Thermal characterization and separation of whey proteins by differential scanning calorimetry

Most commercially available whey products contain a mixture of 6-7 whey proteins; however, there is an increased focus on using the individual whey proteins for their unique biological activities. Before extracting individual whey proteins for use, it is important to quantify how much of a particular protein is present in whey mixtures as well as if the protein is still structurally folded. We first characterized the denaturation temperature and enthalpy values for the six purified whey proteins at six pHs (3-9) and under ion chelation using a nano-differential scanning calorimeter (DSC). From the individual protein scans, we determined the optimal condition for detecting all 6 proteins on a single DSC scan was whey in an EDTA MOPs pH 6.7 buffer.

9,10-KODA, an α-ketol produced by the tonoplast-localized 9-lipoxygenase ZmLOX5, plays a signaling role in maize defense against insect herbivory

13-Lipoxygenases (LOXs) initiate the synthesis of jasmonic acid (JA), the best-understood oxylipin hormone in herbivory defense. However, the roles of 9-LOX-derived oxylipins in insect resistance remain unclear. Here, we report a novel anti-herbivory mechanism mediated by a tonoplast-localized 9-LOX, ZmLOX5, and its linolenic acid-derived product, 9-hydroxy-10-oxo-12(Z),15(Z)-octadecadienoic acid (9,10-KODA). Transposon-insertional disruption of ZmLOX5 resulted in the loss of resistance to insect herbivory. lox5 knockout mutants displayed greatly reduced wound-induced accumulation of multiple oxylipins and defense metabolites, including benzoxazinoids, abscisic acid (ABA), and JA-isoleucine (JA-Ile). However, exogenous JA-Ile failed to rescue insect defense in lox5 mutants, while applications of 1 μM 9,10-KODA or the JA precursor, 12-oxo-phytodienoic acid (12-OPDA), restored wild-type resistance levels. Metabolite profiling revealed that exogenous 9,10-KODA primed the plants for increased production of ABA and 12-OPDA, but not JA-Ile. While none of the 9-oxylipins were able to rescue JA-Ile induction, the lox5 mutant accumulated lower wound-induced levels of Ca2+, suggesting this as a potential explanation for lower wound-induced JA. Seedlings pretreated with 9,10-KODA exhibited rapid or more robust wound-induced defense gene expression. In addition, an artificial diet supplemented with 9,10-KODA arrested fall armyworm larvae growth. Finally, analysis of single and double lox5 and lox10 mutants showed that ZmLOX5 also contributed to insect defense by modulating ZmLOX10-mediated green leaf volatile signaling. Collectively, our study uncovered a previously unknown anti-herbivore defense and hormone-like signaling activity for a major 9-oxylipin α-ketol.

A non-JA producing oxophytodienoate reductase functions in salicylic acid-mediated antagonism with jasmonic acid during pathogen attack

Peroxisome-localized oxo-phytodienoic acid (OPDA) reductases (OPR) are enzymes converting 12-OPDA into jasmonic acid (JA). However, the biochemical and physiological functions of the cytoplasmic non-JA producing OPRs remain largely unknown. Here, we generated Mutator-insertional mutants of the maize OPR2 gene and tested its role in resistance to pathogens with distinct lifestyles. Functional analyses showed that the opr2 mutants were more susceptible to the (hemi)biotrophic pathogens Colletotrichum graminicola and Ustilago maydis, but were more resistant to the necrotrophic fungus Cochliobolus heterostrophus. Hormone profiling revealed that increased susceptibility to C. graminicola was associated with decreased salicylic acid (SA) but increased JA levels. Mutation of the JA-producing lipoxygenase 10 (LOX10) reversed this phenotype in the opr2 mutant background, corroborating the notion that JA promotes susceptibility to this pathogen. Exogenous SA did not rescue normal resistance levels in opr2 mutants, suggesting that this SA-inducible gene is the key downstream component of the SA-mediated defences against C. graminicola. Disease assays of the single and double opr2 and lox10 mutants and the JA-deficient opr7opr8 mutants showed that OPR2 negatively regulates JA biosynthesis, and that JA is required for resistance against C. heterostrophus. Overall, this study uncovers a novel function of a non-JA producing OPR as a major negative regulator of JA biosynthesis during pathogen infection, a function that leads to its contrasting contribution to either resistance or susceptibility depending on pathogen lifestyle.

Liguleless narrow and narrow odd dwarf act in overlapping pathways to regulate maize development and metabolism

Narrow odd dwarf (nod) and Liguleless narrow (Lgn) are pleiotropic maize mutants that both encode plasma membrane proteins, cause similar developmental patterning defects, and constitutively induce stress signaling pathways. To investigate how these mutants coordinate maize development and physiology, we screened for protein interactors of NOD by affinity purification. LGN was identified by this screen as a strong candidate interactor, and we confirmed the NOD-LGN molecular interaction through orthogonal experiments. We further demonstrated that LGN, a receptor-like kinase, can phosphorylate NOD in vitro, hinting that they could act in intersecting signal transduction pathways. To test this hypothesis, we generated Lgn-R;nod mutants in two backgrounds (B73 and A619), and found that these mutations enhance each other, causing more severe developmental defects than either single mutation on its own, with phenotypes including very narrow leaves, increased tillering, and failure of the main shoot. Transcriptomic and metabolomic analyses of the single and double mutants in the two genetic backgrounds revealed widespread induction of pathogen defense genes and a shift in resource allocation away from primary metabolism in favor of specialized metabolism. These effects were similar in each single mutant and heightened in the double mutant, leading us to conclude that NOD and LGN act cumulatively in overlapping signaling pathways to coordinate growth-defense tradeoffs in maize.

Dedicated farnesyl diphosphate synthases circumvent isoprenoid-derived growth-defense tradeoffs in Zea mays

Zea mays (maize) makes phytoalexins such as sesquiterpenoid zealexins, to combat invading pathogens. Zealexins are produced from farnesyl diphosphate in microgram per gram fresh weight quantities. As farnesyl diphosphate is also a precursor for many compounds essential for plant growth, the question arises as to how Z. mays produces high levels of zealexins without negatively affecting vital plant systems. To examine if specific pools of farnesyl diphosphate are made for zealexin synthesis we made CRISPR/Cas9 knockouts of each of the three farnesyl diphosphate synthases (FPS) in Z. mays and examined the resultant impacts on different farnesyl diphosphate-derived metabolites. We found that FPS3 (GRMZM2G098569) produced most of the farnesyl diphosphate for zealexins, while FPS1 (GRMZM2G168681) made most of the farnesyl diphosphate for the vital respiratory co-factor ubiquinone. Indeed, fps1 mutants had strong developmental phenotypes such as reduced stature and development of chlorosis. The replication and evolution of the fps gene family in Z. mays enabled it to produce dedicated FPSs for developmentally related ubiquinone production (FPS1) or defense-related zealexin production (FPS3). This partitioning of farnesyl diphosphate production between growth and defense could contribute to the ability of Z. mays to produce high levels of phytoalexins without negatively impacting its growth.

