Publications by authors named "Alisa Huffaker"

44 Publications

Getting back to the grass roots: harnessing specialized metabolites for improved crop stress resilience.

Curr Opin Biotechnol 2021 Jun 11;70:174-186. Epub 2021 Jun 11.

Section of Cell and Developmental Biology, University of California at San Diego, La Jolla, CA, USA. Electronic address:

Roots remain an understudied site of complex and important biological interactions mediating plant productivity. In grain and bioenergy crops, grass root specialized metabolites (GRSM) are central to key interactions, yet our basic knowledge of the chemical language remains fragmentary. Continued improvements in plant genome assembly and metabolomics are enabling large-scale advances in the discovery of specialized metabolic pathways as a means of regulating root-biotic interactions. Metabolomics, transcript coexpression analyses, forward genetic studies, gene synthesis and heterologous expression assays drive efficient pathway discoveries. Functional genetic variants identified through genome wide analyses, targeted CRISPR/Cas9 approaches, and both native and non-native overexpression studies critically inform novel strategies for bioengineering metabolic pathways to improve plant traits.
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http://dx.doi.org/10.1016/j.copbio.2021.05.010DOI Listing
June 2021

MutRank: an R shiny web-application for exploratory targeted mutual rank-based coexpression analyses integrated with user-provided supporting information.

PeerJ 2020 9;8:e10264. Epub 2020 Nov 9.

Division of Biology, University of California, San Diego, La Jolla, CA, USA.

The rapid assignment of genotypes to phenotypes has been a historically challenging process. The discovery of genes encoding biosynthetic pathway enzymes for defined plant specialized metabolites has been informed and accelerated by the detection of gene clusters. Unfortunately, biosynthetic pathway genes are commonly dispersed across chromosomes or reside in genes clusters that provide little predictive value. More reliably, transcript abundance of genes underlying biochemical pathways for plant specialized metabolites display significant coregulation. By rapidly identifying highly coexpressed transcripts, it is possible to efficiently narrow candidate genes encoding pathway enzymes and more easily predict both functions and functional associations. Mutual Rank (MR)-based coexpression analyses in plants accurately demonstrate functional associations for many specialized metabolic pathways; however, despite the clear predictive value of MR analyses, the application is uncommonly used to drive new pathway discoveries. Moreover, many coexpression databases aid in the prediction of both functional associations and gene functions, but lack customizability for refined hypothesis testing. To facilitate and speed flexible MR-based hypothesis testing, we developed MutRank, an R Shiny web-application for coexpression analyses. MutRank provides an intuitive graphical user interface with multiple customizable features that integrates user-provided data and supporting information suitable for personal computers. Tabular and graphical outputs facilitate the rapid analyses of both unbiased and user-defined coexpression results that accelerate gene function predictions. We highlight the recent utility of MR analyses for functional predictions and discoveries in defining two maize terpenoid antibiotic pathways. Beyond applications in biosynthetic pathway discovery, MutRank provides a simple, customizable and user-friendly interface to enable coexpression analyses relating to a breadth of plant biology inquiries. Data and code are available at GitHub: https://github.com/eporetsky/MutRank.
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http://dx.doi.org/10.7717/peerj.10264DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7659623PMC
November 2020

A receptor-like protein mediates plant immune responses to herbivore-associated molecular patterns.

Proc Natl Acad Sci U S A 2020 12 23;117(49):31510-31518. Epub 2020 Nov 23.

Section of Cell and Developmental Biology, Division of Biological Sciences, University of California San Diego, La Jolla, CA 92093;

Herbivory is fundamental to the regulation of both global food webs and the extent of agricultural crop losses. Induced plant responses to herbivores promote resistance and often involve the perception of specific herbivore-associated molecular patterns (HAMPs); however, precisely defined receptors and elicitors associated with herbivore recognition remain elusive. Here, we show that a receptor confers signaling and defense outputs in response to a defined HAMP common in caterpillar oral secretions (OS). Staple food crops, including cowpea () and common bean (), specifically respond to OS via recognition of proteolytic fragments of chloroplastic ATP synthase, termed inceptins. Using forward-genetic mapping of inceptin-induced plant responses, we identified a corresponding leucine-rich repeat receptor, termed INR, specific to select legume species and sufficient to confer inceptin-induced responses and enhanced defense against armyworms () in tobacco. Our results support the role of plant immune receptors in the perception of chewing herbivores and defense.
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http://dx.doi.org/10.1073/pnas.2018415117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7733821PMC
December 2020

Genetic elucidation of interconnected antibiotic pathways mediating maize innate immunity.

Nat Plants 2020 11 26;6(11):1375-1388. Epub 2020 Oct 26.

Section of Cell and Developmental Biology, University of California at San Diego, La Jolla, CA, USA.

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.
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http://dx.doi.org/10.1038/s41477-020-00787-9DOI Listing
November 2020

Differential activities of maize plant elicitor peptides as mediators of immune signaling and herbivore resistance.

Plant J 2020 12 2;104(6):1582-1602. Epub 2020 Dec 2.

Division of Biology, University of California San Diego, La Jolla, CA, USA.

