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

Leaf Size Determines Damage- and Herbivore-Induced Volatile Emissions in Maize

Stress-induced plant volatiles play an important role in mediating ecological interactions between plants and their environment. The timing and location of the inflicted damage is known to influence the quality and quantity of induced volatile emissions. However, how leaf characteristics and herbivore feeding behaviour interact to shape volatile emissions is not well understood. Using a high-throughput volatile profiling system with high temporal resolution, we examined how mechanical damage and herbivore feeding on different leaves shape plant-level volatile emission patterns in maize. We then tested feeding patterns and resulting consequences on volatile emissions with two generalist herbivores (Spodoptera exigua and Spodoptera littoralis), and assessed whether feeding preferences are associated with enhanced herbivore performance. We found maize seedlings emit more volatiles when larger leaves are damaged. Larger leaves emitted more volatiles locally, which was the determining factor for higher plant-level emissions. Surprisingly, both S. exigua and S. littoralis preferentially consumed larger leaves, and thus maximize plant volatile emission without apparent growth benefits. Together, these findings provide an ecophysiological and behavioural mechanism for plant volatile emission patterns, with potentially important implications for volatile-mediated plant-environment interactions.

Genetic Warfare: The Plant Genome's Role in Fending Off Insect Invaders

The plant defense against insects is multiple layers of interactions. They defend through direct defense and indirect defense. Direct defenses include both physical and chemical barriers that hinder insect growth, development, and reproduction. In contrast, indirect defenses do not affect insects directly but instead suppress them by releasing volatile compounds that attract the natural enemies of herbivores. Insects overcome plant defenses by deactivating biochemical defenses, suppressing defense signaling through effectors, and altering their behavior through chemical regulation. There is always a genetic war between plants and insects. In this genetic war, plant-insect co-evolution act as both weapons and messengers. Because plants always look for new strategies to avoid insects by developing adaptation. There are molecular processes that regulate the interaction between plants and insect. Here, we examine the genes and proteins involved in plant-insect interactions and explore how their discovery has shaped the current model of the plant genome's role. Plants detect damage-associated and herbivore-associated molecular patterns through receptors, which trigger early signaling pathways involving Ca2+, reactive oxygen species, and MAP kinases. The specific defense mechanisms are activated through gene signaling pathways, including phytohormones, secondary metabolites, and transcription factors. Expanding plant genome approaches to unexplored dimensions in fending off insects should be a future priority in order to develop management strategies.

Genotypic Variation and Potential Mechanisms of Resistance against Multiple Insect Herbivores in Cranberries

Plant genotypes often exhibit varying resistance levels to herbivores. However, the impact of this genotypic variation on resistance against multiple herbivores remains poorly understood, especially in crops undergoing recent process of domestication. To address this gap, we studied the magnitude and mechanism of resistance in 12 cranberry (Vaccinium macrocarpon) genotypes to three leaf-chewing herbivores - Sparganothis fruitworm (Sparganothis sulfureana), spotted fireworm (Choristoneura parallela), and spongy moth (Lymantria dispar) - along a domestication gradient (native 'wild' genotypes, 'early hybrid' genotypes, and 'modern hybrid' genotypes). Like cranberries, S. sulfureana and C. parallela are native to the United Sates, while L. dispar is an invasive pest. We measured the survival and growth of larvae on each genotype, as well as variation in plant performance (height and biomass) and leaf defensive chemical traits (C/N ratio, total phenolics, total proanthocyanidins, and flavonols levels) in these genotypes to elucidate potential resistance mechanisms. We found differences in C. parallela and L. dispar larval performance across genotypes, with larvae performing better on the modern hybrid genotypes, while S. sulfureana showed no differences. Morphological and chemical traits varied among genotypes, with total phenolics being the only trait correlated with C. parallela and L. dispar larval performance. Notably, the wild genotypes 'McFarlin' and 'Potter' had higher total phenolics and were more resistant to both herbivores than the modern hybrids 'Demoranville' and 'Mullica Queen.' This research contributes to a comprehensive understanding of the impact of crop domestication on multiple insect herbivores, offering insights for future breeding efforts to enhance host-plant resistance against agricultural pests.