Production of the Green Leaf Volatile (Z)-3-Hexenal by a Zea mays Hydroperoxide Lyase

Plant-produced volatile compounds play important roles in plant signaling and in the communication of plants with other organisms. Many plants emit green leaf volatiles (GLVs) in response to damage or attack, which serve to warn neighboring plants or attract predatory or parasitic insects to help defend against insect pests. GLVs include aldehydes, esters, and alcohols of 6-carbon compounds that are released rapidly following wounding. One GLV produced by maize (Zea mays) is the volatile (Z)-3-hexenal; this volatile is produced from the cleavage of (9Z,11E,15Z)-octadecatrienoic acid by hydroperoxide lyases (HPLs) of the cytochrome P450 CYP74B family. The specific HPL in maize involved in (Z)-3-hexenal production had not been determined. In this study, we used phylogenetics with known HPLs from other species to identify a candidate HPL from maize (ZmHPL). To test the ability of the putative HPL to produce (Z)-3-hexenal, we constitutively expressed the gene in Arabidopsis thaliana ecotype Columbia-0 that contains a natural loss-of-function mutant in AtHPL and examined the transgenic plants for restored (Z)-3-hexenal production. Volatile analysis of leaves from these transgenic plants showed that they did produce (Z)-3-hexenal, confirming that ZmHPL can produce (Z)-3-hexenal in vivo. Furthermore, we used gene expression analysis to show that expression of ZmHPL is induced in maize in response to both wounding and the insect pests Spodoptera frugiperda and Spodoptera exigua. Our study demonstrates that ZmHPL can produce GLVs and highlights its likely role in (Z)-3-hexenal production in response to mechanical damage and herbivory in maize.

RNAi-induced knockdown of white gene in the southern green stink bug (Nezara viridula L.)

The southern green stink bug (SGSB) Nezara viridula L. is one of the most common stink bug species in the United States and can cause significant yield loss in a variety of crops. A suitable marker for the assessment of gene-editing tools in SGSB has yet to be characterized. The white gene, first documented in Drosophila, has been a useful target to assess the efficiency of introduced mutations in many species as it controls pigmentation processes and mutants display readily identifiable phenotypes. In this study we used the RNAi technique to investigate functions and phenotypes associated with the white ortholog in the SGSB and to validate white as a marker for genetic transformation in this species. This study revealed that white may be a suitable marker for germline transformation in the SGSB as white transcript knockdown was not lethal, did not impair embryo development and provided a distinguishable phenotype. Our results demonstrated that the white ortholog in SGSB is involved in the pathway for ommochrome synthesis and suggested additional functions of this gene such as in the integument composition, management of hemolymph compounds and riboflavin mobilization.

Fusarium verticillioides Induces Maize-Derived Ethylene to Promote Virulence by Engaging Fungal G-Protein Signaling

Seed maceration and contamination with mycotoxin fumonisin inflicted by Fusarium verticillioides is a major disease concern for maize producers worldwide. Meta-analyses of quantitative trait loci for Fusarium ear rot resistance uncovered several ethylene (ET) biosynthesis and signaling genes within them, implicating ET in maize interactions with F. verticillioides. We tested this hypothesis using maize knockout mutants of the 1-aminocyclopropane-1-carboxylate (ACC) synthases ZmACS2 and ZmACS6. Infected wild-type seed emitted five-fold higher ET levels compared with controls, whereas ET was abolished in the acs2 and acs6 single and double mutants. The mutants supported reduced fungal biomass, conidia, and fumonisin content. Normal susceptibility was restored in the acs6 mutant with exogenous treatment of ET precursor ACC. Subsequently, we showed that fungal G-protein signaling is required for virulence via induction of maize-produced ET. F. verticillioides G_β subunit and two regulators of G-protein signaling mutants displayed reduced seed colonization and decreased ET levels. These defects were rescued by exogenous application of ACC. We concluded that pathogen-induced ET facilitates F. verticillioides colonization of seed, and, in turn, host ET production is manipulated via G-protein signaling of F. verticillioides to facilitate pathogenesis.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A maize leucine-rich repeat receptor-like protein kinase mediates responses to fungal attack

A maize receptor kinase controls defense response to fungal pathogens by regulating jasmonic acid and antimicrobial phytoalexin production. Plants use a range of pattern recognition receptors to detect and respond to biotic threats. Some of these receptors contain leucine-rich repeat (LRR) domains that recognize microbial proteins or peptides. Maize (Zea mays) has 226 LRR-receptor like kinases, making it challenging to identify those important for pathogen recognition. In this study, co-expression analysis with genes for jasmonic acid and phytoalexin biosynthesis was used to identify a fungal induced-receptor like protein kinase (FI-RLPK) likely involved in the response to fungal pathogens. Loss-of-function mutants in fi-rlpk displayed enhanced susceptibility to the necrotrophic fungal pathogen Cochliobolus heterostrophus and reduced accumulation of jasmonic acid and the anti-microbial phytoalexins -kauralexins and zealexins- in infected tissues. In contrast, fi-rlpk mutants displayed increased resistance to stem inoculation with the hemibiotrophic fungal pathogen Fusarium graminearum. These data indicate that FI-RLPK is important for fungal recognition and activation of defenses, and that F. graminearum may be able to exploit FI-RLPK function to increase its virulence.

Analysis of the transcriptomic, metabolomic, and gene regulatory responses to Puccinia sorghi in maize

Common rust, caused by Puccinia sorghi, is a widespread and destructive disease of maize. The Rp1-D gene confers resistance to the P. sorghi IN2 isolate, mediating a hypersensitive cell death response (HR). To identify differentially expressed genes (DEGs) and metabolites associated with the compatible (susceptible) interaction and with Rp1-D-mediated resistance in maize, we performed transcriptomics and targeted metabolome analyses of P. sorghi IN2-infected leaves from the near-isogenic lines H95 and H95:Rp1-D, which differed for the presence of Rp1-D. We observed up-regulation of genes involved in the defence response and secondary metabolism, including the phenylpropanoid, flavonoid, and terpenoid pathways. Metabolome analyses confirmed that intermediates from several transcriptionally up-regulated pathways accumulated during the defence response. We identified a common response in H95:Rp1-D and H95 with an additional H95:Rp1-D-specific resistance response observed at early time points at both transcriptional and metabolic levels. To better understand the mechanisms underlying Rp1-D-mediated resistance, we inferred gene regulatory networks occurring in response to P. sorghi infection. A number of transcription factors including WRKY53, BHLH124, NKD1, BZIP84, and MYB100 were identified as potentially important signalling hubs in the resistance-specific response. Overall, this study provides a novel and multifaceted understanding of the maize susceptible and resistance-specific responses to P. sorghi.

