Herbivore-Induced Defenses in Tomato Plants Enhance the Lethality of the Entomopathogenic Bacterium, Bacillus thuringiensis var. kurstaki

Tomato Chemical Defenses Intensify Corn Earworm (Helicoverpa zea) Mortality from Opportunistic Bacterial Pathogens

Insect herbivores face multiple challenges to their ability to grow and reproduce. Plants can produce a series of defenses that disrupt and damage the herbivore digestive system, which are heightened upon injury by insect feeding. Additionally, insects face threats from virulent microorganisms that can incur their own set of potential costs to hosts. Microorganisms that invade through the digestive system may function in concert with defenses generated by plants, creating combined assailments on host insects. In our study, we evaluated how tomato defenses interact with an enteric bacterial isolate, Serratia marcescens, in the corn earworm (Helicoverpa zea). We performed bioassays using different tomato cultivars that were induced by methyl jasmonate and larvae orally inoculated with a S. marcescens isolate. Untreated corn earworm larval mortality was low on constitutive tomato, while larvae inoculated with S. marcescens exhibited > 50% mortality within 5 days. Induction treatments elevated both control mortality (~ 45%) and in combination with S. marcescens (> 95%). Larvae also died faster when encountering induced defenses and Serratia. Using a tomato mutant, foliar polyphenol oxidase activity likely had stronger impacts on S. marcescens-mediated larval mortality. Induction treatments also elevated the number of bacterial colony-forming units in the hemolymph of larvae inoculated with Serratia. Larval mortality by S. marcescens was low (< 10%) on artificial diets. Our results demonstrate that plant chemical defenses enhance larval mortality from an opportunistic gut microbe. We propose that the combined damage from both the plant and microbial agent overwhelm the herbivore to increase mortality rates and expedite host death.

Compatibility of mycorrhiza-induced resistance with viral and bacterial entomopathogens in the control of Spodoptera exigua in tomato

BACKGROUND

Arbuscular mycorrhizal fungi (AMF) are soil-borne microorganisms that establish mutualistic associations with roots of most terrestrial plants. This symbiosis results in nutritional and defensive benefits to the host plant, usually conferring protection against biotic stresses, but its indirect impact on third trophic levels is still unknown. In the present work, we explore whether the symbiosis of tomato plants with Funneliformis mosseae (and/or exposition to herbivory) influences the interaction of the generalist pest Spodoptera exigua (Lepidoptera: Noctuidae) with bacterial (Bacillus thuringiensis) and viral (baculovirus, SeMNPV) natural entomopathogens.

RESULTS

Symbiosis with AMF and previous herbivory reduces the relative growth of S. exigua, increases its susceptibility to a sublethal dose of B. thuringiensis and has positive or neutral impact on the lethality of SeMNPV. Reduction of the phenoloxidase activity, a marker of the insect immune response, was associated with the larval feeding on plant material previously exposed to herbivory but not to the AMF. In addition, no changes in the insect gut microbiota could be associated with the observed changes in larval growth and susceptibility to the entomopathogens.

CONCLUSION

Our findings provide the first evidence of compatibility of AMF symbiosis in tomato with the use of bacterial and viral entomopathogens, contributing to the development of novel approaches to combine the beneficial effect of AMF and entomopathogens in biological pest control. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Concerted impacts of antiherbivore defenses and opportunistic Serratia pathogens on the fall armyworm (Spodoptera frugiperda)

Insects frequently confront different microbial assemblages. Bacteria inhabiting an insect gut are often commensal, but some can become pathogenic when the insect is compromised from different stressors. Herbivores are often confronted by various forms of plant resistance, but how defenses generate opportunistic microbial infections from residents in the gut are not well understood. In this study, we evaluated the pathogenic tendencies of Serratia isolated from the digestive system of healthy fall armyworm larvae (Spodoptera frugiperda) and how it interfaces with plant defenses. We initially selected Serratia strains that varied in their direct expression of virulence factors. Inoculation of the different isolates into the fall armyworm body cavity indicated differing levels of pathogenicity, with some strains exhibiting no effects while others causing mortality 24 h after injection. Oral inoculations of pathogens on larvae provided artificial diets caused marginal (< 7%) mortality. However, when insects were provided different maize genotypes, mortality from Serratia increased and was higher on plants exhibiting elevated levels of herbivore resistance (< 50% mortality). Maize defenses facilitated an initial invasion of pathogenic Serratia into the larval hemocoel¸ which was capable of overcoming insect antimicrobial defenses. Tomato and soybean further indicated elevated mortality due to Serratia compared to artificial diets and differences between plant genotypes. Our results indicate plants can facilitate the incipient emergence of pathobionts within gut of fall armyworm. The ability of resident gut bacteria to switch from a commensal to pathogenic lifestyle has significant ramifications for the host and is likely a broader phenomenon in multitrophic interactions facilitated by plant defenses.