Integrated IBD Analysis, GWAS Analysis and Transcriptome Analysis to Identify the Candidate Genes for White Spot Disease in Maize

Foundation parents (FPs) play an irreplaceable role in maize breeding practices. Maize white spot (MWS) is an important disease in Southwest China that always seriously reduces production. However, knowledge about the genetic mechanism of MWS resistance is limited. In this paper, a panel of 143 elite lines were collected and genotyped by using the MaizeSNP50 chip with approximately 60,000 single nucleotide polymorphisms (SNPs) and evaluated for resistance to MWS among 3 environments, and a genome-wide association study (GWAS) and transcriptome analysis were integrated to reveal the function of the identity-by-descent (IBD) segments for MWS. The results showed that (1) 225 IBD segments were identified only in the FP QB512, 192 were found only in the FP QR273 and 197 were found only in the FP HCL645. (2) The GWAS results showed that 15 common quantitative trait nucleotides (QTNs) were associated with MWS. Interestingly, SYN10137 and PZA00131.14 were in the IBD segments of QB512, and the SYN10137-PZA00131.14 region existed in more than 58% of QR273's descendants. (3) By integrating the GWAS and transcriptome analysis, Zm00001d031875 was found to located in the region of SYN10137-PZA00131.14. These results provide some new insights for the detection of MWS's genetic variation mechanisms.

The effects of plant domestication on the foraging and performance of parasitoids

Domestication-related changes in the chemical traits of crop plants affect parasitoid foraging success, development, and survival. For example, herbivore-induced changes in the production of volatiles by domesticated plants can enhance or reduce parasitoid attraction. While the trade-off between nutrient content and chemical defense in cultivated plants can increase the suitability of hosts for parasitoids, their increased health and size can positively affect their immune response against parasitoids. Overall, plant domestication is expected to significantly affect their relationship with parasitoids due to altered plant morphology, physical characteristics, chemical defenses, and new plant associations. This review highlights the need for research on the effects of plant domestication on host-parasitoid interactions in the interest of better controlling insect pests.

Herbivory Protection via Volatile Organic Compounds Is Influenced by Maize Genotype, Not Bacillus altitudinis-Enriched Bacterial Communities

Belowground, plants interact with beneficial soil microbes such as plant growth-promoting rhizobacteria (PGPR). PGPR are rhizosphere bacteria that colonize roots and elicit beneficial effects in plants such as improved plant growth, pathogen resistance, abiotic stress tolerance, and herbivore protection. Treatment of plants with PGPR has been shown to trigger the emission of volatile organic compounds (VOCs). Volatile emissions can also be triggered by herbivory, termed herbivore-induced plant volatiles (HIPV), with important ramifications for chemical-mediated plant and insect interactions. Much of our current understanding of PGPR and herbivore-induced volatiles is based on studies using one plant genotype, yet domestication and modern breeding has led to the development of diverse germplasm with altered phenotypes and chemistry. In this study, we investigated if volatile emissions triggered by PGPR colonization and herbivory varies by maize genotype and microbial community assemblages. Six maize genotypes representing three decades of crop breeding and two heterotic groups were used, with four microbiome treatments: live or sterilized soil, with or without a Bacillus inoculant. Soil sterilization was used to delay microbiome establishment, resulting in low-diversity treatments. At planting, maize seeds were inoculated with PGPR Bacillus altitudinis AP-283 and grown under greenhouse conditions. Four weeks post planting, plants were subjected to feeding by third instar Helicoverpa zea (Lepidoptera: Noctuidae) larvae. Volatiles were collected using solid phase microextraction and analyzed with gas chromatography-mass spectrometry. Illumina NovaSeq 16S rRNA amplicon sequencing was carried out to characterize the rhizosphere microbiome. Maize genotype significantly influenced total volatile emissions, and relative abundance of volatile classes. We did not document a strong influence of microbe treatment on plant VOC emissions. However, inoculating plants with PGPR improved plant growth under sterile conditions. Taken together, our results suggest that genotypic variation is the dominant driver in HIPV composition and individual HIPV abundances, and any bacterial-mediated benefit is genotype and HIPV-specific. Therefore, understanding the interplay of these factors is necessary to fully harness microbially-mediated benefits and improve agricultural sustainability.

Ecology and Evolutionary History of Diabrotica Beetles—Overview and Update

An overview is given on several aspects of evolutionary history, ecology, host plant use, and pharmacophagy of Diabrotica spp. with a focus on the evolution of host plant breadth and effects of plant compounds on natural enemies used for biocontrol of pest species in the group. Recent studies on each aspect are discussed, latest publications on taxonomic grouping of Diabrotica spp., and new findings on variations in the susceptibility of corn varieties to root feeding beetle larvae are presented. The further need for in-depth research on biology and ecology of the large number of non-pest species in the genus is pointed out.

How Does the Application of Beauveria bassiana and Compost on Corn Crops Affect the Survival and Genetic Diversity of Phyllophaga obsoleta (Coleoptera: Melolonthinae)?