Metabolomics by UHPLC-HRMS reveals the impact of heat stress on pathogen-elicited immunity in maize

INTRODUCTION

Studies investigating crop resistance to abiotic and biotic stress have largely focused on plant responses to singular forms of stress and individual biochemical pathways that only partially represent stress responses. Thus, combined abiotic and biotic stress treatments and the global assessment of their elicited metabolic expression remains largely unexplored. In this study, we employed targeted and untargeted metabolomics to investigate the molecular responses of maize (Zea mays) to abiotic, biotic, and combinatorial stress.

OBJECTIVE

We compared the inducible metabolomes of heat-stressed (abiotic) and C. heterostrophus-infected (biotic) maize and examined the effects of heat stress on the ability of maize to defend itself against C. heterostrophus.

METHODS

Ultra-high-performance liquid chromatography-high-resolution mass spectrometry was performed on plants grown under control conditions (28 °C), heat stress (38 °C), Cochliobolus heterostrophus infection, or combinatorial stress [heat (38 °C) + C. heterostrophus infection].

RESULTS

Multivariate analyses revealed differential metabolite expression between heat stress, C. heterostrophus infection, and their respective controls. In combinatorial experiments, treatment with heat stress prior to fungal inoculation negatively impacted maize disease resistance against C. heterostrophus, and distinct metabolome separation between combinatorial stressed plants and the non-heat-stressed infected controls was observed. Targeted analysis revealed inducible primary and secondary metabolite responses to abiotic/biotic stress, and combinatorial experiments indicated that deficiency in the hydroxycinnamic acid, p-coumaric acid, may contribute to the heat-induced susceptibility of maize to C. heterostrophus.

CONCLUSION

These findings demonstrate that abiotic stress can predispose crops to more severe disease symptoms, underlining the increasing need to investigate defense chemistry in plants under combinatorial stress.

Genetic elucidation of interconnected antibiotic pathways mediating maize innate immunity

Specialized metabolites constitute key layers of immunity that underlie disease resistance in crops; however, challenges in resolving pathways limit our understanding of the functions and applications of these metabolites. In maize (Zea mays), the inducible accumulation of acidic terpenoids is increasingly considered to be a defence mechanism that contributes to disease resistance. Here, to understand maize antibiotic biosynthesis, we integrated association mapping, pan-genome multi-omic correlations, enzyme structure-function studies and targeted mutagenesis. We define ten genes in three zealexin (Zx) gene clusters that encode four sesquiterpene synthases and six cytochrome P450 proteins that collectively drive the production of diverse antibiotic cocktails. Quadruple mutants in which the ability to produce zealexins (ZXs) is blocked exhibit a broad-spectrum loss of disease resistance. Genetic redundancies ensuring pathway resiliency to single null mutations are combined with enzyme substrate promiscuity, creating a biosynthetic hourglass pathway that uses diverse substrates and in vivo combinatorial chemistry to yield complex antibiotic blends. The elucidated genetic basis of biochemical phenotypes that underlie disease resistance demonstrates a predominant maize defence pathway and informs innovative strategies for transferring chemical immunity between crops.

The 13-lipoxygenase MSD2 and the ω-3 fatty acid desaturase MSD3 impact Spodoptera frugiperda resistance in Sorghum

Linolenic acid produced by the ω-3 fatty acid desaturase MSD3 in sorghum is used for insect-induced jasmonic acid production and is important for resistance against Spodoptera frugiperda. Jasmonic acid (JA) is a phytohormone that regulates both plant development and stress responses. In sorghum (Sorghum bicolor), the ω-3 fatty acid desaturase Multiseeded3 (MSD3) and the 13-lipoxygenase Multiseeded2 (MSD2) are important for producing JA to regulate panicle development and spikelet fertility, but their function in plant defense remains unknown. In this study, we examined whether these genes are important for the production of JA in response to herbivory by the insect pest Spodoptera frugiperda. Compared to wild-type controls, the msd3 mutant accumulated less JA in leaves of both infested and uninfested plants, revealing that MSD3 is involved in stress-induced JA production. In contrast, herbivore-induced JA production in the msd2 mutant was indistinguishable from wild type, indicating that MSD2 does not function in herbivore-induced JA production. An increase of S. frugiperda growth was observed on both the msd3 and msd2 mutants, hinting at roles for both JA and additional oxylipins in sorghum's defense responses.

Green leaf volatiles and jasmonic acid enhance susceptibility to anthracnose diseases caused by Colletotrichum graminicola in maize

Colletotrichum graminicola is a hemibiotrophic fungus that causes anthracnose leaf blight (ALB) and anthracnose stalk rot (ASR) in maize. Despite substantial economic losses caused by these diseases, the defence mechanisms against this pathogen remain poorly understood. Several hormones are suggested to aid in defence against C. graminicola, such as jasmonic acid (JA) and salicylic acid (SA), but supporting genetic evidence was not reported. Green leaf volatiles (GLVs) are a group of well-characterized volatiles that induce JA biosynthesis in maize and are known to function in defence against necrotrophic pathogens. Information regarding the role of GLVs and JA in interactions with (hemi)biotrophic pathogens remains limited. To functionally elucidate GLVs and JA in defence against a hemibiotrophic pathogen, we tested GLV- and JA-deficient mutants, lox10 and opr7 opr8, respectively, for resistance to ASR and ALB and profiled jasmonates and SA in their stalks and leaves throughout infection. Both mutants were resistant and generally displayed elevated levels of SA and low amounts of jasmonates, especially at early stages of infection. Pretreatment with GLVs restored susceptibility of lox10 mutants, but not opr7 opr8 mutants, which coincided with complete rescue of JA levels. Exogenous methyl jasmonate restored susceptibility in both mutants when applied before inoculation, whereas methyl salicylate did not induce further resistance in either of the mutants, but did induce mutant-like resistance in the wild type. Collectively, this study reveals that GLVs and JA contribute to maize susceptibility to C. graminicola due to suppression of SA-related defences.

Setaria viridis as a model for translational genetic studies of jasmonic acid-related insect defenses in Zea mays

Little is known regarding insect defense pathways in Setaria viridis (setaria), a model system for panicoid grasses, including Zea mays (maize). It is thus of interest to compare insect herbivory responses of setaria and maize. Here we use metabolic, phylogenetic, and gene expression analyses to measure a subset of jasmonic acid (JA)-related defense responses to leaf-chewing caterpillars. Phylogenetic comparisons of known defense-related maize genes were used to identify putative orthologs in setaria, and candidates were tested by quantitative PCR to determine transcriptional responses to insect challenge. Our findings show that while much of the core JA-related metabolic and genetic responses appear conserved between setaria and maize, production of downstream secondary metabolites such as benzoxazinoids and herbivore-induced plant volatiles are dissimilar. This diversity of chemical defenses and gene families involved in secondary metabolism among grasses presents new opportunities for cross species engineering. The high degree of genetic similarity and ease of orthologous gene identification between setaria and maize make setaria an excellent species for translational genetic studies, but the species specificity of downstream insect defense chemistry makes some pathways unamenable to cross-species comparisons.