Plant elicitor peptides (Peps) are conserved regulators of defense responses and models for the study of damage-associated molecular pattern-induced immunity. Although present as multigene families in most species, the functional relevance of these multigene families remains largely undefined. While Arabidopsis Peps appear largely redundant in function, previous work examining Pep-induced responses in maize (Zm) implied specificity of function. To better define the function of individual ZmPeps and their cognate receptors (ZmPEPRs), activities were examined by assessing changes in defense-associated phytohormones, specialized metabolites and global gene expression patterns, in combination with heterologous expression assays and analyses of CRISPR/Cas9-generated knockout plants. Beyond simply delineating individual ZmPep and ZmPEPR activities, these experiments led to a number of new insights into Pep signaling mechanisms. ZmPROPEP and other poaceous precursors were found to contain multiple active Peps, a phenomenon not previously observed for this family. In all, seven new ZmPeps were identified and the peptides were found to have specific activities defined by the relative magnitude of their response output rather than by uniqueness. A striking correlation was observed between individual ZmPep-elicited changes in levels of jasmonic acid and ethylene and the magnitude of induced defense responses, indicating that ZmPeps may collectively regulate immune output through rheostat-like tuning of phytohormone levels. Peptide structure-function studies and ligand-receptor modeling revealed structural features critical to the function of ZmPeps and led to the identification of ZmPep5a as a potential antagonist peptide able to competitively inhibit the activity of other ZmPeps, a regulatory mechanism not previously observed for this family.
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http://dx.doi.org/10.1111/tpj.15022DOI Listing
December 2020

Dynamic regulation of Pep-induced immunity through post-translational control of defence transcript splicing.

Nat Plants 2020 08 20;6(8):1008-1019. Epub 2020 Jul 20.

Section of Cell and Developmental Biology, UC San Diego, San Diego, CA, USA.

The survival of all living organisms requires the ability to detect attacks and swiftly counter them with protective immune responses. Despite considerable mechanistic advances, the interconnectivity of signalling modules often remains unclear. A newly characterized protein, IMMUNOREGULATORY RNA-BINDING PROTEIN (IRR), negatively regulates immune responses in both maize and Arabidopsis, with disrupted function resulting in enhanced disease resistance. IRR associates with and promotes canonical splicing of transcripts encoding defence signalling proteins, including the key negative regulator of pattern-recognition receptor signalling complexes, CALCIUM-DEPENDENT PROTEIN KINASE 28 (CPK28). On immune activation by Plant Elicitor Peptides (Peps), IRR is dephosphorylated, disrupting interaction with CPK28 transcripts and resulting in the accumulation of an alternative splice variant encoding a truncated CPK28 protein with impaired kinase activity and diminished function as a negative regulator. We demonstrate a new mechanism linking Pep-induced post-translational modification of IRR with post-transcriptionally mediated attenuation of CPK28 function to dynamically amplify Pep signalling and immune output.
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http://dx.doi.org/10.1038/s41477-020-0724-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7482133PMC
August 2020

Multiple genes recruited from hormone pathways partition maize diterpenoid defences.

Nat Plants 2019 10 16;5(10):1043-1056. Epub 2019 Sep 16.

Section of Cell and Developmental Biology, University of California San Diego, La Jolla, CA, USA.

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.
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http://dx.doi.org/10.1038/s41477-019-0509-6DOI Listing
October 2019

Chemical genetic identification of a lectin receptor kinase that transduces immune responses and interferes with abscisic acid signaling.

Plant J 2019 05 7;98(3):492-510. Epub 2019 Mar 7.

Division of Biological Sciences, Cell and Developmental Biology Section, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA, 92093-0116, USA.

Insight into how plants simultaneously cope with multiple stresses, for example, when challenged with biotic stress from pathogen infection and abiotic stress from drought, is important both for understanding evolutionary trade-offs and optimizing crop responses to these stresses. Mechanisms by which initial plant immune signaling antagonizes abscisic acid (ABA) signal transduction require further investigation. Using a chemical genetics approach, the small molecule [5-(3,4-dichlorophenyl)furan-2-yl]-piperidine-1-ylmethanethione (DFPM) has previously been identified due to its ability to suppress ABA signaling via plant immune signaling components. Here, we have used forward chemical genetics screening to identify DFPM-insensitive loci by monitoring the activity of ABA-inducible pRAB18::GFP in the presence of DFPM and ABA. The ability of DFPM to attenuate ABA signaling was reduced in rda mutants (resistant to DFPM inhibition of ABA signaling). One of the mutants, rda2, was mapped and is defective in a gene encoding a lectin receptor kinase. RDA2 functions in DFPM-mediated inhibition of ABA-mediated reporter expression. RDA2 is required for DFPM-mediated activation of immune signaling, including phosphorylation of mitogen-activated protein kinase (MAPK) 3 (MPK3) and MPK6, and induction of immunity marker genes. Our study identifies a previously uncharacterized receptor kinase gene that is important for DFPM-mediated immune signaling and inhibition of ABA signaling. We demonstrate that the lectin receptor kinase RDA2 is essential for perceiving the DFPM signal and activating MAPKs, and that MKK4 and MKK5 are required for DFPM interference with ABA signal transduction.
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http://dx.doi.org/10.1111/tpj.14232DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6488365PMC
May 2019

A fragmentation study of isoflavones by IT-TOF-MS using biosynthesized isotopes.

Biosci Biotechnol Biochem 2018 Aug 26;82(8):1309-1315. Epub 2018 Apr 26.

a Graduate School of Agriculture , Kyoto University , Kyoto , Japan.