The knowledge of the diversity and genetic structure of pest insects under management contributes to the improvement of control strategies. An experiment was run to investigate whether the addition of the fungus Beauveria bassiana (Balsamo) Vuillemin (Hypocreales: Cordycipitaceae) (BB) and compost to soil affects the presence and genetic diversity of adults and larvae of Phyllophaga obsoleta Blanch (Coleoptera: Melolonthinae) larvae in maize crops. We collected adults in and used mating pairs under four treatments (BB, compost, soil, blank). Genetic diversity and structure were determined through five allo/iso-enzymatic loci. Beauveria bassiana affected the presence and mortality of P. obsoleta in the laboratory but not under field conditions. The genetic diversity of P. obsoleta ranged from moderate to high (Ho = 0.26-0.31), with a low genetic differentiation among localities or treatments (Phi < 0.05), indicating high levels of gene flow. Our results showed a weak effect of B. bassiana on P. obsoleta in the field. Still, our laboratory observations suggest that the fungus may be a suitable alternative for biological control.

Determining relevant traits for selecting landrace accessions of Phaseolus lunatus L. for insect resistance

Plant-insect interactions are a determining factor for sustainable crop production. Although plants can resist or tolerate herbivorous insects to varying degrees, even with the use of pesticides, insects can reduce plant net productivity by as much as 20%, so sustainable strategies for pest control with less dependence on chemicals are needed. Selecting plants with optimal resistance and photosynthetic traits can help minimize damage and maintain productivity. Here, 27 landrace accessions of lima beans, Phaseolus lunatus L., from the Yucatan Peninsula were evaluated in the field for morphological resistance traits, photosynthetic characteristics, insect damage and seed yield. Variation was found in physical leaf traits (number, area, and dry mass of leaves; trichome density, specific leaf thickness and hardness) and in physiological traits (photosynthetic rate, stomatal conductance, intercellular carbon, water-use efficiency, and transpiration). Five accessions (JMC1325, JMC1288, JMC1339, JMC1208 and JMC1264) had the lowest index for cumulative damage with the highest seed yield, although RDA analysis uncovered two accessions (JMC1339, JMC1288) with strong positive association of seed yield and the cumulative damage index with leaf production, specific leaf area (SLA) and total leaf area. Leaf traits, including SLA and total leaf area are important drivers for optimizing seed yield. This study identified 12 important morphological and physiological leaf traits for selecting landrace accessions of P. lunatus for high yields (regardless of damage level) to achieve sustainable, environmentally safe crop production.

Maize biochemistry in response to root herbivory was mediated by domestication, spread, and breeding

With domestication, northward spread, and breeding, maize defence against root-herbivores relied on induced defences, decreasing levels of phytohormones involved in resistance, and increasing levels of a phytohormone involved in tolerance. We addressed whether a suite of maize (Zea mays mays) phytohormones and metabolites involved in herbivore defence were mediated by three successive processes: domestication, spread to North America, and modern breeding. With those processes, and following theoretical predictions, we expected to find: a change in defence strategy from reliance on induced defences to reliance on constitutive defences; decreasing levels of phytohormones involved in herbivore resistance, and; increasing levels of a phytohormone involved in herbivore tolerance. We tested those predictions by comparing phytohormone levels in seedlings exposed to root herbivory by Diabrotica virgifera virgifera among four plant types encompassing those processes: the maize ancestor Balsas teosinte (Zea mays parviglumis), Mexican maize landraces, USA maize landraces, and USA inbred maize cultivars. With domestication, maize transitioned from reliance on induced defences in teosinte to reliance on constitutive defences in maize, as predicted. One subset of metabolites putatively involved in herbivory defence (13-oxylipins) was suppressed with domestication, as predicted, though another was enhanced (9-oxylipins), and both were variably affected by spread and breeding. A phytohormone (indole-3-acetic acid) involved in tolerance was enhanced with domestication, and with spread and breeding, as predicted. These changes are consistent with documented changes in herbivory resistance and tolerance, and occurred coincidentally with cultivation in increasingly resource-rich environments, i.e., from wild to highly enriched agricultural environments. We concluded that herbivore defence evolution in crops may be mediated by processes spanning thousands of generations, e.g., domestication and spread, as well as by processes spanning tens of generations, e.g., breeding and agricultural intensification.