Fighting on two fronts: Elevated insect resistance in flooded maize

To grow and thrive plants must be able to adapt to both adverse environmental conditions and attack by a variety of pests. Elucidating the sophisticated mechanisms plants have developed to achieve this has been the focus of many studies. What is less well understood is how plants respond when faced with multiple stressors simultaneously. In this study, we assess the response of Zea mays (maize) to the combinatorial stress of flooding and infestation with the insect pest Spodoptera frugiperda (fall armyworm). This combined stress leads to elevated production of the defence hormone salicylic acid, which does not occur in the individual stresses, and the resultant salicylic acid-dependent increase in S. frugiperda resistance. Remodelling of phenylpropanoid pathways also occurs in response to this combinatorial stress leading to increased production of the anti-insect C-glycosyl flavones (maysins) and the herbivore-induced volatile phenolics, benzyl acetate, and phenethyl acetate. Furthermore, changes in cellular redox status also occur, as indicated by reductions in peroxidase and polyphenol oxidase activity. These data suggest that metabolite changes important for flooding tolerance and anti-insect defence may act both additively and synergistically to provide extra protection to the plant.

Sorghum MSD3 Encodes an ω-3 Fatty Acid Desaturase that Increases Grain Number by Reducing Jasmonic Acid Levels

Grain number per panicle is an important component of grain yield in sorghum (Sorghum bicolor (L.)) and other cereal crops. Previously, we reported that mutations in multi-seeded 1 (MSD1) and MSD2 genes result in a two-fold increase in grain number per panicle due to the restoration of the fertility of the pedicellate spikelets, which invariably abort in natural sorghum accessions. Here, we report the identification of another gene, MSD3, which is also involved in the regulation of grain numbers in sorghum. Four bulked F₂ populations from crosses between BTx623 and each of the independent msd mutants p6, p14, p21, and p24 were sequenced to 20× coverage of the whole genome on a HiSeq 2000 system. Bioinformatic analyses of the sequence data showed that one gene, Sorbi_3001G407600, harbored homozygous mutations in all four populations. This gene encodes a plastidial ω-3 fatty acid desaturase that catalyzes the conversion of linoleic acid (18:2) to linolenic acid (18:3), a substrate for jasmonic acid (JA) biosynthesis. The msd3 mutants had reduced levels of linolenic acid in both leaves and developing panicles that in turn decreased the levels of JA. Furthermore, the msd3 panicle phenotype was reversed by treatment with methyl-JA (MeJA). Our characterization of MSD1, MSD2, and now MSD3 demonstrates that JA-regulated processes are critical to the msd phenotype. The identification of the MSD3 gene reveals a new target that could be manipulated to increase grain number per panicle in sorghum, and potentially other cereal crops, through the genomic editing of MSD3 functional orthologs.

Fertility of Pedicellate Spikelets in Sorghum Is Controlled by a Jasmonic Acid Regulatory Module

As in other cereal crops, the panicles of sorghum (Sorghum bicolor (L.) Moench) comprise two types of floral spikelets (grass flowers). Only sessile spikelets (SSs) are capable of producing viable grains, whereas pedicellate spikelets (PSs) cease development after initiation and eventually abort. Consequently, grain number per panicle (GNP) is lower than the total number of flowers produced per panicle. The mechanism underlying this differential fertility is not well understood. To investigate this issue, we isolated a series of ethyl methane sulfonate (EMS)-induced multiseeded (msd) mutants that result in full spikelet fertility, effectively doubling GNP. Previously, we showed that MSD1 is a TCP (Teosinte branched/Cycloidea/PCF) transcription factor that regulates jasmonic acid (JA) biosynthesis, and ultimately floral sex organ development. Here, we show that MSD2 encodes a lipoxygenase (LOX) that catalyzes the first committed step of JA biosynthesis. Further, we demonstrate that MSD1 binds to the promoters of MSD2 and other JA pathway genes. Together, these results show that a JA-induced module regulates sorghum panicle development and spikelet fertility. The findings advance our understanding of inflorescence development and could lead to new strategies for increasing GNP and grain yield in sorghum and other cereal crops.

Multiple genes recruited from hormone pathways partition maize diterpenoid defences

Duplication and divergence of primary pathway genes underlie the evolution of plant specialized metabolism; however, mechanisms partitioning parallel hormone and defence pathways are often speculative. For example, the primary pathway intermediate ent-kaurene is essential for gibberellin biosynthesis and is also a proposed precursor for maize antibiotics. By integrating transcriptional coregulation patterns, genome-wide association studies, combinatorial enzyme assays, proteomics and targeted mutant analyses, we show that maize kauralexin biosynthesis proceeds via the positional isomer ent-isokaurene formed by a diterpene synthase pair recruited from gibberellin metabolism. The oxygenation and subsequent desaturation of ent-isokaurene by three promiscuous cytochrome P450s and a new steroid 5α reductase indirectly yields predominant ent-kaurene-associated antibiotics required for Fusarium stalk rot resistance. The divergence and differential expression of pathway branches derived from multiple duplicated hormone-metabolic genes minimizes dysregulation of primary metabolism via the circuitous biosynthesis of ent-kaurene-related antibiotics without the production of growth hormone precursors during defence.

A maize polygalacturonase functions as a suppressor of programmed cell death in plants

BACKGROUND

The hypersensitive defense response (HR) in plants is a fast, localized necrotic response around the point of pathogen ingress. HR is usually triggered by a pathogen recognition event mediated by a nucleotide-binding site, leucine-rich repeat (NLR) protein. The autoactive maize NLR gene Rp1-D21 confers a spontaneous HR response in the absence of pathogen recognition. Previous work identified a set of loci associated with variation in the strength of Rp1-D21-induced HR. A polygalacturonase gene homolog, here termed ZmPGH1, was identified as a possible causal gene at one of these loci on chromosome 7.

RESULTS

Expression of ZmPGH1 inhibited the HR-inducing activity of both Rp1-D21 and that of another autoactive NLR, RPM1(D505V), in a Nicotiana benthamiana transient expression assay system. Overexpression of ZmPGH1 in a transposon insertion line of maize was associated with suppression of chemically-induced programmed cell death and with suppression of HR induced by Rp1-D21 in maize plants grown in the field.