To aid in the identification and quantification of biologically and agriculturally significant natural products, tandem mass spectrometry can provide accurate structural information with high selectivity and sensitivity. In this study, diagnostic fragmentation patterns of isoflavonoids were examined by liquid chromatography-ion trap-time of flight-mass spectrometry (LC-IT-TOF-MS). The fragmentation scheme for [M+H-2CO] ions derived from isoflavones and [M+H-B-ring-CO] ions derived from 5-hydroxyisoflavones, were investigated using different isotopically labeled isoflavones, specifically [1',2',3',4',5',6',2,3,4-C] and [2',3',5',6',2-D] isoflavones. Specific isotopically labeled isoflavones were prepared through the biosynthetic incorporation of pharmacologically applied C- and D-labelled L-phenylalanine precursors in soybean plants following the application of insect elicitors. Using this approach, we empirically demonstrate that the [M+H-2CO] ion is generated by an intramolecular proton rearrangement during fragmentation. Furthermore, [M+H-B-ring-CO] ion is demonstrated to contain a CH moiety derived from C-ring of 5-hydroxyisoflavones. A mechanistic understanding of characteristic isoflavone fragmentation patterns contributes to the efficacy and confidence in identifying related isoflavones by LC-MS.
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http://dx.doi.org/10.1080/09168451.2018.1465810DOI Listing
August 2018

An apoplastic peptide activates salicylic acid signalling in maize.

Nat Plants 2018 03 26;4(3):172-180. Epub 2018 Feb 26.

Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), University of Cologne, BioCenter, Cologne, Germany.

Localized control of cell death is crucial for the resistance of plants to pathogens. Papain-like cysteine proteases (PLCPs) regulate plant defence to drive cell death and protection against biotrophic pathogens. In maize (Zea mays), PLCPs are crucial in the orchestration of salicylic acid (SA)-dependent defence signalling. Despite this central role in immunity, it remains unknown how PLCPs are activated, and which downstream signals they induce to trigger plant immunity. Here, we discover an immune signalling peptide, Z. mays immune signalling peptide 1 (Zip1), which is produced after salicylic acid (SA) treatment. In vitro studies demonstrate that PLCPs are required to release bioactive Zip1 from its propeptide precursor. Conversely, Zip1 treatment strongly elicits SA accumulation in leaves. Moreover, transcriptome analyses revealed that Zip1 and SA induce highly overlapping transcriptional changes. Consequently, Zip1 promotes the infection of the necrotrophic fungus Botrytis cinerea, while it reduces virulence of the biotrophic fungus Ustilago maydis. Thus, Zip1 represents the previously missing signal that is released by PLCPs to activate SA defence signalling.
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http://dx.doi.org/10.1038/s41477-018-0116-yDOI Listing
March 2018

Discovery, Biosynthesis and Stress-Related Accumulation of Dolabradiene-Derived Defenses in Maize.

Plant Physiol 2018 04 23;176(4):2677-2690. Epub 2018 Feb 23.

Department of Plant Biology, University of California, Davis, California

Terpenoids are a major component of maize () chemical defenses that mediate responses to herbivores, pathogens, and other environmental challenges. Here, we describe the biosynthesis and elicited production of a class of maize diterpenoids, named dolabralexins. Dolabralexin biosynthesis involves the sequential activity of two diterpene synthases, -COPALYL DIPHOSPHATE SYNTHASE (ZmAN2) and KAURENE SYNTHASE-LIKE4 (ZmKSL4). Together, ZmAN2 and ZmKSL4 form the diterpene hydrocarbon dolabradiene. In addition, we biochemically characterized a cytochrome P450 monooxygenase, ZmCYP71Z16, which catalyzes the oxygenation of dolabradiene to yield the epoxides 15,16-epoxydolabrene (epoxydolabrene) and 3β-hydroxy-15,16-epoxydolabrene (epoxydolabranol). The absence of dolabradiene and epoxydolabranol in mutants under elicited conditions confirmed the in vivo biosynthetic requirement of ZmAN2. Combined mass spectrometry and NMR experiments demonstrated that much of the epoxydolabranol is further converted into 3β,15,16-trihydroxydolabrene (trihydroxydolabrene). Metabolite profiling of field-grown maize root tissues indicated that dolabralexin biosynthesis is widespread across common maize cultivars, with trihydroxydolabrene as the predominant diterpenoid. Oxidative stress induced dolabralexin accumulation and transcript expression of and in root tissues, and metabolite and transcript accumulation were up-regulated in response to elicitation with the fungal pathogens and Consistently, epoxydolabranol significantly inhibited the growth of both pathogens in vitro at 10 µg mL, while trihydroxydolabrene-mediated inhibition was specific to These findings suggest that dolabralexins have defense-related roles in maize stress interactions and expand the known chemical space of diterpenoid defenses as genetic targets for understanding and ultimately improving maize resilience.
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http://dx.doi.org/10.1104/pp.17.01351DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5884620PMC
April 2018

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

J Exp Bot 2018 03;69(7):1693-1705

Section of Cell and Developmental Biology, University of California at San Diego, La Jolla, CA, USA.

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.
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http://dx.doi.org/10.1093/jxb/erx495DOI Listing
March 2018

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

Planta 2018 Apr;247(4):863-873

Section of Cell and Developmental Biology, University of California at San Diego, La Jolla, CA, 92093-0380, USA.