Variation in production of cyanogenic glucosides during early plant development: A comparison of wild and domesticated sorghum

Domestication has narrowed the genetic diversity found in crop wild relatives, potentially reducing plasticity to cope with a changing climate. The tissues of domesticated sorghum (Sorghum bicolor), especially in younger plants, are cyanogenic and potentially toxic. Species of wild sorghum produce lower levels of the cyanogenic glucoside (CNglc) dhurrin than S. bicolor at maturity, but it is not known if this is also the case during germination and early growth. CNglcs play multiple roles in primary and specialised metabolism in domesticated sorghum and other crop plants. In this study, the temporal and spatial distribution of dhurrin in wild and domesticated sorghum at different growth stages was monitored in leaf, sheath and root tissues up to 35 days post germination using S. bicolor and the wild species S. brachypodum and S. macrospermum as the experimental systems. Growth parameters were also measured and allocation of plant total nitrogen (N%) to both dhurrin and nitrate (NO₃^-) was calculated. Negligible amounts of dhurrin were produced in the leaves of the two wild species compared to S. bicolor. The morphology of the two wild sorghums also differed from S. bicolor, with the greatest differences observed for the more distantly related S. brachypodum. S. bicolor had the highest leaf N% whilst the wild species had significantly higher root N%. Allocation of nitrogen to dhurrin in aboveground tissue was significantly higher in S. bicolor compared to the wild species but did not differ in the roots across the three species. The differences in plant morphology, dhurrin content and re-mobilisation, and nitrate/nitrogen allocation suggest that domestication has affected the functional roles of dhurrin in sorghum.

The Effects of Ozone on Herbivore-Induced Volatile Emissions of Cultivated and Wild Brassica Rapa

Since preindustrial times, concentrations of tropospheric ozone, a phytotoxic pollutant, have risen in the Northern Hemisphere. Selective breeding has intentionally modified crop plant traits to improve yield but may have altered plant defenses against abiotic and biotic stresses. This study aims to determine if cultivated and wild plants respond differently to herbivory under elevated ozone. We studied the volatile emissions of four cultivated Brassica rapa ssp. oleifera varieties and one wild population after exposure to ozone or Plutella xylostella larval feeding either individually or together. Ozone modulated the volatiles emitted in response to herbivory by all plant varieties to different extents. We did not observe a clear difference in the effects of ozone on wild and cultivated plants, but cultivated plants had higher volatile emission rates in response to herbivory and ozone had either no effect or increased the herbivore-induced response. Larvae tended to feed more on elevated ozone-treated plants; however, we could not link the increase of feeding to the change in volatile emissions. Our study complements recent studies reporting that selective breeding might not have weakened chemical defenses to biotic and abiotic stresses of cultivated plants.

Does ozone exposure affect herbivore-induced plant volatile emissions differently in wild and cultivated plants?

Concentrations of tropospheric ozone have more than doubled in the Northern Hemisphere since pre-industrial times. Plant responses to single abiotic or biotic stresses, such as ozone exposure and herbivore-feeding, have received substantial attention, especially for cultivated plants. Modern cultivated plants have been subjected to selective breeding that has altered plant chemical defences. To understand how ozone might affect plant responses to herbivore-feeding in wild and cultivated plants, we studied the volatile emissions of brassicaceous plants after exposure to ambient (~ 15 ppb) or elevated ozone (80 ppb), with and without Plutella xylostella larvae-feeding. Results indicated that most of the wild and cultivated plants increased volatile emissions in response to herbivore-feeding. Ozone alone had a weaker and less consistent effect on volatile emissions, but appeared to have a greater effect on wild plants than cultivated plants. This study highlights that closely related species of the Brassicaceae have variable responses to ozone and herbivore-feeding stresses and indicates that the effect of ozone may be stronger in wild than cultivated plants. Further studies should investigate the mechanisms by which elevated ozone modulates plant volatile emissions in conjunction with biotic stressors.

Modern Maize Hybrids Have Lost Volatile Bottom-Up and Top-Down Control of Dalbulus maidis, a Specialist Herbivore

Following damage by herbivores, many plants release volatiles that dissuade future conspecifics from feeding. In many crop plants however, induced volatiles mediating this kind of interactions among plants, herbivores and also their natural enemies have been altered through the process of domestication. The selection of crops for increased yield may have gone at a cost of defense, possibly including defense-related volatiles. Dalbulus maidis (Hemiptera: Cicadellidae), a specialist leafhopper that only feeds on Zea spp., is a vector of Corn Stunt Spiroplasma, a serious maize disease. Here, we compared the volatiles released following D. maidis attack by a maize landrace and two maize hybrids of temperate and tropical background. Also, we performed behavioral assays with the leafhopper contrasting healthy non-attacked maize seedlings versus attacked seedlings. The maize landrace produced more than 6-fold larger quantities of induced volatiles compared to the maize hybrids after herbivory. Corn leafhopper females were able to detect and significantly preferred the odors of healthy seedlings over the attacked ones only in the landrace. They did not discriminate between the attacked and non-attacked hybrids. Additionally, we found that the attraction of the parasitoid wasp Anagrus virlai (Hymenoptera: Mymaridae) to its host was diminished in the tested hybrids. The parasitoid was able to detect the odors of the attacked landrace, however it was unable to discriminate between healthy and attacked maize hybrid plants. These results suggest that those more domesticated germplasms may have lost the ability not only to release volatiles that avoid colonization of future herbivores, but also to attract their natural enemies in a tritrophic system.

