Oviposition by a moth suppresses constitutive and herbivore-induced plant volatiles in maize

Plant volatiles function as important signals for herbivores, parasitoids, predators, and neighboring plants. Herbivore attack can dramatically increase plant volatile emissions in many species. However, plants do not only react to herbivore-inflicted damage, but also already start adjusting their metabolism upon egg deposition by insects. Several studies have found evidence that egg deposition itself can induce the release of volatiles, but little is known about the effects of oviposition on the volatiles released in response to subsequent herbivory. To study this we measured the effect of oviposition by Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) moths on constitutive and herbivore-induced volatiles in maize (Zea mays L.). Results demonstrate that egg deposition reduces the constitutive emission of volatiles and suppresses the typical burst of inducible volatiles following mechanical damage and application of caterpillar regurgitant, a treatment that mimics herbivory. We discuss the possible mechanisms responsible for reducing the plant's signaling capacity triggered by S. frugiperda oviposition and how suppression of volatile organic compounds can influence the interaction between the plant, the herbivore, and other organisms in its environment. Future studies should consider oviposition as a potential modulator of plant responses to insect herbivores. © 2011 Springer-Verlag.

Metabolism of sulphonated anthraquinones in rhubarb, maize and celery: the role of cytochromes P450 and peroxidases

Sulphonated anthraquinones are precursors of many synthetic dyes and pigments, recalcitrant to biodegradation, and thus contaminating many industrial effluents and rivers. In the development of a phytotreatment to remove sulphonated aromatic compounds, rhubarb (Rheum rhaponticum), a plant producing natural anthraquinones, as well as maize (Zea mays) and celery (Apium graveolens), plants not producing anthraquinones, were tested for their ability to metabolise these xenobiotics. Plants were cultivated under hydroponic conditions, with or without sulphonated anthraquinones, and were harvested at different times. Either microsomal or cytosolic fractions were prepared. The monooxygenase activity of cytochromes P450 towards several sulphonated anthraquinones was tested using a new method based on the fluorimetric detection of oxygen consumed during cytochromes P450-catalysed reactions. The activity of cytosolic peroxidases was measured by spectrophotometry, using guaiacol as a substrate. Results indicated that the activity of cytochromes P450 and peroxidases significantly increased in rhubarb plants cultivated in the presence of sulphonated anthraquinones. A higher activity of cytochromes P450 was also detected in maize and celery exposed to the pollutants. In these two plants, a peroxidase activity was also detected, but without a clear difference between the control plants and the plants exposed to the organic contaminants. This research demonstrated the existence in rhubarb, maize and celery of biochemical mechanisms involved in the metabolism and detoxification of sulphonated anthraquinones. Taken together, results confirmed that rhubarb might be the most appropriate plant for the phytotreatment of these organic pollutants.

Transport of iodide in structured clay–loam soil under maize during irrigation experiments analyzed using HYDRUS model

Transport of radioactive iodide 131I− in a structured clay loam soil under maize in a final growing phase was monitored during five consecutive irrigation experiments under ponding. Each time, 27 mm of water were applied. The water of the second experiment was spiked with 200 MBq of 131I− tracer. Its activity was monitored as functions of depth and time with Geiger-Müller (G-M) detectors in 11 vertically installed access tubes. The aim of the study was to widen our current knowledge of water and solute transport in unsaturated soil under different agriculturally cultivated settings. It was supposed that the change in 131I− activity (or counting rate) is proportional to the change in soil water content. Rapid increase followed by a gradual decrease in 131I− activity occurred at all depths and was attributed to preferential flow. The iodide transport through structured soil profile was simulated by the HYDRUS 1D model. The model predicted relatively deep percolation of iodide within a short time, in a good agreement with the observed vertical iodide distribution in soil. We found that the top 30 cm of the soil profile is the most vulnerable layer in terms of water and solute movement, which is the same depth where the root structure of maize can extend.

Metabolomics reveals herbivore-induced metabolites of resistance and susceptibility in maize leaves and roots

Plants respond to herbivory by reprogramming their metabolism. Most research in this context has focused on locally induced compounds that function as toxins or feeding deterrents. We developed an ultra-high-pressure liquid chromatography time-of-flight mass spectrometry (UHPLC-TOF-MS)-based metabolomics approach to evaluate local and systemic herbivore-induced changes in maize leaves, sap, roots and root exudates without any prior assumptions about their function. Thirty-two differentially regulated compounds were identified from Spodoptera littoralis-infested maize seedlings and isolated for structure assignment by microflow nuclear magnetic resonance (CapNMR). Nine compounds were quantified by a high throughput direct nano-infusion tandem mass spectrometry/mass spectrometry (MS/MS) method. Leaf infestation led to a marked local increase of 1,3-benzoxazin-4-ones, phospholipids, N-hydroxycinnamoyltyramines, azealic acid and tryptophan. Only few changes were found in the root metabolome, but 1,3-benzoxazin-4-ones increased in the vascular sap and root exudates. The role of N-hydroxycinnamoyltyramines in plant–herbivore interactions is unknown, and we therefore tested the effect of the dominating p-coumaroyltyramine on S. littoralis. Unexpectedly, p-coumaroyltyramine was metabolized by the larvae and increased larval growth, possibly by providing additional nitrogen to the insect. Taken together, this study illustrates that herbivore attack leads to the induction of metabolites that can have contrasting effects on herbivore resistance in the leaves and roots.