CONCLUSIONS

ZmPGH1 functions as a suppressor of programmed cell death induced by at least two autoactive NLR proteins and by two chemical inducers. These findings deepen our understanding of the control of the HR in plants.

Biosynthesis and function of terpenoid defense compounds in maize (Zea mays)

Maize produces an array of herbivore-induced terpene volatiles that attract parasitoids to infested plants and a suite of pathogen-induced non-volatile terpenoids with antimicrobial activity to defend against pests. Plants rely on complex blends of constitutive and dynamically produced specialized metabolites to mediate beneficial ecological interactions and protect against biotic attack. One such class of metabolites are terpenoids, a large and structurally diverse class of molecules shown to play significant defensive and developmental roles in numerous plant species. Despite this, terpenoids have only recently been recognized as significant contributors to pest resistance in maize (Zea mays), a globally important agricultural crop. The current review details recent advances in our understanding of biochemical structures, pathways and functional roles of maize terpenoids. Dependent upon the lines examined, maize can harbor more than 30 terpene synthases, underlying the inherent diversity of maize terpene defense systems. Part of this defensive arsenal is the inducible production of volatile bouquets that include monoterpenes, homoterpenes and sesquiterpenes, which often function in indirect defense by enabling the attraction of parasitoids and predators. More recently discovered are a subset of sesquiterpene and diterpene hydrocarbon olefins modified by cytochrome P450s to produce non-volatile end-products such kauralexins, zealexins, dolabralexins and β-costic acid. These non-volatile terpenoid phytoalexins often provide effective defense against both microbial and insect pests via direct antimicrobial and anti-feedant activity. The diversity and promiscuity of maize terpene synthases, coupled with a variety of secondary modifications, results in elaborate defensive layers whose identities, regulation and precise functions are continuing to be elucidated.

Interactions Among Plants, Insects, and Microbes: Elucidation of Inter-Organismal Chemical Communications in Agricultural Ecology

The last 2 decades have witnessed a sustained increase in the study of plant-emitted volatiles and their role in plant-insect, plant-microbe, and plant-plant interactions. While each of these binary systems involves complex chemical and biochemical processes between two organisms, the progression of increasing complexity of a ternary system (i.e., plant-insect-microbe), and the study of a ternary system requires nontrivial planning. This planning can include an experimental design that factors in potential overarching ecological interactions regarding the binary or ternary system, correctly identifying and understanding unexpected observations that may occur during the experiment and thorough interpretation of the resultant data. This challenge of planning, performing, and interpreting a plant's defensive response to multiple biotic stressors will be even greater when abiotic stressors (i.e., temperature or water) are factored into the system. To fully understand the system, we need to not only continue to investigate and understand the volatile profiles but also include and understand the biochemistry of the plant's response to these stressors. In this review, we provide examples and discuss interaction considerations with respect to how readers and future authors of the Journal of Agricultural and Food Chemistry can contribute their expertise toward the extraction and interpretation of chemical information exchanged between agricultural commodities and their associated pests. This holistic, multidisciplinary, and thoughtful approach to interactions of plants, insects, and microbes, and the resultant response of the plants can lead to a better understanding of agricultural ecology, in turn leading to practical and viable solutions to agricultural problems.

Contrasting insect attraction and herbivore-induced plant volatile production in maize

The maize inbred line W22 has lower herbivore-induced volatile production than B73 but both fall armyworm larvae and the wasps that parasitize them prefer W22 over B73. Maize inbred line W22 is an important resource for genetic studies due to the availability of the UniformMu mutant population and a complete genome sequence. In this study, we assessed the suitability of W22 as a model for tritrophic interactions between maize, Spodoptera frugiperda (fall armyworm) and the parasitoid wasp Cotesia marginiventris. W22 was found to be a good model for studying the interaction as S. frugiperda prefers W22 over B73 and a higher parasitism rate by C. marginiventris was observed on W22 compared to the inbred line B73. W22 also produced lower amounts of many herbivore-induced volatile terpenes and indole emission upon treatment with S. frugiperda oral secretions. We propose that some of the major herbivore-induced terpene volatiles are perhaps impeding S. frugiperda and C. marginiventris preference and that as yet unidentified compounds are produced at low abundance may be positively impacting these interactions.

Fungal and herbivore elicitation of the novel maize sesquiterpenoid, zealexin A4, is attenuated by elevated CO2

Chemical isolation and NMR-based structure elucidation revealed a novel keto-acidic sesquiterpenoid, termed zealexin A4 (ZA4). ZA4 is elicited by pathogens and herbivory, but attenuated by heightened levels of CO ₂ . The identification of the labdane-related diterpenoids, termed kauralexins and acidic sesquiterpenoids, termed zealexins, demonstrated the existence of at least ten novel stress-inducible maize metabolites with diverse antimicrobial activity. Despite these advances, the identity of co-occurring and predictably related analytes remains largely unexplored. In the current effort, we identify and characterize the first sesquiterpene keto acid derivative of β-macrocarpene, named zealexin A4 (ZA4). Evaluation of diverse maize inbreds revealed that ZA4 is commonly produced in maize scutella during the first 14 days of seedling development; however, ZA4 production in the scutella was markedly reduced in seedlings grown in sterile soil. Elevated ZA4 production was observed in response to inoculation with adventitious fungal pathogens, such as Aspergillus flavus and Rhizopus microsporus, and a positive relationship between ZA4 production and expression of the predicted zealexin biosynthetic genes, terpene synthases 6 and 11 (Tps6 and Tps11), was observed. ZA4 exhibited significant antimicrobial activity against the mycotoxigenic pathogen A. flavus; however, ZA4 activity against R. microsporus was minimal, suggesting the potential of some fungi to detoxify ZA4. Significant induction of ZA4 production was also observed in response to infestation with the stem tunneling herbivore Ostrinia nubilalis. Examination of the interactive effects of elevated CO₂ (E-CO₂) on both fungal and herbivore-elicited ZA4 production revealed significantly reduced levels of inducible ZA4 accumulation, consistent with a negative role for E-CO₂ on ZA4 production. Collectively, these results describe a novel β-macrocarpene-derived antifungal defense in maize and expand the established diversity of zealexins that are differentially regulated in response to biotic/abiotic stress.