Fertilizing Corn With Manure Decreases Caterpillar Performance but Increases Slug Damage

Many farmers use manure as an alternative to inorganic fertilizer. Previous research has shown that manure can decrease plant susceptibility to herbivores, but the mechanisms remain unclear. To determine how manure affects herbivore performance in a greenhouse setting, we fertilized corn with stacked cow manure or an equivalent amount of NPK fertilizer and measured caterpillar development, plant nutritional content, and defenses. After 4 wk of growth, we allowed fall armyworm (Spodoptera frugiperda) or black cutworm (Agrotis ipsilon) caterpillars to feed on these plants for 6 d. Compared to inorganic fertilizer, manure reduced mass-gain of black cutworm caterpillars and smaller fall armyworms. We paired this greenhouse experiment with a 3-yr field experiment, which incorporated a wheat cover-crop treatment crossed with the two fertilizer treatments in a 2 × 2 factorial design. We measured plant damage early in the season from naturally occurring herbivores and measured neonate fall armyworm performance on field-collected leaf tissue. In 2017, corn in manure-fertilized plots sustained more herbivore damage, primarily driven by a higher incidence of slug damage. Fall armyworm performance, however, was lower on leaves collected from manure-fertilized plants. In contrast to previous studies, we did not find increased micronutrients or enhanced defenses in manure treated plants. While manure can offer resistance to some herbivores, our results suggest that this resistance can be overshadowed by habitat conditions.

RNA-Seq identification of candidate defense genes by analyzing Mythimna separata feeding-damage induced systemic resistance in balsas teosinte

BACKGROUND

Armyworm (Mythimna separata) is a destructive herbivore for maize. Balsas teosinte (Zea mays ssp. parviglumis), the direct wild ancestor of cultivated maize, has shown great potential to defend against herbivory. Here, based on armyworm bioassay, we compared responses of teosinte and B73 maize inbred during armyworm attack in their transcriptome profiles to elucidate the gene expression changes involved in teosinte responses to armyworm attack. The goal of this study was to identify novel resistance alleles that could serve as valuable resources for modern maize breeding.

RESULTS

Our bioassay revealed that armyworm larvae grew less on teosinte than on maize. A follow-up transcriptomic comparison showed more down-regulated genes in maize B73 and similar numbers of up-regulated genes in both genotypes under armyworm attack. The up-regulated genes in teosinte were markedly enriched in MAPK cascade-mediated signaling pathway and phytohormone pathway. Gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis showed that phytohormones jasmonic acid, ethylene, salicylic acid and abscisic acid (ABA) were actively involved in armyworm resistance of teosinte plants, and so were transcription factors such as MYBs, WRKYs and TIFYs. Interestingly, teosinte also showed high regulation in three ABA receptor PYLs. Based on differential expression analysis, we identified 30 candidate defense-related genes in teosinte, which belong to 11 gene families and the majority of the genes were up-regulated, while some of them were nonresponsive in maize.

CONCLUSION

This study demonstrates that teosinte showed more vigorous defense response than maize toward armyworm attack and might be a beneficial genetic resource to improve pest resistance in cultivated maize. © 2019 Society of Chemical Industry.

Large-scale metabolite quantitative trait locus analysis provides new insights for high-quality maize improvement

It is generally recognized that many favorable genes which were lost during domestication, including those related to both nutritional value and stress resistance, remain hidden in wild relatives. To uncover such genes in teosinte, an ancestor of maize, we conducted metabolite profiling in a BC₂ F₇ population generated from a cross between the maize wild relative (Zea mays ssp. mexicana) and maize inbred line Mo17. In total, 65 primary metabolites were quantified in four tissues (seedling-stage leaf, grouting-stage leaf, young kernel and mature kernel) with clear tissue-specific patterns emerging. Three hundred and fifty quantitative trait loci (QTLs) for these metabolites were obtained, which were distributed unevenly across the genome and included two QTL hotspots. Metabolite concentrations frequently increased in the presence of alleles from the teosinte genome while the opposite was observed for grain yield and shape trait QTLs. Combination of the multi-tissue transcriptome and metabolome data provided considerable insight into the metabolic variations between maize and its wild relatives. This study thus identifies favorable genes hidden in the wild relative which should allow us to balance high yield and quality in future modern crop breeding programs.

