Carotenoids moderate the effectiveness of a Bt gene against the European corn borer, Ostrinia nubilalis

Jasmonic Acid and Salicylic Acid improved resistance against Spodoptera frugiperda Infestation in maize by modulating growth and regulating redox homeostasis

Exploring host plant resistance and elevating plant defense mechanisms through the application of exogenous elicitors stands as a promising strategy for integrated pest management. The fall armyworm, a pernicious menace to grain crops in tropical and subtropical regions, stands as a formidable threat due to its capacity for devastation and a wide-ranging spectrum of host plants. There is no literature regarding artificially induced resistance in maize against fall armyworm (Spodoptera frugiperda) by exogenous application of phytohormones. The present investigation was performed to evaluate the role of jasmonic acid (JA) and salicylic acid (SA) on two maize hybrids namely FH-1046 and YH-1898 against fall armyworm. Results showed that plant height, biomass and lengths, fresh and dry weight of root shoot which decreased with armyworm infestation improved with phytohormonal application. JA treatment resulted in a higher increase in all attributes as compared to SA treatment. Improvement in relative water contents, photosynthetic pigments and pronounced levels of phenol and proline accumulation were observed in infested plants after JA treatment. Infested plants recovered from oxidative stress as JA application activated and increased the antioxidant enzyme activity of superoxide dismutase, peroxidase and polyphenol oxidase activity in both FH-1046 and YH-1898 . The oxidative stress reduction in infested plants after JA treatment was also evident from a fair decrease in MDA and H2O2 in both varieties. The SA and JA mediated genes expression was studied and it was found that in FH1046 maize cultivar, JA dependent genes, particularly marker genes PR1 and Lox5 were highly expressed along with TPS10 and BBT12. Whereas SPI, WRKY28, ICS and PAL were shown to be activated upon SA application. Evidently, both JA and SA elicited a robust defensive response within the maize plants against the voracious S. frugiperda, which in consequence exerted a discernible influence over the pest's developmental trajectory and physiological dynamics. A decrease in detoxification enzyme activity of the insects was observed after feeding on treated plants. Moreover, it was recorded that the survival and weight gain of FAW feeding on phytohormone treated maize plants also decelerated. In conclusion, FH-1046 was found to be more tolerant than YH-1898 against fall armyworm infestation and 1 mM JA was more effective than 1 mM SA for alleviation of fall armyworm stress. Therefore, it was inferred that phytohormones regulated redox homeostasis to circumvent oxidative damage and mediate essential metabolic events in maize under stress. To our current understanding, this study is the very first presentation of induced resistance in maize against S. frugiperda with the phytohormonal application (JA and SA).

Development of a bioassay method to test activity of cry insecticidal proteins against Diatraea spp. (Lepidoptera: Crambidae) sugarcane stem borers

The genus Diatraea (Lepidoptera: Crambidae) includes stem borers representing the most critical sugarcane pests in the Americas. Colombia's most widely distributed and damaging Diatraea species include Diatraea saccharalis, D. indigenella, D. busckella, and D. tabernella. The reduced efficacy of biological tools commonly used in controlling several species highlights the importance of evaluating alternative management strategies, such as transgenic plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt). The selection of optimal Bt insecticidal proteins for Diatraea control depends on bioassays with purified Bt proteins. Because there is no described artificial diet for borer species other than D. saccharalis and availability of most purified Bt toxins is restricted, this study aimed at developing a bioassay method using fresh corn tissue and providing proof of concept by testing susceptibility to the Cry1Ac insecticidal protein from Bt. Toxicity was evaluated with a single Cry1Ac dose applied directly to corn discs. Stem borer mortality after seven days was higher than 90% for all four tested Diatraea species, while control mortality was below 8%. In addition, we observed that Cry1Ac caused more than 90% weight inhibition in all survivors and delayed development. These results validate the use of this method to determine mortality and growth inhibition due to the consumption of the Cry1Ac protein in each of the Diatraea species. Furthermore, this method could be used to assess other entomopathogenic substances to control these insect pests.