Commercial hybrids and mutant genotypes reveal complex protective roles for inducible terpenoid defenses in maize

Plant defense research is facilitated by the use of genome-sequenced inbred lines; however, a foundational knowledge of interactions in commercial hybrids remains relevant to understanding mechanisms present in crops. Using an array of commercial maize hybrids, we quantified the accumulation patterns of defense-related metabolites and phytohormones in tissues challenged with diverse fungal pathogens. Across hybrids, Southern leaf blight (Cochliobolus heterostrophus) strongly elicited specific sesqui- and diterpenoid defenses, namely zealexin A4 (ZA4) and kauralexin diacids, compared with the stalk-rotting agents Fusarium graminearum and Colletotrichum graminicola. With respect to biological activity, ZA4 and kauralexin diacids demonstrated potent antimicrobial action against F. graminearum. Unexpectedly, ZA4 displayed an opposite effect on C. graminicola by promoting growth. Overall, a negative correlation was observed between total analyzed terpenoids and fungal growth. Statistical analyses highlighted kauralexin A3 and abscisic acid as metabolites most associated with fungal suppression. As an empirical test, mutants of the ent-copalyl diphosphate synthase Anther ear 2 (An2) lacking kauralexin biosynthetic capacity displayed increased susceptibility to C. heterostrophus and Fusarium verticillioides. Our results highlight a widely occurring defensive function of acidic terpenoids in commercial hybrids and the complex nature of elicited pathway products that display selective activities on fungal pathogen species.

Maize w3 disrupts homogentisate solanesyl transferase (ZmHst) and reveals a plastoquinone-9 independent path for phytoene desaturation and tocopherol accumulation in kernels

Maize white seedling 3 (w3) has been used to study carotenoid deficiency for almost 100 years, although the molecular basis of the mutation has remained unknown. Here we show that the w3 phenotype is caused by disruption of the maize gene for homogentisate solanesyl transferase (HST), which catalyzes the first and committed step in plastoquinone-9 (PQ-9) biosynthesis in the plastid. The resulting PQ-9 deficiency prohibits photosynthetic electron transfer and eliminates PQ-9 as an oxidant in the enzymatic desaturation of phytoene during carotenoid synthesis. As a result, light-grown w3 seedlings are albino, deficient in colored carotenoids and accumulate high levels of phytoene. However, despite the absence of PQ-9 for phytoene desaturation, dark-grown w3 seedlings can produce abscisic acid (ABA) and homozygous w3 kernels accumulate sufficient carotenoids to generate ABA needed for seed maturation. The presence of ABA and low levels of carotenoids in w3 nulls indicates that phytoene desaturase is able to use an alternate oxidant cofactor, albeit less efficiently than PQ-9. The observation that tocopherols and tocotrienols are modestly affected in w3 embryos and unaffected in w3 endosperm indicates that, unlike leaves, grain tissues deficient in PQ-9 are not subject to severe photo-oxidative stress. In addition to identifying the molecular basis for the maize w3 mutant, we: (1) show that low levels of phytoene desaturation can occur in w3 seedlings in the absence of PQ-9; and (2) demonstrate that PQ-9 and carotenoids are not required for vitamin E accumulation.

Herbivore-derived fatty-acid amides elicit reactive oxygen species burst in plants

Reactive oxygen species (ROS) can be elicited by many forms of stress, including pathogen attack, abiotic stress, damage and insect infestation. Perception of microbe- or damage-associated elicitors triggers an ROS burst in many plant species; however, the impact of herbivore fatty-acid amides on ROS elicitation remains largely unexplored. In this study we show that the lepidopteran-derived fatty-acid amide elicitor N-linolenoyl-L-glutamine (GLN18:3) can induce a ROS burst in multiple plant species. Furthermore, in Arabidopsis this ROS burst is partially dependent on the plasma membrane localized NADPH oxidases RBOHD and RBOHF, and an Arabidopsis rbohD/F double mutant produces enhanced GLN18:3-induced jasmonic acid. Quantification of GLN18:3-induced ROS in phytohormone-deficient lines revealed that in Arabidopsis reduced levels of jasmonic acid resulted in a larger elicitor-induced ROS burst, while in tomato reduction of either jasmonic acid or salicylic acid led to higher induced ROS production. These data indicate that GLN18:3-induced ROS is antagonistic to jasmonic acid production in these species. In biological assays, rbohD/F mutant plants were more resistant to the generalist herbivores Spodoptera exigua and Trichoplusia ni but not to the specialist Plutella xylostella. Collectively, these results demonstrate that in Arabidopsis herbivore-induced ROS may negatively regulate plant defense responses to herbivory.

RNA-Seq analysis of resistant and susceptible sub-tropical maize lines reveals a role for kauralexins in resistance to grey leaf spot disease, caused by Cercospora zeina

BACKGROUND

Cercospora zeina is a foliar pathogen responsible for maize grey leaf spot in southern Africa that negatively impacts maize production. Plants use a variety of chemical and structural mechanisms to defend themselves against invading pathogens such as C. zeina, including the production of secondary metabolites with antimicrobial properties. In maize, a variety of biotic and abiotic stressors induce the accumulation of the terpenoid phytoalexins, zealexins and kauralexins.

RESULTS

C. zeina-susceptible line displayed pervasive rectangular grey leaf spot lesions, running parallel with the leaf veins in contrast to C. zeina-resistant line that had restricted disease symptoms. Analysis of the transcriptome of both lines indicated that genes involved in primary and secondary metabolism were up-regualted, and although different pathways were prioritized in each line, production of terpenoid compounds were common to both. Targeted phytoalexin analysis revealed that C. zeina-inoculated leaves accumulated zealexins and kauralexins. The resistant line shows a propensity toward accumulation of the kauralexin B series metabolites in response to infection, which contrasts with the susceptible line that preferentially accumulates the kauralexin A series. Kauralexin accumulation was correlated to expression of the kauralexin biosynthetic gene, ZmAn2 and a candidate biosynthetic gene, ZmKSL2. We report the expression of a putative copalyl diphosphate synthase gene that is induced by C. zeina in the resistant line exclusively.

DISCUSSION

This study shows that zealexins and kauralexins, and expression of their biosynthetic genes, are induced by C. zeina in both resistant and susceptible germplasm adapted to the southern African climate. The data presented here indicates that different forms of kauralexins accumulate in the resistant and susceptible maize lines in response to C. zeina, with the accumulation of kauralexin B compounds in a resistant maize line and kauralexin A compounds accumulating in the susceptible line.

Elevated carbon dioxide reduces emission of herbivore-induced volatiles in Zea mays

Terpene volatiles produced by sweet corn (Zea mays) upon infestation with pests such as beet armyworm (Spodoptera exigua) function as part of an indirect defence mechanism by attracting parasitoid wasps; yet little is known about the impact of climate change on this form of plant defence. To investigate how a central component of climate change affects indirect defence, we measured herbivore-induced volatile emissions in plants grown under elevated carbon dioxide (CO₂ ). We found that S. exigua infested or elicitor-treated Z. mays grown at elevated CO₂ had decreased emission of its major sesquiterpene, (E)-β-caryophyllene and two homoterpenes, (3E)-4,8-dimethyl-1,3,7-nonatriene and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. In contrast, inside the leaves, elicitor-induced (E)-β-caryophyllene hyper-accumulated at elevated CO₂ , while levels of homoterpenes were unaffected. Furthermore, gene expression analysis revealed that the induction of terpene synthase genes following treatment was lower in plants grown at elevated CO₂ . Our data indicate that elevated CO₂ leads both to a repression of volatile synthesis at the transcriptional level and to limitation of volatile release through effects of CO₂ on stomatal conductance. These findings suggest that elevated CO₂ may alter the ability of Z. mays to utilize volatile terpenes to mediate indirect defenses.