Natural Variation in Maize Aphid Resistance Is Associated with 2,4-Dihydroxy-7-Methoxy-1,4-Benzoxazin-3-One Glucoside Methyltransferase Activity

Plants differ greatly in their susceptibility to insect herbivory, suggesting both local adaptation and resistance tradeoffs. We used maize (Zea mays) recombinant inbred lines to map a quantitative trait locus (QTL) for the maize leaf aphid (Rhopalosiphum maidis) susceptibility to maize Chromosome 1. Phytochemical analysis revealed that the same locus was also associated with high levels of 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one glucoside (HDMBOA-Glc) and low levels of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one glucoside (DIMBOA-Glc). In vitro enzyme assays with candidate genes from the region of the QTL identified three O-methyltransferases (Bx10a-c) that convert DIMBOA-Glc to HDMBOA-Glc. Variation in HDMBOA-Glc production was attributed to a natural CACTA family transposon insertion that inactivates Bx10c in maize lines with low HDMBOA-Glc accumulation. When tested with a population of 26 diverse maize inbred lines, R. maidis produced more progeny on those with high HDMBOA-Glc and low DIMBOA-Glc. Although HDMBOA-Glc was more toxic to R. maidis than DIMBOA-Glc in vitro, BX10c activity and the resulting decline of DIMBOA-Glc upon methylation to HDMBOA-Glc were associated with reduced callose deposition as an aphid defense response in vivo. Thus, a natural transposon insertion appears to mediate an ecologically relevant trade-off between the direct toxicity and defense-inducing properties of maize benzoxazinoids.

Two enzymes responsible for the formation of herbivore-induced volatiles of maize, the methyltransferase AAMT1 and the terpene synthase TPS23, are regulated by a similar signal transduction pathway

Herbivore-induced volatiles play an important role in the indirect defense of plants. After herbivore damage, volatiles are released from the plant and can attract herbivore enemies that protect the plant from additional damage. The herbivore-induced volatile blend is complex and usually consists of mono- and sesquiterpenes, aromatic compounds, and indole. Although these classes of compounds are generally produced at different times after herbivore damage, the release of the terpene (E)-β-caryophyllene and the aromatic ester methyl anthranilate appear to be tightly coordinated. We have studied the herbivore induction patterns of two terpene synthases from Zea mays L. (Poaceae), TPS23 and TPS10, as well as S-adenosyl-L-methionine:anthranilic acid carboxyl methyltransferases (AAMT1), which are critical for the production of terpenes and anthranilate compounds, respectively. The transcript levels of tps23 and aamt1 displayed the same kinetics after damage by the larvae of Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae), and showed the same organ-specific and haplotype-specific expression patterns. Despite its close functional relation to TPS23, the terpene synthase TPS10 is not expressed in roots and does not display the haplotype-specific expression pattern. The results indicate that the same JA-mediated signaling cascade maycontrol the production of both the terpene (E)-β-caryophyllene and aromatic ester methyl anthranilate.

Less is More: Treatment with BTH and Laminarin Reduces Herbivore-Induced Volatile Emissions in Maize but Increases Parasitoid Attraction

Chemical plant strengtheners find increasing use in agriculture to enhance resistance against pathogens. In an earlier study, it was found that treatment with one such resistance elicitor, BTH (benzo-(1, 2, 3)-thiadiazole-7-carbothioic acid S-methyl ester), increases the attractiveness of maize plants to a parasitic wasp. This surprising additional benefit of treating plants with BTH prompted us to conduct a series of olfactometer tests to find out if BTH and another commercially available plant strengthener, Laminarin, increase the attractiveness of maize to three important parasitic wasps, Cotesia marginventris, Campoletis sonorensis, and Microplitis rufiventris. In each case, plants that were sprayed with the plant strengtheners and subsequently induced to release volatiles by real or mimicked attack by Spodoptera littoralis caterpillars became more attractive to the parasitoids than water treated plants. The elicitors alone or in combination with plants that were not induced by herbivory were not attractive to the wasps. Interestingly, plants treated with the plant strengtheners did not show any consistent increase in volatile emissions. On the contrary, treated plants released less herbivore-induced volatiles, most notably indole, which has been reported to interfere with parasitoid attraction. The emission of the sesquiterpenes (E)-β-caryophyllene, β-bergamotene, and (E)-β-farnesene was similarly reduced by the treatment. Expression profiles of marker genes showed that BTH and Laminarin induced several pathogenesis related (PR) genes. The results support the notion that, as yet undetectable and unidentified compounds, are of major importance for parasitoid attraction, and that these attractants may be masked by some of the major compounds in the volatile blends. This study confirms that elicitors of pathogen resistance are compatible with the biological control of insect pests and may even help to improve it.

