Leaf profile of maize resistance factors to European corn borer,Ostrinia nubilalis

Metabolomic fingerprint of cabbage resistance to Mamestra brassicae L. (Lepidoptera: Noctuidae)

BACKGROUND

Plants defend themselves from insect feeding by activating specific metabolic pathways. We performed a metabolomic analysis to compare the metabolome reorganization that occurs in the leaves of two genotypes of cabbage (one partially resistant and one susceptible) when attacked by Mamestra brassicae caterpillars.

RESULTS

The comparison of the metabolomic reorganization of both genotypes allowed us to identify 43 metabolites that are specifically associated with the insect feeding response in the resistant genotype. Of these, 19% are lipids or lipid-related compounds, most of which are modified fatty acids. These include glycosylated, glycerol-binding and oxidized fatty acids, the majority being associated with the oxylipin pathway. Some of the identified lipids are unlikely to be produced by plants and may be the result of biochemical reactions in the caterpillar oral secretions. A further 16% are phenylpropanoids. Interestingly, some phenylpropanoids were not present in the susceptible genotype, making them possible candidates for specific resistance-related compounds.

CONCLUSION

Our results suggest that glucosinolates do not have a clear role in the resistance to M. brassicae feeding on cabbage. Using an untargeted metabolomics approach, we associated the regulation of metabolic pathways related to lipid signalling and phenylpropanoid compounds with the resistance to this pest. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Morphoanatomical and biochemical factors associated with rice resistance to the South American rice water weevil, Oryzophagus oryzae (Coleoptera: Curculionidae)

The rice water weevil, Oryzophagus oryzae (Coleoptera: Curculionidae), is an economically important pest of flooded rice paddies throughout South America, and species with similar life histories are present in many rice-producing regions globally (collectively referred to here as RWWs). Plant resistance is a key strategy for management of RWWs; however, the mechanisms responsible for rice resistance to RWWs are poorly understood. We investigated morphoanatomical and biochemical plant traits potentially involved in rice resistance to O. oryzae. Resistance-associated traits were characterized in two cultivars, 'Dawn' (resistant) and 'BRS Pampa CL' ('Pamp' = susceptible), which were selected from among six cultivars on 2-year field screenings. Anatomical and morphological traits of leaf tissues from 'Pamp' and 'Dawn' were similar, which perhaps explains the lack of antixenosis during host plant selection. However, significant antibiosis effects were found. The activities of antioxidant enzymes involved in plant defense, as well the content of hydroxycinnamic and hydroxybenzoic acids derivatives and lignin, were higher in roots of 'Dawn' than in 'Pamp', over the period of larval infestation in the field. Additionally, 'Dawn' exhibited a root sclerenchyma arranged in three layers of lignified cells, which differed from the arrangement of cells in 'Pamp', regardless of larval infestation. Our results provide the first evidence for specific resistance-related traits associated with mortality and malnutrition of RWWs in rice.

Elucidating compositional factors of maize cell walls contributing to stalk strength and lodging resistance

Lodging is one of the causes of maize (Zea mays L.) production losses worldwide and, at least, the resistance to stalk lodging has been positively correlated with stalk strength. In order to elucidate the putative relationship between cell wall, stalk strength and lodging resistance, twelve maize inbreds varying in rind penetration strength and lodging resistance were characterized for cell wall composition and structure. Stepwise multiple regression indicates that H lignin subunits confer a greater rind penetration strength. Besides, the predictive model for lodging showed that a high ferulic acid content increases the resistance to lodging, whereas those of diferulates decrease it. These outcomes highlight that the strength and lodging susceptibility of maize stems may be conditioned by structural features of cell wall rather than by the net amount of cellulose, hemicelluloses and lignin. The results presented here provide biotechnological targets in breeding programs aimed at improving lodging in maize.

QTL Mapping of Traits Associated with Dual Resistance to the African Stem Borer (Busseola fusca) and Spotted Stem Borer (Chilo partellus) in Sorghum (Sorghum bicolor)