The effects of a myrmecochore-produced chemical on entomopathogenic fungal growth and seed-dispersing ant survival rates and foraging patterns

Myrmecochory, a type of ant-mediated seed dispersal, is a diffuse, widespread mutualism in which both partners are purported to benefit from the services or rewards of the other. However, ant benefits in this interaction are conflicted and understudied, especially in the context of microbial third parties. Here, we investigate the effect of a myrmecochore plant-produced antimicrobial chemical (sanguinarine) on the growth of a common entomopathogenic fungus (Beauveria bassiana). We then explore whether sanguinarine, through its effect on entomopathogen growth, might influence ant survival and foraging behavior. At high concentrations, sanguinarine increased the growth of B. bassiana, but fungal growth was not affected at concentrations of sanguinarine near natural levels produced in seeds. When ant colonies were exposed to B. bassiana, survival was not affected by a sanguinarine-supplemented diet. Furthermore, ant foraging patterns (preference for or avoidance of food items with sanguinarine) did not change when ants were exposed to the entomopathogen. Though sanguinarine promotes the growth of an entomopathogen at higher concentrations, which might pose an additional risk for ants in myrmecochory, we assert that social immune behavioral defenses (such as grooming or redispersal of seeds after elaiosome consumption) help ants mitigate this risk. By incorporating a microbial third party into this ant-plant interaction, we seek to more fully understand the risks and benefits provided to both partners in this mutualism. We encourage the investigation of third-party influences in reciprocal pairwise interactions to assist in the understanding of the evolution and persistence of mutualisms.

An R1R2R3 MYB Transcription Factor, MnMYB3R1, Regulates the Polyphenol Oxidase Gene in Mulberry (Morus notabilis)

The aim of this study was to determine how the mulberry (Morus notabilis) polyphenol oxidase 1 gene (MnPPO1) is regulated during plant stress responses by exploring the interaction between its promoter region and regulatory transcription factors. First, we analyzed the cis-acting elements in the MnPPO1 promoter. Then, we used the MnPPO1 promoter region [(1268 bp, including an MYB3R-binding cis-element (MSA)] as a probe to capture proteins in DNA pull-down assays. These analyses revealed that the MYB3R1 transcription factor in M. notabilis (encoded by MnMYB3R1) binds to the MnPPO1 promoter region. We further explored the interaction between the MnPPO1 promoter and MYB3R1 with the dual luciferase reporter, yeast one-hybrid, and chromatin immunoprecipitation assays. These analyses verified that MnMYB3R1 binds to the MSA in the MnPPO1 promoter region. The overexpression of MnMYB3R1 in tobacco upregulated the expression of the tobacco PPO gene. This observation as well as the quantitative real-time PCR results implied that MnMYB3R1 and PPO are involved in the abscisic acid-responsive stress response pathway.

Jasmonic acid-induced plant defenses delay caterpillar developmental resistance to a baculovirus: Slow-growth, high-mortality hypothesis in plant-insect-pathogen interactions

Plants damaged by herbivore feeding can induce defensive responses that reduce herbivore growth. The slow-growth, high-mortality hypothesis postulates that these non-lethal plant defenses prolong the herbivore's period of susceptibility to natural enemies, such as predators and parasitoids. While many juvenile animals increase their disease resistance as they grow, direct tests of the slow-growth, high-mortality hypothesis in the context of plant-herbivore-pathogen interactions are lacking. Caterpillars increase their resistance to lethal baculoviruses as they develop within and across instars, a phenomenon termed developmental resistance. Progression of developmental resistance can occur through age-related increases in systemic immune functioning and/or midgut-based resistance. Here, we examined the slow-growth, high-mortality hypothesis in the context of developmental resistance of caterpillars to baculoviruses. Intra-stadial (within-instar) developmental resistance of the fall armyworm, Spodoptera frugiperda, to an oral inoculum of the baculovirus SfMNPV increased more rapidly with age when larvae were fed on non-induced foliage than foliage that was induced by jasmonic acid (a phytohormone that up-regulates plant anti-herbivore defenses). The degree of developmental resistance observed was attributable to larval weight at the time of virus inoculation. Thus, slower growth on induced plants prolonged the window of larval susceptibility to the baculovirus. Developmental resistance on induced and non-induced plants was absent when budded virus was injected intrahemocoelically bypassing the midgut, suggesting that developmental resistance was gut-based. Addition of fluorescent brightener, which weakens midgut-based resistance mechanisms to oral virus challenge, abolished developmental resistance. These results highlight the impact of plant defenses on herbivore growth rate and consequences for disease risk.