Rapid defense responses in maize leaves induced by Spodoptera exigua caterpillar feeding

Insects such as the beet armyworm (Spodoptera exigua) cause extensive damage to maize (Zea mays). Maize plants respond by triggering defense signaling, changes in gene expression, and biosynthesis of specialized metabolites. Leaves of maize inbred line B73, which has an available genome sequence, were infested with S. exigua for 1 to 24 h, followed by comparisons of the transcript and metabolite profiles with those of uninfested controls. The most extensive gene expression responses occurred rapidly, within 4-6 h after caterpillar infestation. However, both gene expression and metabolite profiles were altered within 1 h and continued to change during the entire 24 h experiment. The defensive functions of three caterpillar-induced genes were examined using available Dissociation transposon insertions in maize inbred line W22. Whereas mutations in the benzoxazinoid biosynthesis pathway (Bx1 and Bx2) significantly improved caterpillar growth, the knockout of a 13-lipoxygenase (Lox8) involved in jasmonic acid biosynthesis did not. Interestingly, 9-lipoxygenases, which lead to the production of maize death acids, were more strongly induced by caterpillar feeding than 13-lipoxygenases, suggesting an as yet unknown function in maize defense against herbivory. Together, these results provide a comprehensive view of the dynamic transcriptomic and metabolomic responses of maize leaves to caterpillar feeding.

Systems genetics reveals a transcriptional network associated with susceptibility in the maize-grey leaf spot pathosystem

We used a systems genetics approach to elucidate the molecular mechanisms of the responses of maize to grey leaf spot (GLS) disease caused by Cercospora zeina, a threat to maize production globally. Expression analysis of earleaf samples in a subtropical maize recombinant inbred line population (CML444 × SC Malawi) subjected in the field to C. zeina infection allowed detection of 20 206 expression quantitative trait loci (eQTLs). Four trans-eQTL hotspots coincided with GLS disease QTLs mapped in the same field experiment. Co-expression network analysis identified three expression modules correlated with GLS disease scores. The module (GY-s) most highly correlated with susceptibility (r = 0.71; 179 genes) was enriched for the glyoxylate pathway, lipid metabolism, diterpenoid biosynthesis and responses to pathogen molecules such as chitin. The GY-s module was enriched for genes with trans-eQTLs in hotspots on chromosomes 9 and 10, which also coincided with phenotypic QTLs for susceptibility to GLS. This transcriptional network has significant overlap with the GLS susceptibility response of maize line B73, and may reflect pathogen manipulation for nutrient acquisition and/or unsuccessful defence responses, such as kauralexin production by the diterpenoid biosynthesis pathway. The co-expression module that correlated best with resistance (TQ-r; 1498 genes) was enriched for genes with trans-eQTLs in hotspots coinciding with GLS resistance QTLs on chromosome 9. Jasmonate responses were implicated in resistance to GLS through co-expression of COI1 and enrichment of genes with the Gene Ontology term 'cullin-RING ubiquitin ligase complex' in the TQ-r module. Consistent with this, JAZ repressor expression was highly correlated with the severity of GLS disease in the GY-s susceptibility network.

Interactive Effects of Elevated [CO2] and Drought on the Maize Phytochemical Defense Response against Mycotoxigenic Fusarium verticillioides

Changes in climate due to rising atmospheric carbon dioxide concentration ([CO2]) are predicted to intensify episodes of drought, but our understanding of how these combined conditions will influence crop-pathogen interactions is limited. We recently demonstrated that elevated [CO2] alone enhances maize susceptibility to the mycotoxigenic pathogen, Fusarium verticillioides (Fv) but fumonisin levels remain unaffected. In this study we show that maize simultaneously exposed to elevated [CO2] and drought are even more susceptible to Fv proliferation and also prone to higher levels of fumonisin contamination. Despite the increase in fumonisin levels, the amount of fumonisin produced in relation to pathogen biomass remained lower than corresponding plants grown at ambient [CO2]. Therefore, the increase in fumonisin contamination was likely due to even greater pathogen biomass rather than an increase in host-derived stimulants. Drought did not negate the compromising effects of elevated [CO2] on the accumulation of maize phytohormones and metabolites. However, since elevated [CO2] does not influence the drought-induced accumulation of abscisic acid (ABA) or root terpenoid phytoalexins, the effects elevated [CO2] are negated belowground, but the stifled defense response aboveground may be a consequence of resource redirection to the roots.

A maize death acid, 10-oxo-11-phytoenoic acid, is the predominant cyclopentenone signal present during multiple stress and developmental conditions

Recently we investigated the function of the 9-lipoxygenase (LOX) derived cyclopentenones 10-oxo-11-phytoenoic acid (10-OPEA) and 10-oxo-11,15-phytodienoic acid (10-OPDA) and identified their C-14 and C-12 derivatives. 10-OPEA accumulation is elicited by fungal and insect attack and acts as a strong inhibitor of microbial and herbivore growth. Although structurally similar, comparative analyses between 10-OPEA and its 13-LOX analog 12-oxo-phytodienoic acid (12-OPDA) demonstrate specificity in transcript accumulation linked to detoxification, secondary metabolism, jasmonate regulation, and protease inhibition. As a potent cell death signal, 10-OPEA activates cysteine protease activity leading to ion leakage and apoptotic-like DNA fragmentation. In this study we further elucidate the distribution, abundance, and functional roles of 10-OPEA, 10-OPDA, and 12-OPDA, in diverse organs under pathogen- and insect-related stress.