Sorghum (Sorghum bicolor (L.) Moench) is an important food crop in semi-arid tropics. The crop grain yield ranges from 0.5 t/ha to 0.8 t/ha compared to potential yields of 10 t/ha. The African stem borer Busseola fusca Fuller (Noctuidae) and the spotted stem borer Chilo partellus Swinhoe (Crambidae), are among the most economically important insect pests of sorghum. The two borers can cause 15% - 80% grain yield loss in sorghum. Mapping of QTLs associated with resistance traits to the two stem borers is important towards marker-assisted breeding. The objective of this study was to map QTLs associated with resistance traits to B. fusca and C. partellus in sorghum. 243 F9:10 sorghum RILs derived from ICSV 745 (S) and PB 15520-1 (R) were selected for the study with 4,955 SNP markers. The RILs were evaluated in three sites. Data was collected on leaf feeding, deadheart, exit holes, stem tunnels, leaf toughness, seedling vigour, bloom waxiness, and leaf glossiness. ANOVA for all the traits was done using Genstat statistical software. Insect damage traits and morphological traits were correlated using Pearson's correlation coefficients. Genetic mapping was done using JoinMap 4 software, while QTL analysis was done using PLABQTL software. A likelihood odds ratio (LOD) score of 3.0 was used to declare linkage. Joint analyses across borer species and sites revealed 4 QTLs controlling deadheart formation; 6 controlling leaf feeding damage; 5 controlling exit holes and stem tunneling damages; 2 controlling bloom waxiness, leaf glossiness, and seedling vigour; 4 conditioning trichome density; and 6 conditioning leaf toughness. Joint analyses for B. fusca and C. partellus further revealed that marker CS132-2 colocalised for leaf toughness and stem tunneling traits on QTLs 1 and 2, respectively; thus, the two traits can be improved using the same linked marker. This study recommended further studies to identify gene(s) underlying the mapped QTLs.

Assortments of Digestive Enzymes Induced in First Instar Larvae of Busseola fusca Feeding on Different Plants

The stem borer Busseola fusca (Fuller) (Lepidoptera: Noctuidae) is an important pest of maize and sorghum in sub-Saharan Africa. This insect has oligophagous feeding habits, feeding mostly on maize and sorghum with a narrow range of wild Poaceous plant species. We hypothesised that first instar B. fusca larvae, the critical stage for successful establishment on a host plant, can establish and then grow on a particular plant as a result of induction of a complement of digestive enzymes that mediates host acceptance at first instars. A fast semi-quantitative analysis of potentially digestive enzymatic activities present in the first larvae previously fed for 4 days on leaves of host and non-host plants was performed using the API-ZYM kit system able to detect a multiplex of enzyme activities. Regardless of the plant species, the larvae exhibited higher activities of the carbohydrate metabolising enzymes than of aminopeptidases and proteases. In addition, highest activities of carbohydrates degrading enzymes were exhibited by larvae that consumed leaves of the most preferred plant species of B. fusca. Conversely, esterases were only detected in neonate larvae that consumed leaves of the less preferred and non-host plants. No alkaline phosphatase and lipase activities were detected. The significance of these results was discussed in terms of food requirements of first instar larvae when settling on a plant.

Plant defense and herbivore counter-defense: benzoxazinoids and insect herbivores

Benzoxazinoids are a class of indole-derived plant chemical defenses comprising compounds with a 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one skeleton and their derivatives. These phytochemicals are widespread in grasses, including important cereal crops such as maize, wheat and rye, as well as a few dicot species, and display a wide range of antifeedant, insecticidal, antimicrobial, and allelopathic activities. Although their overall effects against insect herbivores are frequently reported, much less is known about how their modes of action specifically influence insect physiology. The present review summarizes the biological activities of benzoxazinoids on chewing, piercing-sucking, and root insect herbivores. We show how within-plant distribution modulates the exposure of different herbivore feeding guilds to these defenses, and how benzoxazinoids may act as toxins, feeding deterrents and digestibility-reducing compounds under different conditions. In addition, recent results on the metabolism of benzoxazinoids by insects and their consequences for plant-herbivore interactions are addressed, as well as directions for future research.

Plant defense and herbivore counter-defense: benzoxazinoids and insect herbivores

Benzoxazinoids are a class of indole-derived plant chemical defenses comprising compounds with a 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one skeleton and their derivatives. These phytochemicals are widespread in grasses, including important cereal crops such as maize, wheat and rye, as well as a few dicot species, and display a wide range of antifeedant, insecticidal, antimicrobial, and allelopathic activities. Although their overall effects against insect herbivores are frequently reported, much less is known about how their modes of action specifically influence insect physiology. The present review summarizes the biological activities of benzoxazinoids on chewing, piercing-sucking, and root insect herbivores. We show how within-plant distribution modulates the exposure of different herbivore feeding guilds to these defenses, and how benzoxazinoids may act as toxins, feeding deterrents and digestibility-reducing compounds under different conditions. In addition, recent results on the metabolism of benzoxazinoids by insects and their consequences for plant-herbivore interactions are addressed, as well as directions for future research.