Main Conclusion: 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 . 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 CO (E-CO) on both fungal and herbivore-elicited ZA4 production revealed significantly reduced levels of inducible ZA4 accumulation, consistent with a negative role for E-CO 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.
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http://dx.doi.org/10.1007/s00425-017-2830-5DOI Listing
April 2018

Selinene Volatiles Are Essential Precursors for Maize Defense Promoting Fungal Pathogen Resistance.

Plant Physiol 2017 Nov 20;175(3):1455-1468. Epub 2017 Sep 20.

Section of Cell and Developmental Biology, University of California San Diego, La Jolla, California 92093-0380

To ensure food security, maize () is a model crop for understanding useful traits underlying stress resistance. In contrast to foliar biochemicals, root defenses limiting the spread of disease remain poorly described. To better understand belowground defenses in the field, we performed root metabolomic profiling and uncovered unexpectedly high levels of the sesquiterpene volatile β-selinene and the corresponding nonvolatile antibiotic derivative β-costic acid. The application of metabolite-based quantitative trait locus mapping using biparental populations, genome-wide association studies, and near-isogenic lines enabled the identification of terpene synthase21 () on chromosome 9 as a β-costic acid pathway candidate gene. Numerous closely examined β-costic acid-deficient inbred lines were found to harbor pseudogenes lacking conserved motifs required for farnesyl diphosphate cyclase activity. For biochemical validation, a full-length was cloned, heterologously expressed in , and demonstrated to cyclize farnesyl diphosphate, yielding β-selinene as the dominant product. Consistent with microbial defense pathways, transcripts strongly accumulate following fungal elicitation. Challenged field roots containing functional alleles displayed β-costic acid levels over 100 μg g fresh weight, greatly exceeding in vitro concentrations required to inhibit the growth of five different fungal pathogens and rootworm larvae (). In vivo disease resistance assays, using and near-isogenic lines, further support the endogenous antifungal role of selinene-derived metabolites. Involved in the biosynthesis of nonvolatile antibiotics, exists as a useful gene for germplasm improvement programs targeting optimized biotic stress resistance.
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http://dx.doi.org/10.1104/pp.17.00879DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5664469PMC
November 2017

Extracellular Alkalinization Assay for the Detection of Early Defense Response.

Curr Protoc Plant Biol 2017 Sep;2(3):210-220

Department of Plant Pathology, Washington State University, Pullman, Washington.

Plant recognition of invading organisms occurs through identification of foreign molecules associated with attackers and of self-derived, damage-associated molecules. Perception of these molecules activates signaling processes including dynamic changes in ion balance, production of second messengers such as reactive oxygen species and nitric oxide, increased levels of plant hormones, and map kinase cascade activation. Together these signaling events stimulate transcriptional changes to initiate plant defense responses. Among the earliest detectable signaling events is a rapid increase in apoplastic pH, i.e., extracellular alkalinization. Here, an assay for quantification of this alkalinization response using suspension-cultured cell lines for Arabidopsis, potato, and maize is described. This assay is an inexpensive, fast, simple, and reproducible method to quantify defense signaling output, providing a powerful tool for evaluating early plant responses to elicitors and pathogens. Results from the alkalinization assay are comparable to other more costly and time-consuming methods for assessing defense signaling, such as measurement of the oxidative burst, calcium influx, and marker gene expression. This bioassay is a quantitative and robust method for evaluation of plant defense output. © 2017 by John Wiley & Sons, Inc.
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http://dx.doi.org/10.1002/cppb.20057DOI Listing
September 2017

Plant elicitor peptides promote plant defences against nematodes in soybean.

Mol Plant Pathol 2018 04 22;19(4):858-869. Epub 2017 Aug 22.

Department of Entomology, University of Arkansas, Fayetteville, AR 72701, USA.

Plant elicitor peptides (Peps) are widely distributed among angiosperms, and have been shown to amplify immune responses in multiple plant families. Here, we characterize three Peps from soybean (Glycine max) and describe their effects on plant defences against two damaging agricultural pests, the root-knot nematode (Meloidogyne incognita) and the soybean cyst nematode (Heterodera glycines). Seed treatments with exogenous GmPep1, GmPep2 or GmPep3 significantly reduced the reproduction of both nematodes. Pep treatment also protected plants from the inhibitory effects of root-knot nematodes on above-ground growth, and up-regulated basal expression levels of nematode-responsive defence genes. GmPep1 induced the expression of its propeptide precursor (GmPROPEP1), a nucleotide-binding site leucine-rich repeat protein (NBS-LRR), a pectin methylesterase inhibitor (PMEI), Respiratory Burst Oxidase Protein D (RBOHD) and the accumulation of reactive oxygen species (ROS) in leaves. In addition, GmPep2 and GmPep3 seed treatments up-regulated RBOHD expression and ROS accumulation in roots and leaves. These results suggest that GmPeps activate plant defences through systemic transcriptional reprogramming and ROS signalling, and that Pep seed treatments represent a potential strategy for nematode management.
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http://dx.doi.org/10.1111/mpp.12570DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6638146PMC
April 2018

Foundational and Translational Research Opportunities to Improve Plant Health.