Molecular Interactions Between Plants and Insect Herbivores

Diverse molecular processes regulate the interactions between plants and insect herbivores. Here, we review genes and proteins that are involved in plant-herbivore interactions and discuss how their discovery has structured the current standard model of plant-herbivore interactions. Plants perceive damage-associated and, possibly, herbivore-associated molecular patterns via receptors that activate early signaling components such as Ca²⁺, reactive oxygen species, and MAP kinases. Specific defense reprogramming proceeds via signaling networks that include phytohormones, secondary metabolites, and transcription factors. Local and systemic regulation of toxins, defense proteins, physical barriers, and tolerance traits protect plants against herbivores. Herbivores counteract plant defenses through biochemical defense deactivation, effector-mediated suppression of defense signaling, and chemically controlled behavioral changes. The molecular basis of plant-herbivore interactions is now well established for model systems. Expanding molecular approaches to unexplored dimensions of plant-insect interactions should be a future priority.

Integration of two herbivore-induced plant volatiles results in synergistic effects on plant defence and resistance

Plants can use induced volatiles to detect herbivore- and pathogen-attacked neighbors and prime their defenses. Several individual volatile priming cues have been identified, but whether plants are able to integrate multiple cues from stress-related volatile blends remains poorly understood. Here, we investigated how maize plants respond to two herbivore-induced volatile priming cues with complementary information content, the green leaf volatile (Z)-3-hexenyl acetate (HAC) and the aromatic volatile indole. In the absence of herbivory, HAC directly induced defence gene expression, whereas indole had no effect. Upon induction by simulated herbivory, both volatiles increased jasmonate signalling, defence gene expression, and defensive secondary metabolite production and increased plant resistance. Plant resistance to caterpillars was more strongly induced in dual volatile-exposed plants than plants exposed to single volatiles.. Induced defence levels in dual volatile-exposed plants were significantly higher than predicted from the added effects of the individual volatiles, with the exception of induced plant volatile production, which showed no increase upon dual-exposure relative to single exposure. Thus, plants can integrate different volatile cues into strong and specific responses that promote herbivore defence induction and resistance. Integrating multiple volatiles may be beneficial, as volatile blends are more reliable indicators of future stress than single cues.

The Asian corn borer Ostrinia furnacalis feeding increases the direct and indirect defence of mid-whorl stage commercial maize in the field

The Asian corn borer (Ostrinia furnacalis Guenée) is a destructive pest of maize (Zea mays L.). Despite large-scale commercial maize production, little is known about the defensive responses of field-grown commercial maize to O. furnacalis herbivory, and how these responses result in direct and indirect defence against this pest. To elucidate the maize transcriptome response to O. furnacalis feeding, leaves of maize hybrid Jingke968 were infested with O. furnacalis for 0, 2, 4, 12 and 24 h. Ostrinia furnacalis feeding elicited stronger and more rapid changes in the defence-related gene expression (i.e. after 2 h), and more differentially expressed genes (DEGs) were up-regulated than down-regulated at all times post-induction (i.e. 2, 4, 12 and 24 h) in the O. furnacalis pre-infested maize plants. KEGG pathway analysis indicated that the DEGs in the O. furnacalis pre-infested maize are involved in benzoxazinoids, phytohormones, volatiles, and other metabolic pathways related to maize resistance to herbivores. In addition, the maize leaves previously infested by O. furnacalis for 24 h showed an obvious inhibition of the subsequent O. furnacalis performance, and maize volatiles induced by O. furnacalis feeding for 24 and 48 h attracted the parasitic wasp, Macrocentrus cingulum Brischke. The increased direct and indirect defences induced by O. furnacalis feeding were correlated with O. furnacalis-induced phytohormones, benzoxazinoids, and volatiles. Together, our findings provide new insights into how commercial maize orchestrates its transcriptome and metabolome to directly and indirectly defend against O. furnacalis at the mid-whorl stage in the field.

Plant iron acquisition strategy exploited by an insect herbivore

Insect herbivores depend on their host plants to acquire macro- and micronutrients. Here we asked how a specialist herbivore and damaging maize pest, the western corn rootworm, finds and accesses plant-derived micronutrients. We show that the root-feeding larvae use complexes between iron and benzoxazinoid secondary metabolites to identify maize as a host, to forage within the maize root system, and to increase their growth. Maize plants use these same benzoxazinoids for protection against generalist herbivores and, as shown here, for iron uptake. We identify an iron transporter that allows the corn rootworm to benefit from complexes between iron and benzoxazinoids. Thus, foraging for an essential plant-derived complex between a micronutrient and a secondary metabolite shapes the interaction between maize and a specialist herbivore.