Covalent cross-linking of cell-wall polysaccharides through esterified diferulates as a maize resistance mechanism against corn borers

There is strong evidence to suggest that cross-linking of cell-wall polymers through ester-linked diferulates has a key role in plant resistance to pests; however, direct experimentation to provide conclusive proof is lacking. This study presents an evaluation of the damage caused by two corn borer species on six maize populations particularly selected for divergent diferulate concentrations in pith stem tissues. Maize populations selected for high total diferulate concentration had 31% higher diferulates than those selected for low diferulates. Stem tunneling by corn borer species was 29% greater in the population with the lowest diferulates than in the population with the highest diferulates (31.7 versus 22.6 cm), whereas total diferulate concentration was negatively correlated with stem tunneling by corn borers. Moreover, orthogonal contrasts between groups of populations evaluated showed that larvae fed in laboratory bioassays on pith stem tissues from maize populations with higher diferulates had 30-40% lower weight than larvae fed on the same tissues from maize populations with lower diferulates. This is the first report that shows a direct relationship between diferulate deposition in maize cell walls and corn borer resistance. Current findings will help to develop adapted maize varieties with an acceptable level of resistance against borers and be useful in special kinds of agriculture, such as organic farming.

Differential performance and parasitism of caterpillars on maize inbred lines with distinctly different herbivore-induced volatile emissions

Plant volatiles induced by insect feeding are known to attract natural enemies of the herbivores. Six maize inbred lines that showed distinctly different patterns of volatile emission in laboratory assays were planted in randomized plots in the Central Mexican Highlands to test their ability to recruit parasitic wasps under field conditions. The plants were artificially infested with neonate larvae of the fall armyworm Spodoptera frugiperda, and two of its main endoparasitoids, Campoletis sonorensis and Cotesia marginiventris, were released in the plots. Volatiles were collected from equally treated reference plants in the neighbourhood of the experimental field. The cumulative amount of 36 quantified volatile compounds determined for each line was in good accordance with findings from the laboratory; there was an almost 15-fold difference in total emission between the two extreme lines. We found significant differences among the lines with respect to the numbers of armyworms recovered from the plants, their average weight gain and parasitism rates. Average weight of the caterpillars was negatively correlated with the average total amount of volatiles released by the six inbred lines. However, neither total volatile emission nor any specific single compound within the blend could explain the differential parasitism rates among the lines, with the possible exception of (E)-2-hexenal for Campoletis sonorensis and methyl salicylate for Cotesia marginiventris. Herbivore-induced plant volatiles and/or correlates thereof contribute to reducing insect damage of maize plants through direct plant defence and enhanced attraction of parasitoids, alleged indirect defence. The potential to exploit these volatiles for pest control deserves to be further evaluated.

QTL mapping for Mediterranean corn borer resistance in European flint germplasm using recombinant inbred lines

BACKGROUND

Ostrinia nubilalis (ECB) and Sesamia nonagrioides (MCB) are two maize stem borers which cause important losses in temperate maize production, but QTL analyses for corn borer resistance were mostly restricted to ECB resistance and maize materials genetically related (mapping populations derived from B73). Therefore, the objective of this work was to identify and characterize QTLs for MCB resistance and agronomic traits in a RILs population derived from European flint inbreds.

RESULTS

Three QTLs were detected for stalk tunnel length at bins 1.02, 3.05 and 8.05 which explained 7.5% of the RILs genotypic variance. The QTL at bin 3.05 was co-located to a QTL related to plant height and grain humidity and the QTL at bin 8.05 was located near a QTL related to yield.

CONCLUSIONS

Our results, when compared with results from other authors, suggest the presence of genes involved in cell wall biosynthesis or fortification with effects on resistance to different corn borer species and digestibility for dairy cattle. Particularly, we proposed five candidate genes related to cell wall characteristics which could explain the QTL for stalk tunnelling in the region 3.05. However, the small proportion of genotypic variance explained by the QTLs suggest that there are also many other genes of small effect regulating MCB resistance and we conclude that MAS seems not promising for this trait. Two QTLs detected for stalk tunnelling overlap with QTLs for agronomic traits, indicating the presence of pleitropism or linkage between genes affecting resistance and agronomic traits.

