared to plants treated with the B. pumilus strain INR-7 (24%). A follow up no-choice feeding test showed that D. v. virgifera larvae fed INR-7 treated plants weighed significantly less than larvae fed untreated plants or plants treated with bacilli blends. Overall, the results demonstrate that B. pumilus INR-7 can enhance resistance of corn against damage by D. v. virgifera larvae. Following the result of chapter III where B. pumilus strain INR-7 showed promise as biological control agent against D. v. virgifera larvae, I conducted a study with natural enemies of D. v. virgifera larvae in chapter IV. The preference of Heterorhabditis bacteriophora to D. v. virgifera-infested corn roots was investigated using belowground four-choice olfactometer. Treatments for this study were B. pumilus strain INR-7, heat-killed INR-7 (HK), untreated or sand control. Intact plants were presented in four-choice olfactometer in the presence or absence of D. v. virgifera larvae in growth chambers. Furthermore, volatile root extract was sampled from plants with similar treatments above and effects were evaluated using four-choice olfactometer in a laboratory setting. The results showed that significantly higher numbers of H. bacteriophora choose B. pumilus strain INR-7 inoculated intact corn roots compared to untreated plants or sand control both in the presence or absence of D. v. virgifera larvae. A further test of corn root VOC extract showed that a higher number of H. bacteriophora was recovered from the arm containing VOC from INR-7 treated plants without infestation but the number was not significantly different from other treatments. Also, H. bacteriophora recovered from the arm containing VOC from INR-7 treated plants that were infested with D. v. virgifera larvae was not statistically different from other treatments, suggesting that other factors than volatile cues may have affected H. bacteriophora choice to bacillus-treated plants. These studies have increased our understanding of the role of rhzobacteria in mediation of host plant indirect resistance. In chapter V, I studied the molecular mechanisms of cis-Jasmone (CJ) in mediating changes in terpenoid genes and emission of VOCs. The role of VOC in oviposition behavior of insects was also tested. First, I quantified VOC and the transcript levels of key genes that encode VOC biosynthesis in CJ-treated plants with Spodoptera exigua caterpillar infestation (CJI), untreated plants with S. exigua caterpillar infestation (UI), CJ-treated plants without S. exigua caterpillar infestation (CJ), and untreated plants without S. exigua caterpillar infestation (U). In addition, oviposition preference of S. exigua was compared between CJI and UI, and between CJ and U. The result of GC-MS analyses showed qualitative and quantitative differences in CJI compared to UI, CJ or U. The result also showed that the transcript levels of certain terpene synthase genes involved in the biosynthesis of many VOC were higher in CJI plants. Consequently, S. exigua laid fewer numbers of eggs on CJI than UI. Moreover, in an in vitro oviposition choice test using filter paper, S. exigua laid significantly fewer eggs on filter papers containing VOC from CJI compared to UI. These results indicate that CJ treatment followed by caterpillar infestation can prime tomato plant defense with potential ramifications for insect oviposition. In chapter VI, I provided a general conclusion that summed up the entire project. Opportunities for future research were identified.