Root exudate metabolites drive plant-soil feedbacks on growth and defense by shaping the rhizosphere microbiota

By changing soil properties, plants can modify their growth environment. Although the soil microbiota is known to play a key role in the resulting plant-soil feedbacks, the proximal mechanisms underlying this phenomenon remain unknown. We found that benzoxazinoids, a class of defensive secondary metabolites that are released by roots of cereals such as wheat and maize, alter root-associated fungal and bacterial communities, decrease plant growth, increase jasmonate signaling and plant defenses, and suppress herbivore performance in the next plant generation. Complementation experiments demonstrate that the benzoxazinoid breakdown product 6-methoxy-benzoxazolin-2-one (MBOA), which accumulates in the soil during the conditioning phase, is both sufficient and necessary to trigger the observed phenotypic changes. Sterilization, fungal and bacterial profiling and complementation experiments reveal that MBOA acts indirectly by altering root-associated microbiota. Our results reveal a mechanism by which plants determine the composition of rhizosphere microbiota, plant performance and plant-herbivore interactions of the next generation.

Plants can modify soil microbiota through root exudation, but how this process influences plant health in turn is often unclear. Here, Hu et al. show that maize benzoxazinoids released into the soil modify root-associated microbiota and thereby increase leaf defenses of the next plant generation.

Wild Relatives of Maize, Rice, Cotton, and Soybean: Treasure Troves for Tolerance to Biotic and Abiotic Stresses

Global food demand is expected to nearly double by 2050 due to an increase in the world's population. The Green Revolution has played a key role in the past century by increasing agricultural productivity worldwide, however, limited availability and continued depletion of natural resources such as arable land and water will continue to pose a serious challenge for global food security in the coming decades. High yielding varieties with proven tolerance to biotic and abiotic stresses, superior nutritional profiles, and the ability to adapt to the changing environment are needed for continued agricultural sustainability. The narrow genetic base of modern cultivars is becoming a major bottleneck for crop improvement efforts and, therefore, the use of crop wild relatives (CWRs) is a promising approach to enhance genetic diversity of cultivated crops. This article provides a review of the efforts to date on the exploration of CWRs as a source of tolerance to multiple biotic and abiotic stresses in four global crops of importance; maize, rice, cotton, and soybean. In addition to the overview of the repertoire and geographical spread of CWRs in each of the respective crops, we have provided a comprehensive discussion on the morphological and/or genetic basis of the traits along with some examples, when available, of the research in the transfer of traits from CWRs to cultivated varieties. The emergence of modern molecular and genomic technologies has not only accelerated the pace of dissecting the genetics underlying the traits found in CWRs, but also enabled rapid and efficient trait transfer and genome manipulation. The potential and promise of these technologies has also been highlighted in this review.

Nonsensical choices? Fall armyworm moths choose seemingly best or worst hosts for their larvae, but neonate larvae make their own choices

Selecting optimal host plants is critical for herbivorous insects, such as fall armyworm (Spodoptera frugiperda), an important maize pest in the Americas and Africa. Fall armyworm larvae are presumed to have limited mobility, hence female moths are presumed to be largely responsible for selecting hosts. We addressed host selection by fall armyworm moths and neonate and older (3^rd-instar) larvae, as mediated by resistance and herbivory in maize plants. Thus, we compared discrimination among three maize cultivars with varying degrees of resistance to fall armyworm, and between plants subjected or not to two types of herbivory. The cultivars were: (i) susceptible, and deficient in jasmonic acid (JA) production and green leaf volatiles (GLV) emissions (inbred line B73-lox10); (ii) modestly resistant (B73), and; (iii) highly resistant (Mp708). The herbivory types were: (i) ongoing (= fall armyworm larvae present), and; (ii) future (= fall armyworm eggs present). In choice tests, moths laid more eggs on the highly resistant cultivar, and least on the susceptible cultivar, though on those cultivars larvae performed poorest and best, respectively. In the context of herbivory, moths laid more eggs: (i) on plants subject to versus free of future herbivory, regardless of whether plants were deficient or not in JA and GLV production; (ii) on plants subject versus free of ongoing herbivory, and; (iii) on plants not deficient in compared to deficient in JA and GLV production. Neonate larvae dispersed aerially from host plants (i.e. ballooned), and most larvae colonized the modestly resistant cultivar, and fewest the highly resistant cultivar, suggesting quasi-directional, directed aerial descent. Finally, dispersing older larvae did not discriminate among the three maize cultivars, nor between maize plants and (plastic) model maize plants, suggesting random, visually-oriented dispersal. Our results were used to assemble a model of host selection by fall armyworm moths and larvae, including recommendations for future research.