3-β-d-Glucopyranosyl-6-methoxy-2-benzoxazolinone (MBOA-N-Glc) is an insect detoxification product of maize 1,4-benzoxazin-3-ones

In order to defend themselves against arthropod herbivores, maize plants produce 1,4-benzoxazin-3-ones (BXs), which are stored as weakly active glucosides in the vacuole. Upon tissue disruption, BXs come into contact with β-glucosidases, resulting in the release of active aglycones and their breakdown products. While some aglycones can be reglucosylated by specialist herbivores, little is known about how they detoxify BX breakdown products. Here we report on the structure of an N-glucoside, 3-β-d-glucopyranosyl-6-methoxy-2-benzoxazolinone (MBOA-N-Glc), purified from Spodoptera frugiperda faeces. In vitro assays showed that MBOA-N-Glc is formed enzymatically in the insect gut using the BX breakdown product 6-methoxy-2-benzoxazolinone (MBOA) as precursor. While Spodoptera littoralis and S. frugiperda caterpillars readily glucosylated MBOA, larvae of the European corn borer Ostrinia nubilalis were hardly able to process the molecule. Accordingly, Spodoptera caterpillar growth was unaffected by the presence of MBOA, while O. nubilalis growth was reduced. We conclude that glucosylation of MBOA is an important detoxification mechanism that helps insects tolerate maize BXs.

Maize Domestication and Anti-Herbivore Defences: Leaf-Specific Dynamics during Early Ontogeny of Maize and Its Wild Ancestors

As a consequence of artificial selection for specific traits, crop plants underwent considerable genotypic and phenotypic changes during the process of domestication. These changes may have led to reduced resistance in the cultivated plant due to shifts in resource allocation from defensive traits to increased growth rates and yield. Modern maize (Zea mays ssp. mays) was domesticated from its ancestor Balsas teosinte (Z. mays ssp. parviglumis) approximately 9000 years ago. Although maize displays a high genetic overlap with its direct ancestor and other annual teosintes, several studies show that maize and its ancestors differ in their resistance phenotypes with teosintes being less susceptible to herbivore damage. However, the underlying mechanisms are poorly understood. Here we addressed the question to what extent maize domestication has affected two crucial chemical and one physical defence traits and whether differences in their expression may explain the differences in herbivore resistance levels. The ontogenetic trajectories of 1,4-benzoxazin-3-ones, maysin and leaf toughness were monitored for different leaf types across several maize cultivars and teosinte accessions during early vegetative growth stages. We found significant quantitative and qualitative differences in 1,4-benzoxazin-3-one accumulation in an initial pairwise comparison, but we did not find consistent differences between wild and cultivated genotypes during a more thorough examination employing several cultivars/accessions. Yet, 1,4-benzoxazin-3-one levels tended to decline more rapidly with plant age in the modern maize cultivars. Foliar maysin levels and leaf toughness increased with plant age in a leaf-specific manner, but were also unaffected by domestication. Based on our findings we suggest that defence traits other than the ones that were investigated are responsible for the observed differences in herbivore resistance between teosinte and maize. Furthermore, our results indicate that single pairwise comparisons may lead to false conclusions regarding the effects of domestication on defensive and possibly other traits.

Belowground herbivore tolerance involves delayed overcompensatory root regrowth in maize

Plants can tolerate leaf-herbivore attack through metabolic reconfigurations that allow for the rapid regrowth of lost leaves. Several studies indicate that root-attacked plants can re-allocate resources to the aboveground parts. However, the connection between tolerance and root regrowth remains poorly understood. We investigated the timing and extent of root regrowth of tolerant and susceptible lines of maize, Zea mays L. (Poaceae), attacked by the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), in the laboratory and the field. Infested tolerant maize plants produced more root biomass and even overcompensated for the lost roots, whereas this effect was absent in susceptible lines. Furthermore, the tolerant plants slowed growth of new roots in the greenhouse and in the field 4–8 days after infestation, whereas susceptible plants slowed growth of new roots only in the field and only after 12 days of infestation. The quick response of tolerant lines may have enabled them to escape root attack by starving the herbivores and by saving resources for regrowth after the attack had ceased. We conclude that both timing and the extent of regrowth may determine plant tolerance to root herbivory.