Parental effects of Bt toxin and vitamin A on Helicoverpa armigera

The increase in the area cultivated with vitamin-enriched transgenic crops producing Bt toxin raises the question of whether the addition of vitamins will in any way mitigates the effect of the toxin on the phytophagous insects that feed on those crops. On the other hand, the parental effect that feeding on these enriched transgenic crops may have on the offspring of the phytophagous that survive on them is not well known. In this work, the effect of vitamin A (β-carotene) addition to diets with or without Bt toxin on Helicoverpa armigera larvae and their offspring was determined. The addition of vitamin A did not have any beneficial effect either for the larvae fed on enriched diets nor for their offspring. However, parental effects due to dietary feeding with the toxin were detected since adults from larvae fed on the Bt diet had higher mating success than those fed on the toxin-free diet, although there were no differences on the fertility of mated females regardless of whether their previous larvae fed on the Bt or non-Bt diet. A certain adaptive effect to the toxin was also noted since the mortality of larvae whose previous generation fed on diet with Bt was lower than that of the larvae that came from larvae fed on a non-Bt diet. It would be interesting to determine if H. armigera adults prefer to mate and lay eggs in the same type of crops in which they have developed or if feeding on different crops, such as corn or alfalfa, causes different paternal effects on the offspring. These aspects can be of great importance in the development of resistance of this species to the Bt toxin.

Fitness Costs of Two Maize Lepidopteran Pests Fed on Bacillus thuringiensis (Bt) Diets Enriched with Vitamins A and C

Simple Summary

Biotechnologists are designing new transgenic plants enriched with micronutrients and vitamins that are resistant to insects. These new plants could favor the development of some pest insects. This work aims to discover the effect of adding two vitamins, A and C, to insect diets prepared with Bt and no-Bt maize in two maize insect pests. M. unipuncta was less sensitive to the toxin, although ingestion of the Bt diet resulted in longer larval development and lower pupal weight, which were not mitigated by any of the vitamins. However, the two vitamins reduced the mortality of H. armigera larvae fed on the Bt diet. In addition, we found evidence of the antioxidant function of vitamin A. The results obtained here indicate that crops enriched with these vitamins will hardly favor the development of H. armigera and suggest that they do not affect M. unipuncta’s development at all.

Abstract

Serious malnutrition problems occur in developing countries where people’s diets are mainly based on staple crops. To alleviate this, high-production crops are being developed that are better adapted to climate change, enriched in micronutrients and vitamins, or resistant to pests. In some cases, new varieties have been developed with several of the characteristics mentioned above, such as biofortified and pest-resistant crops. The development of biofortified Bacillus thuringiensis (Bt) crops raises the question of whether vitamin enrichment of Bt crops can in any way favor those pests that are not very susceptible to the Bt toxin that feed on these crops, such as Helicoverpa armigera (Hübner) or Mythimna unipuncta (Haworth) (Lepidoptera: Noctuidae). In this study, the response to a Bt diet enriched with vitamins A (β-carotene) and C (ascorbic acid) was somewhat different between the two species. M. unipuncta was less sensitive to the toxin than H. armigera, although the ingestion of the Bt diet resulted in oxidative stress (longer larval development and lower pupal weight) which was not mitigated by the vitamins. However, the two vitamins reduced the mortality of H. armigera larvae fed on a Bt-enriched diet; in addition, ß-carotene reduced the activity of the antioxidant glutathione S-transferase (GST) of both species, suggesting it has an antioxidant role. The results obtained here indicate that biofortified Bt crops will not favor the development of H. armigera very much and will not affect M. unipuncta’s development at all, although the effect of the increase in vitamins may be very variable and should be studied for each specific phytophagous.

Nitrogen inputs influence vegetative metabolism in maize engineered with a seed-specific carotenoid pathway

Metabolomic profiling of a maize line engineered with an endosperm-specific carotenogenic pathway revealed unexpected metabolic readjustments of primary metabolism in leaves and roots. High-carotenoid (HC) maize was engineered to accumulate high levels of carotenoids in the endosperm. The metabolic interventions influenced the flux through non-target pathways in tissues that were not affected by the targeted intervention. HC maize at the vegetative stage also showed a reduced susceptibility to insect feeding. It is unknown, however, whether the metabolic history of the embryo has any impact on the metabolite composition in vegetative tissues. We, therefore, compared HC maize and its isogenic counterpart (M37W) to test the hypothesis that boosting the carotenoid content in the endosperm triggers compensatory effects in core metabolism in vegetative tissues. Specifically, we investigated whether the metabolite composition of leaves and roots at the V6 stage differs between HC and M37W, and whether N inputs further alter the core metabolism of HC compared to M37W. We found an increase in the abundance of organic acids from the tricarboxylic acid (TCA) cycle in HC even under restricted N conditions. In contrast, low levels of carotenoids and chlorophyll were measured regardless of N levels. Sugars were also significantly depleted in HC under low N. We propose a model explaining the observed genotype-dependent and input-dependent effects, in which organic acids derived from the TCA cycle accumulate during vegetative growth and contribute to the increased demand for pyruvate and/or acetyl-CoA in the endosperm and embryo. This response may in part reflect the transgenerational priming of vegetative tissues in the embryo induced by the increased demand for metabolic precursors during seed development in the previous generation.