A Bird in the Hand Versus Two in the Bush? The Specialist Leafhopper Dalbulus maidis (Hemiptera: Cicadellidae) Does Not Discriminate Against Sub-optimal Host Plants (Zea spp.)

The corn leafhopper [Dalbulus maidis (DeLong & Wolcott)] is a specialist on Zea (Poaceae) that coevolved with maize (Zea mays mays) and its teosinte (Zea spp.) relatives. This study tested the hypothesis that host acceptance by females varies among Zea hosts, and is correlated with variation in defensive levels across those hosts. Prior studies revealed differences in plant defenses among Zea hosts and corresponding differences in corn leafhopper performance. Thus, host acceptance was expected to be correlated with defensive levels and offspring performance across Zea hosts, following the hypothesis that offspring performance mediates host preference. In parallel, host acceptance was expected to be correlated with transitions in life history strategy (perennial to annual life cycle), domestication status (wild to domesticated), and breeding intensity (landrace to hybrid variety) in Zea because variation in defensive levels and corn leafhopper performance were shown in prior studies to be correlated with those transitions. The study's hypotheses were tested by comparing, under no-choice conditions, host acceptance by corn leafhopper of a suite of Zea hosts encompassing those transitions: perennial teosinte (Zea diploperennis), Balsas teosinte (Zea mays parviglumis), and landrace and commercial hybrid maize. The results did not show differences in host acceptance for oviposition or feeding among the hosts. Thus, under no-choice conditions, all Zea hosts may be similarly acceptable for feeding and oviposition, despite marked ovipositional preferences under choice conditions and poorer offspring performance on teosintes relative to maize shown previously. The results suggested also that oviposition frequency per plant by females was not correlated with their offspring's performance.

Fine-tuning the 'plant domestication-reduced defense' hypothesis: specialist vs generalist herbivores

Domesticated plants are assumed to have weakened chemical defenses. We argue, however, that artificial selection will have maintained defense traits against specialized herbivores that have coexisted with the crops throughout their domestication. We assessed the performance of eight species of insect herbivores from three feeding guilds on six European maize lines and six populations of their wild ancestor, teosinte. A metabolomics approach was used in an attempt to identify compounds responsible for observed differences in insect performance. Insects consistently performed better on maize than on teosinte. As hypothesized, this difference was greater for generalist herbivores that are normally not found on teosinte. We also found clear differences in defense metabolites among the different genotypes, but none that consistently correlated with differences in performance. Concentrations of benzoxazinoids, the main chemical defense in maize, tended to be higher in leaves of teosinte, but the reverse was true for the roots. It appears that chemical defenses that target specialized insects are still present at higher concentrations in cultivated maize than compounds that are more effective against generalists. These weakened broad-spectrum defenses in crops may explain the successes of novel pests.

Distribution of the tryptophan pathway-derived defensive secondary metabolites gramine and benzoxazinones in Poaceae

The Poaceae is a large taxonomic group consisting of approximately 12,000 species and is classified into 12 subfamilies. Gramine and benzoxazinones (Bxs), which are biosynthesized from the tryptophan pathway, are well-known defensive secondary metabolites in the Poaceae. We analyzed the presence or absence of garamine and Bxs in 64 species in the Poaceae by LC-MS/MS. We found that Hordeum brachyantherum and Hakonechloa macra accumulated gramine, but the presence of gramine was limited to small groups of species. We also detected Bxs in four species in the Pooideae and six species in the Panicoideae. In particular, four species in the Paniceae tribe in Panicoideae accumulaed Bxs, indicating that this tribe is a center of the Bx distribution. Bxs were absent in the subfamilies other than Pooideae and Panicoideae. These findings provide an overview of biased distribution of gramine and Bxs in Poaceae species.

Physiological function and ecological aspects of fatty acid-amino acid conjugates in insects†

In tritrophic interactions, plants recognize herbivore-produced elicitors and release a blend of volatile compounds (VOCs), which work as chemical cues for parasitoids or predators to locate their hosts. From detection of elicitors to VOC emissions, plants utilize sophisticated systems that resemble the plant–microbe interaction system. Fatty acid–amino acid conjugates (FACs), a class of insect elicitors, resemble compounds synthesized by microbes in nature. Recent evidence suggests that the recognition of insect elicitors by an ancestral microbe-associated defense system may be the origin of tritrophic interactions mediated by FACs. Here we discuss our findings in light of how plants have customized this defense to be effective against insect herbivores, and how some insects have successfully adapted to these defenses.