Mol Plant Microbe Interact 2017 07 12;30(7):515-516. Epub 2017 Jun 12.

27 Cornell University, Ithaca, NY, U.S.A.

Reader Comments | Submit a Comment The white paper reports the deliberations of a workshop focused on biotic challenges to plant health held in Washington, D.C. in September 2016. Ensuring health of food plants is critical to maintaining the quality and productivity of crops and for sustenance of the rapidly growing human population. There is a close linkage between food security and societal stability; however, global food security is threatened by the vulnerability of our agricultural systems to numerous pests, pathogens, weeds, and environmental stresses. These threats are aggravated by climate change, the globalization of agriculture, and an over-reliance on nonsustainable inputs. New analytical and computational technologies are providing unprecedented resolution at a variety of molecular, cellular, organismal, and population scales for crop plants as well as pathogens, pests, beneficial microbes, and weeds. It is now possible to both characterize useful or deleterious variation as well as precisely manipulate it. Data-driven, informed decisions based on knowledge of the variation of biotic challenges and of natural and synthetic variation in crop plants will enable deployment of durable interventions throughout the world. These should be integral, dynamic components of agricultural strategies for sustainable agriculture.
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http://dx.doi.org/10.1094/MPMI-01-17-0010-CRDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5810936PMC
July 2017

Inducible De Novo Biosynthesis of Isoflavonoids in Soybean Leaves by Spodoptera litura Derived Elicitors: Tracer Techniques Aided by High Resolution LCMS.

J Chem Ecol 2016 Dec 8;42(12):1226-1236. Epub 2016 Nov 8.

Graduate School of Agriculture, Kyoto University, Kitashirakawa, Sakyo, Kyoto, Kyoto, 606-8502, Japan.

Isoflavonoids are a characteristic family of natural products in legumes known to mediate a range of plant-biotic interactions. For example, in soybean (Glycine max: Fabaceae) multiple isoflavones are induced and accumulate in leaves following attack by Spodoptera litura (Lepidoptera: Noctuidae) larvae. To quantitatively examine patterns of activated de novo biosynthesis, soybean (Var. Enrei) leaves were treated with a combination of plant defense elicitors present in S. litura gut content extracts and L-α-[C, N]phenylalanine as a traceable isoflavonoid precursor. Combined treatments promoted significant increases in C-labeled isoflavone aglycones (daidzein, formononetin, and genistein), C-labeled isoflavone 7-O-glucosides (daidzin, ononin, and genistin), and C-labeled isoflavone 7-O-(6″-O-malonyl-β-glucosides) (malonyldaidzin, malonylononin, and malonylgenistin). In contrast levels of C-labeled flavones and flavonol (4',7-dihydroxyflavone, kaempferol, and apigenin) were not significantly altered. Curiously, application of fatty acid-amino acid conjugate (FAC) elicitors present in S. litura gut contents, namely N-linolenoyl-L-glutamine and N-linoleoyl-L-glutamine, both promoted the induced accumulation of isoflavone 7-O-glucosides and isoflavone 7-O-(6″-O-malonyl-β-glucosides), but not isoflavone aglycones in the leaves. These results demonstrate that at least two separate reactions are involved in elicitor-induced soybean leaf responses to the S. litura gut contents: one is the de novo biosynthesis of isoflavone conjugates induced by FACs, and the other is the hydrolysis of the isoflavone conjugates to yield isoflavone aglycones. Gut content extracts alone displayed no hydrolytic activity. The quantitative analysis of isoflavone de novo biosynthesis, with respect to both aglycones and conjugates, affords a useful bioassay system for the discovery of additional plant defense elicitor(s) in S. litura gut contents that specifically promote hydrolysis of isoflavone conjugates.
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http://dx.doi.org/10.1007/s10886-016-0786-8DOI Listing
December 2016

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

PLoS One 2016 13;11(7):e0159270. Epub 2016 Jul 13.

Chemistry Research Unit, Center of Medical, Agricultural, and Veterinary Entomology, United States Department of Agriculture, Agricultural Research Service, 1600 SW 23rd Drive, Gainesville, Florida, 32608, United States of America.

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.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0159270PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4943682PMC
August 2017

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

Plant Signal Behav 2016 ;11(2):e1120395

b Section of Cell and Developmental Biology, University of California at San Diego , La Jolla , CA , USA.

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.
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http://dx.doi.org/10.1080/15592324.2015.1120395DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4883972PMC
December 2016

Dynamic Maize Responses to Aphid Feeding Are Revealed by a Time Series of Transcriptomic and Metabolomic Assays.

Plant Physiol 2015 Nov 16;169(3):1727-43. Epub 2015 Sep 16.

Boyce Thompson Institute for Plant Research, Ithaca, New York 14853 (V.T., N.F.-P., K.R.A., L.N.M., H.K., L.A.M., G.J.);Martin Luther University Halle, Wittenberg Institute for Pharmacy, D-06108 Halle, Germany (A.R., J.D.);Division of Biological Sciences, University of California, La Jolla, California 92093 (E.A.S., A.H.);Department of Molecular Ecology, Max Planck Institute for Chemical Ecology, D-07745 Jena, Germany (M.Scho., M.Schä.); andGraduate School of Agriculture, Kyoto University, Kyoto 808-8502, Japan (N.M.)