Gross vs. net income: How plant toughness affects performance of an insect herbivore

Leaf biomechanical properties are thought to impose a significant obstacle to herbivores and as such influence patterns of herbivory more than leaf chemistry. However, evidence for the role of structural traits in influencing herbivore food choice and performance has come from correlative studies, whereas the underlying mechanisms have been given little attention. By manipulating the biomechanical properties of a host grass species through a combination of lyophilization and milling, and providing water separately, we were able to compare behavioral, physiological, and developmental responses of the Australian plague locust, Chortoicetes terminifera, to the biomechanical properties of plant food (exemplified by toughness) independently of the food's macronutrient content and the insect's demand for water. Increasing leaf toughness was associated with reduced rates of locust growth and prolonged development, with potential ecological consequences. Poorer performance on the tougher foods was primarily a consequence of a reduced rate of nutrient supply, which occurred as a result of (1) smaller meals being eaten more slowly, (2) slowed gut passage rates, which limited how quickly the next meal could be taken, and (3) reduced efficiency of assimilation of nutrients from food in the gut. In addition, there were deleterious changes in the ratio of protein to carbohydrate assimilated from the gut. Prolonged development time was associated with increased total nutrient demands throughout the extended developmental period. Because these demands could not be met by increased consumption, there was a decreased efficiency of conversion of assimilated nutrients to growth. By disentangling the effects of biomechanical properties from macronutrient and water content we have shown that leaf biomechanical traits can influence chewing herbivores independently of leaf chemical traits.

Role of dehydrodiferulates in maize resistance to pests and diseases

Phenolic esters have attracted considerable interest due to the potential they offer for peroxidase catalysed cross-linking of cell wall polysaccharides. Particularly, feruloyl residues undergo radical coupling reactions that result in cross-linking (intra-/intermolecular) between polysaccharides, between polysaccharides and lignin and, between polysaccharides and proteins. This review addresses for the first time different studies in which it is established that cross-linking by dehydrodiferulates contributes to maize's defences to pests and diseases. Dehydrodiferulate cross-links are involved in maize defence mechanisms against insects such as the European, Mediterranean, and tropical corn borers and, storage pest as the maize weevil. In addition, cross-links are also discussed to be involved in genetic resistance of maize to fungus diseases as Gibberella ear and stalk rot. Resistance against insects and fungus attending dehydrodiferulates could go hand in hand. Quantitative trait loci mapping for these cell wall components could be a useful tool for enhancing resistance to pest and diseases in future breeding programs.

Hydroxamic acids derived from 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one: key defense chemicals of cereals

Many cereals accumulate hydroxamic acids derived from 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one. These benzoxazinoid hydroxamic acids are involved in defense of maize against various lepidopteran pests, most notably the European corn borer, in defense of cereals against various aphid species, and in allelopathy affecting the growth of weeds associated with rye and wheat crops. The role of benzoxazinoid hydroxamic acids in defense against fungal infection is less clear and seems to depend on the nature of the interactions at the plant-fungus interface. Efficient use of benzoxazinoid hydroxamic acids as resistance factors has been limited by the inability to selectively increase their levels at the plant growth stage and the plant tissues where they are mostly needed for a given pest. Although the biosynthesis of benzoxazinoid hydroxamic acids has been elucidated, the genes and mechanisms controlling their differential expression in different plant tissues and along plant ontogeny remain to be unraveled.

Changes in phenolic concentrations during recurrent selection for resistance to the Mediterranean corn borer (Sesamia nonagrioides Lef.)

Recurrent selection has been reported as successful for improving maize resistance against corn borers. This study was conducted to determine if phenolics concentration in maize changes during recurrent selection to improve stalk resistance to the Mediterranean corn borer. Three cycles of selection [EPS12(S)C0, ESP12(S)C2, and EPS12(S)C3] from the maize synthetic population EPS12 and test crosses to inbred lines A639, B93, and EP42 were field grown and artificially infested with Mediterranean corn borer larvae, and the pith tissues were sampled for biochemical analyses. Two major simple phenolic acids [p-coumaric (p-CA) and trans-ferulic (E-FA) acids] were identified in free and cell-wall fractions, whereas four isomers of diferulic acid (DFA) (8-5'l, 5-5', 8-o-4', and 8-5' benzofuran form) were present in the cell-wall bound fraction. The selection cycles EPS12(S)C0 and EPS12(S)C3 showed less damage and higher cell wall phenolics concentrations than the cycle EPS12(S)C2. In addition, higher concentrations of total DFAs were associated with shorter tunnel length and lower numbers of larvae per stem. The current study shows new and concrete evidence that the cell-wall bound phenolics could have a determinative role in the resistance to the Mediterranean corn borer, although future development of recurrent and divergent selection cycles will clarify this point.