The novel monocot-specific 9-lipoxygenase ZmLOX12 is required to mount an effective jasmonate-mediated defense against Fusarium verticillioides in maize

Fusarium verticillioides is a major limiting factor for maize production due to ear and stalk rot and the contamination of seed with the carcinogenic mycotoxin fumonisin. While lipoxygenase (LOX)-derived oxylipins have been implicated in defense against diverse pathogens, their function in maize resistance against F. verticillioides is poorly understood. Here, we functionally characterized a novel maize 9-LOX gene, ZmLOX12. This gene is distantly related to known dicot LOX genes, with closest homologs found exclusively in other monocot species. ZmLOX12 is predominantly expressed in mesocotyls in which it is strongly induced in response to F. verticillioides infection. The Mutator transposon-insertional lox12-1 mutant is more susceptible to F. verticillioides colonization of mesocotyls, stalks, and kernels. The infected mutant kernels accumulate a significantly greater amount of the mycotoxin fumonisin. Reduced resistance to the pathogen is accompanied by diminished levels of the jasmonic acid (JA) precursor 12-oxo phytodienoic acid, JA-isoleucine, and expression of jasmonate-biosynthetic genes. Supporting the strong defense role of jasmonates, the JA-deficient opr7 opr8 double mutant displayed complete lack of immunity to F. verticillioides. Unexpectedly, the more susceptible lox12 mutant accumulated higher levels of kauralexins, suggesting that F. verticillioides is tolerant to this group of antimicrobial phytoalexins. This study demonstrates that this unique monocot-specific 9-LOX plays a key role in defense against F. verticillioides in diverse maize tissues and provides genetic evidence that JA is the major defense hormone against this pathogen.

Biosynthesis, elicitation and roles of monocot terpenoid phytoalexins

A long-standing goal in plant research is to optimize the protective function of biochemical agents that impede pest and pathogen attack. Nearly 40 years ago, pathogen-inducible diterpenoid production was described in rice, and these compounds were shown to function as antimicrobial phytoalexins. Using rice and maize as examples, we discuss recent advances in the discovery, biosynthesis, elicitation and functional characterization of monocot terpenoid phytoalexins. The recent expansion of known terpenoid phytoalexins now includes not only the labdane-related diterpenoid superfamily but also casbane-type diterpenoids and β-macrocarpene-derived sequiterpenoids. Biochemical approaches have been used to pair pathway precursors and end products with cognate biosynthetic genes. The number of predicted terpenoid phytoalexins is expanding through advances in cereal genome annotation and terpene synthase characterization that likewise enable discoveries outside the Poaceae. At the cellular level, conclusive evidence now exists for multiple plant receptors of fungal-derived chitin elicitors, phosphorylation of membrane-associated signaling complexes, activation of mitogen-activated protein kinase, involvement of phytohormone signals, and the existence of transcription factors that mediate the expression of phytoalexin biosynthetic genes and subsequent accumulation of pathway end products. Elicited production of terpenoid phytoalexins exhibit additional biological functions, including root exudate-mediated allelopathy and insect antifeedant activity. Such findings have encouraged consideration of additional interactions that blur traditionally discrete phytoalexin classifications. The establishment of mutant collections and increasing ease of genetic transformation assists critical examination of further biological roles. Future research directions include examination of terpenoid phytoalexin precursors and end products as potential signals mediating plant physiological processes.

Two closely related members of Arabidopsis 13-lipoxygenases (13-LOXs), LOX3 and LOX4, reveal distinct functions in response to plant-parasitic nematode infection

The responses of two closely related members of Arabidopsis 13-lipoxygenases (13-LOXs), LOX3 and LOX4, to infection by the sedentary nematodes root-knot nematode (Meloidogyne javanica) and cyst nematode (Heterodera schachtii) were analysed in transgenic Arabidopsis seedlings. The tissue localization of LOX3 and LOX4 gene expression using β-glucuronidase (GUS) reporter gene constructs showed local induction of LOX3 expression when second-stage juveniles reached the vascular bundle and during the early stages of plant-nematode interaction through gall and syncytia formation. Thin sections of nematode-infested knots indicated LOX3 expression in mature giant cells, and high expression in neighbouring cells and those surrounding the female body. LOX4 promoter was also activated by nematode infection, although the GUS signal weakened as infection and disease progressed. Homozygous insertion mutants lacking LOX3 were less susceptible than wild-type plants to root-knot nematode infection, as reflected by a decrease in female counts. Conversely, deficiency in LOX4 function led to a marked increase in females and egg mass number and in the female to male ratio of M. javanica and H. schachtii, respectively. The susceptibility of lox4 mutants was accompanied by increased expression of allene oxide synthase, allene oxide cyclase and ethylene-responsive transcription factor 4, and the accumulation of jasmonic acid, measured in the roots of lox4 mutants. This response was not found in lox3 mutants. Taken together, our results reveal that LOX4 and LOX3 interfere differentially with distinct metabolic and signalling pathways, and that LOX4 plays a major role in controlling plant defence against nematode infection.

The maize lipoxygenase, ZmLOX10, mediates green leaf volatile, jasmonate and herbivore-induced plant volatile production for defense against insect attack

Fatty acid derivatives are of central importance for plant immunity against insect herbivores; however, major regulatory genes and the signals that modulate these defense metabolites are vastly understudied, especially in important agro-economic monocot species. Here we show that products and signals derived from a single Zea mays (maize) lipoxygenase (LOX), ZmLOX10, are critical for both direct and indirect defenses to herbivory. We provide genetic evidence that two 13-LOXs, ZmLOX10 and ZmLOX8, specialize in providing substrate for the green leaf volatile (GLV) and jasmonate (JA) biosynthesis pathways, respectively. Supporting the specialization of these LOX isoforms, LOX8 and LOX10 are localized to two distinct cellular compartments, indicating that the JA and GLV biosynthesis pathways are physically separated in maize. Reduced expression of JA biosynthesis genes and diminished levels of JA in lox10 mutants indicate that LOX10-derived signaling is required for LOX8-mediated JA. The possible role of GLVs in JA signaling is supported by their ability to partially restore wound-induced JA levels in lox10 mutants. The impaired ability of lox10 mutants to produce GLVs and JA led to dramatic reductions in herbivore-induced plant volatiles (HIPVs) and attractiveness to parasitoid wasps. Because LOX10 is under circadian rhythm regulation, this study provides a mechanistic link to the diurnal regulation of GLVs and HIPVs. GLV-, JA- and HIPV-deficient lox10 mutants display compromised resistance to insect feeding, both under laboratory and field conditions, which is strong evidence that LOX10-dependent metabolites confer immunity against insect attack. Hence, this comprehensive gene to agro-ecosystem study reveals the broad implications of a single LOX isoform in herbivore defense.

Effects of vanadium on immunologic functions

Exposure of mice to vanadium caused a dose-related but nonsignificant decrease in the antibody-forming cells in the spleen of animals challenged with sheep erythrocytes. Delayed hypersensitivity reaction was not affected in similarly sensitized animals. Serum immunoglobulins were also not altered by the vanadium treatment. Splenic lymphocytes obtained at 1, 4, 8 and 13 wk of exposure to 0, 1, 10, and 50 mg of vanadium per liter of drinking water showed an increased DNA synthesis related to vanadium treatment when cultured in the presence of phytohemagglutinin and pokeweed mitogen but not with bacterial lipopolysaccharide. Addition of vanadium to splenic cultures in vitro caused a marked enhancement of lymphocyte transformation at low concentrations, whereas a decreased cellular proliferation was observed at high concentrations of vanadium.