As a response to insect attack, maize (Zea mays) has inducible defenses that involve large changes in gene expression and metabolism. Piercing/sucking insects such as corn leaf aphid (Rhopalosiphum maidis) cause direct damage by acquiring phloem nutrients as well as indirect damage through the transmission of plant viruses. To elucidate the metabolic processes and gene expression changes involved in maize responses to aphid attack, leaves of inbred line B73 were infested with corn leaf aphids for 2 to 96 h. Analysis of infested maize leaves showed two distinct response phases, with the most significant transcriptional and metabolic changes occurring in the first few hours after the initiation of aphid feeding. After 4 d, both gene expression and metabolite profiles of aphid-infested maize reverted to being more similar to those of control plants. Although there was a predominant effect of salicylic acid regulation, gene expression changes also indicated prolonged induction of oxylipins, although not necessarily jasmonic acid, in aphid-infested maize. The role of specific metabolic pathways was confirmed using Dissociator transposon insertions in maize inbred line W22. Mutations in three benzoxazinoid biosynthesis genes, Bx1, Bx2, and Bx6, increased aphid reproduction. In contrast, progeny production was greatly decreased by a transposon insertion in the single W22 homolog of the previously uncharacterized B73 terpene synthases TPS2 and TPS3. Together, these results show that maize leaves shift to implementation of physical and chemical defenses within hours after the initiation of aphid feeding and that the production of specific metabolites can have major effects in maize-aphid interactions.
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http://dx.doi.org/10.1104/pp.15.01039DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4634079PMC
November 2015

Maize death acids, 9-lipoxygenase-derived cyclopente(a)nones, display activity as cytotoxic phytoalexins and transcriptional mediators.

Proc Natl Acad Sci U S A 2015 Sep 24;112(36):11407-12. Epub 2015 Aug 24.

Section of Cell and Developmental Biology, University of California at San Diego, La Jolla, CA 92093-0380;

Plant damage promotes the interaction of lipoxygenases (LOXs) with fatty acids yielding 9-hydroperoxides, 13-hydroperoxides, and complex arrays of oxylipins. The action of 13-LOX on linolenic acid enables production of 12-oxo-phytodienoic acid (12-OPDA) and its downstream products, termed "jasmonates." As signals, jasmonates have related yet distinct roles in the regulation of plant resistance against insect and pathogen attack. A similar pathway involving 9-LOX activity on linolenic and linoleic acid leads to the 12-OPDA positional isomer, 10-oxo-11-phytodienoic acid (10-OPDA) and 10-oxo-11-phytoenoic acid (10-OPEA), respectively; however, physiological roles for 9-LOX cyclopentenones have remained unclear. In developing maize (Zea mays) leaves, southern leaf blight (Cochliobolus heterostrophus) infection results in dying necrotic tissue and the localized accumulation of 10-OPEA, 10-OPDA, and a series of related 14- and 12-carbon metabolites, collectively termed "death acids." 10-OPEA accumulation becomes wound inducible within fungal-infected tissues and at physiologically relevant concentrations acts as a phytoalexin by suppressing the growth of fungi and herbivores including Aspergillus flavus, Fusarium verticillioides, and Helicoverpa zea. Unlike previously established maize phytoalexins, 10-OPEA and 10-OPDA display significant phytotoxicity. Both 12-OPDA and 10-OPEA promote the transcription of defense genes encoding glutathione S transferases, cytochrome P450s, and pathogenesis-related proteins. In contrast, 10-OPEA only weakly promotes the accumulation of multiple protease inhibitor transcripts. Consistent with a role in dying tissue, 10-OPEA application promotes cysteine protease activation and cell death, which is inhibited by overexpression of the cysteine protease inhibitor maize cystatin-9. Unlike jasmonates, functions for 10-OPEA and associated death acids are consistent with specialized roles in local defense reactions.
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http://dx.doi.org/10.1073/pnas.1511131112DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4568653PMC
September 2015

Plant elicitor peptides in induced defense against insects.

Authors:
Alisa Huffaker

Curr Opin Insect Sci 2015 Jun 4;9:44-50. Epub 2015 Jul 4.

University of California, San Diego, Section of Cell & Developmental Biology, 9500 Gilman Drive, #0116, La Jolla, CA 92093-0116, USA. Electronic address:

Over 20 years ago the peptide systemin was discovered to be an integral regulator of anti-herbivore defense responses in Solanaceous plants. However, other peptides of similar function have remained elusive. Plant Elicitor Peptides (Peps) were initially discovered in Arabidopsis as mediators of basal immune responses protective against invading pathogens. Recently a Pep from maize, ZmPep3, was demonstrated to be a potent regulator of anti-herbivore defenses. ZmPep3 was as active as the Lepidopteran elicitor N-linolenoyl-l-glutamine (Gln-18:3) in stimulating volatile emission and accumulation of defense transcripts and metabolites, resulting in both attraction of the parasitoid Cotesia marginiventris and suppressed growth of Spodoptera exigua larvae. Orthologues of Peps in Solanaceous and Fabaceous plants also trigger emission of herbivore-associated volatiles, indicating that Peps have a conserved role as regulators of plant defense against herbivores in diverse species. This conservation of a peptide signal and cognate receptor for activation of plant defense responses reveals a widespread regulatory motif and provides opportunities for manipulation of plant resistance.
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http://dx.doi.org/10.1016/j.cois.2015.06.003DOI Listing
June 2015

Accumulation of terpenoid phytoalexins in maize roots is associated with drought tolerance.