β-carotene and Bacillus thuringiensis insecticidal protein differentially modulate feeding behaviour, mortality and physiology of European corn borer (Ostrinia nubilalis)

Maize with enhanced β-carotene production was engineered to counteract pervasive vitamin A deficiency in developing countries. Second-generation biofortified crops are being developed with additional traits that confer pest resistance. These include crops that can produce Bacillus thuringiensis Berliner (Bt) insecticidal proteins. Currently, it is unknown whether β-carotene can confer fitness benefits through to insect pests, specifically through altering Ostrinia nubilalis foraging behaviour or development in the presence of Bt insecticidal toxin. Therefore the effects of dietary β-carotene plus Bt insecticidal protein on feeding behaviour, mortality, and physiology in early and late instars of O. nubilalis larvae were investigated. The results of two-choice experiments showed that irrespective of β-carotene presence, at day five 68%-90% of neonates and 69%-77% of fifth-instar larvae avoided diets with Cry1A protein. Over 65% of neonate larvae preferred to feed on diets with β-carotene alone compared to 39% of fifth-instar larvae. Higher mortality (65%-97%) in neonates fed diets supplemented with β-carotene alone and in combination with Bt protein was found, whereas <36% mortality was observed when fed diets without supplemented β-carotene or Bt protein. Diets with both β-carotene and Bt protein extended 25 days the larval developmental duration from neonate to fifth instar (compared to Bt diets) but did not impair larval or pupal weight. Juvenile hormone and 20-hydroxyecdysone regulate insect development and their levels were at least 3-fold higher in larvae fed diets with β-carotene for 3 days. Overall, these results suggest that the effects of β-carotene and Bt protein on O. nubilalis is dependent on larval developmental stage. This study is one of the first that provides insight on how the interaction of novel traits may modulate crop susceptibility to insect pests. This understanding will in turn inform the development of crop protection strategies with greater efficacy.

Transcriptional changes in the aphid species Myzus cerasi under different host and environmental conditions

Aphids feature complex life cycles, which in the case of many agriculturally important species involve primary and secondary host plant species. Whilst host alternation between primary and secondary host can occur in the field depending on host availability and the environment, aphid populations maintained as laboratory stocks generally are kept under conditions that allow asexual reproduction by parthenogenesis on secondary hosts. We used Myzus cerasi (black cherry aphid) to assess aphid transcriptional differences between populations collected from primary hosts in the field and those adapted to secondary hosts under controlled environment conditions. Transfer of M. cerasi collected from local cherry trees to reported secondary host species resulted in low survival rates. Moreover, aphids were unable to survive on the secondary host land cress, unless first adapted to another secondary host, cleavers. Transcriptome analyses of the different aphid populations (field collected and adapted) revealed extensive transcriptional plasticity to a change in environment, with predominantly genes involved in redox reactions differentially regulated. Most of the differentially expressed genes were duplicated and we found evidence for differential exon usage. Our data suggest that aphid adaptation to different environments may pose a major hurdle and leads to extensive gene expression changes.

The Impact of Environmental Stress on Bt Crop Performance

Bt crops have been grown commercially for more than two decades. They have proven remarkably effective in the control of target insect pests. However, Bt crops can become less effective under various forms of environmental stress. Most studies in this area have considered the effect of environmental stress on Bt insecticidal protein levels or target pest mortality, but not both, resulting in a lack of mechanistic analysis. In this review, we critically examine previous research addressing the impact of environmental stress on the effectiveness of Bt crops. We find that the body of research data is not sufficiently robust to allow the reliable prediction of the performance of Bt crops under extreme climatic conditions.