Diferulate content of maize sheaths is associated with resistance to the Mediterranean corn borer Sesamia nonagrioides (Lepidoptera: Noctuidae)

The leaf sheaths of selected inbred lines of maize (Zea mays L.) with variable levels of stem resistance to the Mediterranean corn borer Sesamia nonagrioides (Lefèvbre) were evaluated for antibiotic effect on insect development. Phytochemical analyses of leaf sheaths were conducted for cell wall phenylpropanoid content to gain a better understanding of maize-resistance mechanisms. Laboratory bioassays established that sheath tissues from different genotypes significantly affected the growth of neonate larvae. Three hydroxycinnamates, p-coumaric, trans-ferulic, and cis-ferulic acids, and three isomers of diferulic acid, 8-5', 8-O-4', and 8-5' b (benzofuran form), were identified. Significant negative correlations were found between larvae weight and diferulic acid content for six genotypes. These results are in agreement with previous studies concerning the role of cell wall structural components in stem borer resistance.

Examination of the biological effects of high anionic peroxidase production in tobacco plants grown under field conditions. I. Insect pest damage

At least 25 wild type and high peroxidase tobacco Nicotiana tabacum L. plants were examined semiweekly over several weeks for pest insect distribution and damage in a 2 year field study. Incidence and/or severity of naturally occurring caterpillar damage (dingy cutworm (Feltia ducens Walker), black cutworm (Agrotis ipsilon (Hufnagel), tobacco hornworm (Manduca sexta L.), and false tobacco budworm (= corn earworm Helicoverpa zea (Boddie)) was significantly reduced at several sample dates for high peroxidase vs. wild type plants. These results parallel those of prior laboratory studies with caterpillars. The number of adult whiteflies (Trialeurodes vaporariorum (Westwood) per plant was significantly reduced on high peroxidase compared to wild type plants on most sample dates in both years. The number of plants with leaves containing >100 aphids (primarily Myzus persicae Sulzer) per leaf on high peroxidase plants was significantly lower that on wild type plants after an equivalent invasion period in both years. A significantly higher proportion of aphids were found dead on leaf five of high peroxidase compared to wild type plants at most sample dates in both years. These results indicate that high peroxidase plants have resistance to a wide range of insects, implicating this enzyme as a broad range resistance mechanism.

Putative role of pith cell wall phenylpropanoids in Sesamia nonagrioides (Lepidoptera: Noctuidae) resistance

The stem borer Sesamia nonagrioides (Lefèbvre) is the most important insect pest that attacks maize, Zea mays L., in northwestern Spain. Host plant resistance to this borer was investigated in relation to the cell wall phenylpropanoids content in the pith. Eight inbred lines that differ in resistance were analyzed. Three major simple phenolic acids, p-coumaric, trans-ferulic, and cis-ferulic acids, and three isomers of diferulic acid, 8-5', 8-O-4', and 8-5'b (benzofuran form), were identified. The amount of all these compounds was correlated with the resistance level in the genotypes, with the resistant inbreds having the highest concentrations. The role of these compounds in cell wall fortification and lignification is well-documented, suggesting their possible intervention in S. nonagrioides resistance. Future studies that focus on these compounds could be useful to enhance S. nonagroides resistance.

Effect of transgenic plants expressing high levels of a tobacco anionic peroxidase on the toxicity of Anagrapha falcifera nucleopolyhedrovirus to Helicoverpa zea (Lepidoptera: Noctuidae)

Wild type and corresponding transgenic tomato (Lycopersicon esculentum Miller) and two tobacco (Nicotiana spp.) plants that express high levels of a tobacco anionic peroxidase were used to determine what type of interactions occurred between peroxidase altered plant chemistry and the baculovirus Anagrapha falcifera nucleopolyhedrovirus (AfMNPV) for control of neonate corn earworms, Helicoverpa zea (Boddie). Transgenic plants expressed approximately five to 400 times higher peroxidase activity than corresponding tissues of wild type plants. The H. zea larvae typically fed 1.5 times less on transgenic compared with wild type leaf disks. There was only one experiment (of three with tomato leaves) where the larvae that fed on transgenic leaves were less susceptible to the virus based on nonoverlapping 95% confidence intervals for LC50 values. When the exposure dose was corrected for reduced feeding on the transgenic leaf disks, the insecticidal activity of the virus was not significantly different for larvae fed on transgenic versus wild type plants. Eight other experiments (with tomato and two species of tobacco) indicated either no significant effect or enhanced susceptibility (when corrected for feeding rates) to the virus of larvae fed on the transgenic leaves. These results indicate enhanced insect resistance in plants expressing high levels of a specific anionic peroxidase may be compatible with applications of AfMNPV. Potential reasons for this compatibility are discussed.