Plant Cell Environ 2015 Nov 23;38(11):2195-207. Epub 2015 Jan 23.

Chemistry Research Unit, Center of Medical, Agricultural, and Veterinary Entomology, Agricultural Research Service, United States Department of Agriculture, Gainesville, FL, 32608, USA.

Maize (Zea mays) production, which is of global agro-economic importance, is largely limited by herbivore pests, pathogens and environmental conditions, such as drought. Zealexins and kauralexins belong to two recently identified families of acidic terpenoid phytoalexins in maize that mediate defence against both pathogen and insect attacks in aboveground tissues. However, little is known about their function in belowground organs and their potential to counter abiotic stress. In this study, we show that zealexins and kauralexins accumulate in roots in response to both biotic and abiotic stress including, Diabrotica balteata herbivory, Fusarium verticillioides infection, drought and high salinity. We find that the quantity of drought-induced phytoalexins is positively correlated with the root-to-shoot ratio of different maize varieties, and further demonstrate that mutant an2 plants deficient in kauralexin production are more sensitive to drought. The induction of phytoalexins in response to drought is root specific and does not influence phytoalexin levels aboveground; however, the accumulation of phytoalexins in one tissue may influence the induction capacity of other tissues.
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http://dx.doi.org/10.1111/pce.12482DOI Listing
November 2015

Insect-induced daidzein, formononetin and their conjugates in soybean leaves.

Metabolites 2014 Jul 4;4(3):532-46. Epub 2014 Jul 4.

Graduate School of Agriculture, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-8502, Japan.

In response to attack by bacterial pathogens, soybean (Gylcine max) leaves accumulate isoflavone aglucones, isoflavone glucosides, and glyceollins. In contrast to pathogens, the dynamics of related insect-inducible metabolites in soybean leaves remain poorly understood. In this study, we analyzed the biochemical responses of soybean leaves to Spodoptera litura (Lepidoptera: Noctuidae) herbivory and also S. litura gut contents, which contain oral secretion elicitors. Following S. litura herbivory, soybean leaves displayed an induced accumulation of the flavone and isoflavone aglycones 4',7-dihyroxyflavone, daidzein, and formononetin, and also the isoflavone glucoside daidzin. Interestingly, foliar application of S. litura oral secretions also elicited the accumulation of isoflavone aglycones (daidzein and formononetin), isoflavone 7-O-glucosides (daidzin, ononin), and isoflavone 7-O-(6'-O-malonyl-β-glucosides) (malonyldaidzin, malonylononin). Consistent with the up-regulation of the isoflavonoid biosynthetic pathway, folair phenylalanine levels also increased following oral secretion treatment. To establish that these metabolitic changes were the result of de novo biosynthesis, we demonstrated that labeled (13C9) phenylalanine was incorporated into the isoflavone aglucones. These results are consistent with the presence of soybean defense elicitors in S. litura oral secretions. We demonstrate that isoflavone aglycones and isoflavone conjugates are induced in soybean leaves, not only by pathogens as previously demonstrated, but also by foliar insect herbivory.
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http://dx.doi.org/10.3390/metabo4030532DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4192678PMC
July 2014

Influence of brown stink bug feeding, planting date and sampling time on common smut infection of maize.

Insect Sci 2014 Oct 20;21(5):564-71. Epub 2014 Aug 20.

USDA-ARS, Crop Genetics and Breeding Research Unit, Tifton, GA, 31793.

Phytopathogen infections are frequently influenced by both biotic and abiotic factors in a crop field. The effect of brown stink bug, Euschistus servus (Hemiptera: Pentatomidae), feeding and planting date and sampling time on common smut (Ustilago maydis) infection percentage of maize plants was examined in 2005 and 2006, and 2010 and 2011, respectively. Brown stink bug adult feeding on maize hybrid "DKC6971" at flowering in 2005 and 2006 did not influence smut infection percentage when examined using 3 treatments (i.e., 0 adult, 5 adults, and 5 adults mixed with the smut spores). The smut infection percentages were <3% (n = 12) in the 3 treatments. The smut infection percentage among the 4 weekly samplings was the same, so was natural aflatoxin contamination at harvest among the treatments. The 2nd experiment showed that planting date did not affect the smut infection percentage in either 2010 or 2011. But, the smut infection percentage from the postflowering sampling was greater than preflowering sampling in both years. The smut infection percentage varied among the germplasm lines in 2010, but not in 2011. This study demonstrated that brown stink bug feeding at flowering had no effect on smut infection in maize, and the best time for smut evaluation would be after flowering. The temperature and precipitation might have also influenced the percentage of smut-infected maize plants during the 4 years when the experiments were conducted. The similarity between kernel-colonizing U. maydis and Aspergillus flavus infections and genotype × environment interaction were also discussed.
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http://dx.doi.org/10.1111/1744-7917.12149DOI Listing
October 2014

Effects of elevated [CO2 ] on maize defence against mycotoxigenic Fusarium verticillioides.

Plant Cell Environ 2014 Dec 13;37(12):2691-706. Epub 2014 May 13.

Chemistry Research Unit, Center of Medical, Agricultural, and Veterinary Entomology, U.S. Department of Agriculture, Agricultural Research Service, Gainesville, FL, 32608, USA.

Maize is by quantity the most important C4 cereal crop; however, future climate changes are expected to increase maize susceptibility to mycotoxigenic fungal pathogens and reduce productivity. While rising atmospheric [CO2 ] is a driving force behind the warmer temperatures and drought, which aggravate fungal disease and mycotoxin accumulation, our understanding of how elevated [CO2 ] will effect maize defences against such pathogens is limited. Here we report that elevated [CO2 ] increases maize susceptibility to Fusarium verticillioides proliferation, while mycotoxin levels are unaltered. Fumonisin production is not proportional to the increase in F. verticillioides biomass, and the amount of fumonisin produced per unit pathogen is reduced at elevated [CO2 ]. Following F. verticillioides stalk inoculation, the accumulation of sugars, free fatty acids, lipoxygenase (LOX) transcripts, phytohormones and downstream phytoalexins is dampened in maize grown at elevated [CO2 ]. The attenuation of maize 13-LOXs and jasmonic acid production correlates with reduced terpenoid phytoalexins and increased susceptibility. Furthermore, the attenuated induction of 9-LOXs, which have been suggested to stimulate mycotoxin biosynthesis, is consistent with reduced fumonisin per unit fungal biomass at elevated [CO2 ]. Our findings suggest that elevated [CO2 ] will compromise maize LOX-dependent signalling, which will influence the interactions between maize and mycotoxigenic fungi.
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http://dx.doi.org/10.1111/pce.12337DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4278449PMC
December 2014

A 13-lipoxygenase, TomloxC, is essential for synthesis of C5 flavour volatiles in tomato.

J Exp Bot 2014 Feb 22;65(2):419-28. Epub 2014 Jan 22.

Horticultural Sciences, University of Florida, PO Box 110690, Gainesville, FL 32611-0690, USA.

C5 volatile compounds, derived from fatty acids, are among the most important contributors to consumer liking of fresh tomatoes. Despite their important roles in flavour, the genes responsible for C5 volatile synthesis have yet to be identified. This work shows that their synthesis is catalysed in part by a 13-lipoxygenase (LOX), TomloxC, the same enzyme responsible for synthesis of C6 volatiles. C5 synthesis is independent of hydroperoxide lyase (HPL); moreover, HPL knockdown significantly increased C5 volatile synthesis. This LOX-dependent, HPL-independent pathway functions in both fruits and leaves. Synthesis of C5 volatiles increases in leaves following mechanical wounding but does not increase in response to infection with Xanthomonas campestris pv. vesicatoria. Large reductions in C5 and C6 volatiles in antisense TomloxC knockdown plants were observed but those reductions did not alter the development of disease symptoms, indicating that these volatiles do not have an important defensive function against this bacterial pathogen.
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http://dx.doi.org/10.1093/jxb/ert382DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3904703PMC
February 2014

Biosynthesis, elicitation and roles of monocot terpenoid phytoalexins.

Plant J 2014 Aug 26;79(4):659-78. Epub 2014 Mar 26.

Center for Medical, Agricultural, and Veterinary Entomology, US Department of Agriculture, Agricultural Research Service, Chemistry Research Unit, Gainesville, FL, 32608, USA.

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.
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http://dx.doi.org/10.1111/tpj.12436DOI Listing
August 2014

European corn borer (Ostrinia nubilalis) induced responses enhance susceptibility in maize.

PLoS One 2013 2;8(9):e73394. Epub 2013 Sep 2.

Center of Medical, Agricultural and Veterinary Entomology, Agricultural Research Service, U.S. Department of Agriculture, Gainesville, Florida, United States of America.

Herbivore-induced plant responses have been widely described following attack on leaves; however, less attention has been paid to analogous local processes that occur in stems. Early studies of maize (Zea mays) responses to stem boring by European corn borer (ECB, Ostrinianubilalis) larvae revealed the presence of inducible acidic diterpenoid phytoalexins, termed kauralexins, and increases in the benzoxazinoid 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one-glucose (HDMBOA-Glc) after 24 h of herbivory. Despite these rapidly activated defenses, larval growth was not altered in short-term feeding assays. Unexpectedly, ECB growth significantly improved in assays using stem tissue preconditioned by 48 h of larval tunneling. Correspondingly, measures of total soluble protein increased over 2.6-fold in these challenged tissues and were accompanied by elevated levels of sucrose and free linoleic acid. While microarray analyses revealed up-regulation of over 1100 transcripts, fewer individual protein increases were demonstrable. Consistent with induced endoreduplication, both wounding and ECB stem attack resulted in similar significant expansion of the nucleus, nucleolus and levels of extractable DNA from challenged tissues. While many of these responses are triggered by wounding alone, biochemical changes further enhanced in response to ECB may be due to larval secreted effectors. Unlike other Lepidoptera examined, ECB excrete exceedingly high levels of the auxin indole-3-acetic acid (IAA) in their frass which is likely to contact and contaminate the surrounding feeding tunnel. Stem exposure to a metabolically stable auxin, such as 2,4-dichlorophenoxyacetic acid (2,4-D), promoted significant protein accumulation above wounding alone. As a future testable hypothesis, we propose that ECB-associated IAA may function as a candidate herbivore effector promoting the increased nutritional content of maize stems.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0073394PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3759431PMC
April 2014
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