Gregg Howe - Academia.edu (original) (raw)

Papers by Gregg Howe

Journal of the American Society for Horticultural Science

Six-carbon aldehydes and alcohols formed by tomato (Lycopersicon esculentum Mill.) leaf and fruit... more Six-carbon aldehydes and alcohols formed by tomato (Lycopersicon esculentum Mill.) leaf and fruit tissue following disruption are believed to be derived from the degradation of lipids and free fatty acids. Collectively, these C-6 volatiles comprise some of the most important aroma impact compounds. If fatty acids are the primary source of tomato volatiles, then an alteration in the fatty acid composition such as that caused by a mutation in the chloroplastic omega-3-fatty acid desaturase (ω-3 FAD), referred to as LeFAD7, found in the mutant line of `Castlemart' termed Lefad7, would be reflected in the volatile profile of disrupted leaf and fruit tissue. Leaves and fruit of the Lefad7 mutant had ≈10% to 15% of the linolenic acid (18:3) levels and about 1.5- to 3-fold higher linoleic acid (18:2) levels found in the parent line. Production of unsaturated C-6 aldehydes Z-3-hexenal, Z-3-hexenol, and E-2-hexenal and the alcohol Z-3-hexenol derived from 18:3 was markedly reduced in dis...

The Plant cell, Jan 9, 2018

The evolution of transcriptional regulatory mechanisms is central to how stress response and tole... more The evolution of transcriptional regulatory mechanisms is central to how stress response and tolerance differ between species. However, it remains largely unknown how divergence in cis-regulatory sites and, subsequently, transcription factor (TF) binding specificity contribute to stress-responsive expression divergence, particularly between wild and domesticated spe-cies. By profiling wound-responsive gene transcriptomes in wild Solanum pennellii and do-mesticated S. lycopersicum, we found extensive wound-response divergence and identified 493 S. lycopersicum and 278 S. pennellii putative cis-regulatory elements (pCREs) that were predictive of wound-responsive gene expression. Only 24-52% of these wound-response pCREs (depending on wound-response patterns) were consistently enriched in the putative promoter regions of wound-responsive genes across species. In addition, between these two species, their differences in pCRE site sequences were significantly and positively correlated wi...

Nature chemical biology, Jan 16, 2018

The New phytologist, Jan 26, 2017

The plant hormone jasmonate (JA) promotes the degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins ... more The plant hormone jasmonate (JA) promotes the degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins to relieve repression on diverse transcription factors (TFs) that execute JA responses. However, little is known about how combinatorial complexity among JAZ-TF interactions maintains control over myriad aspects of growth, development, reproduction, and immunity. We used loss-of-function mutations to define epistatic interactions within the core JA signaling pathway and to investigate the contribution of MYC TFs to JA responses in Arabidopsis thaliana. Constitutive JA signaling in a jaz quintuple mutant (jazQ) was largely eliminated by mutations that block JA synthesis or perception. Comparison of jazQ and a jazQ myc2 myc3 myc4 octuple mutant validated known functions of MYC2/3/4 in root growth, chlorophyll degradation, and susceptibility to the pathogen Pseudomonas syringae. We found that MYC TFs also control both the enhanced resistance of jazQ leaves to insect herbivory and restricted...

Nature communications, Jan 30, 2016

Plants resist infection and herbivory with innate immune responses that are often associated with... more Plants resist infection and herbivory with innate immune responses that are often associated with reduced growth. Despite the importance of growth-defense tradeoffs in shaping plant productivity in natural and agricultural ecosystems, the molecular mechanisms that link growth and immunity are poorly understood. Here, we demonstrate that growth-defense tradeoffs mediated by the hormone jasmonate are uncoupled in an Arabidopsis mutant (jazQ phyB) lacking a quintet of Jasmonate ZIM-domain transcriptional repressors and the photoreceptor phyB. Analysis of epistatic interactions between jazQ and phyB reveal that growth inhibition associated with enhanced anti-insect resistance is likely not caused by diversion of photoassimilates from growth to defense but rather by a conserved transcriptional network that is hardwired to attenuate growth upon activation of jasmonate signalling. The ability to unlock growth-defense tradeoffs through relief of transcription repression provides an approach...

ABSTRACT It estimated that over one million insect species feed on plants. During the coevolution... more ABSTRACT It estimated that over one million insect species feed on plants. During the coevolution of plant-insect relationships, plants developed defensive strategies to minimize insect herbivory, which in turn led to the development of counter adaptations by herbivores. Anti-nutritional and toxic host defenses that act post-ingestively often involve plant proteins that are adapted to the harsh environment of the insect gut. The plant hormone jasmonate (JA) plays a central role in regulating the induced expression of many of these defensive proteins, most notably proteinase inhibitors. We are employing a shotgun proteomic approach to identify JA-regulated proteins that are stable during passage through the digestive tract of various insect herbivores. Application of this approach to Manduca sexta larvae reared on tomato led to the identification of two JA-regulated enzymes, arginase and threonine deaminase, that retard the growth of lepidopteran insects by degrading essential amino acids (arginine and threonine, respectively) in the midgut. To gain insight into potential mechanisms that enable lepidopteran insects to adapt to JA-regulated plant defenses, we prepared cDNA libraries from midgut tissue of larvae reared either on wild-type plants or on mutant plants that are defective in JA signaling. High throughput sequencing of these libraries, using next-generation sequencing technology (e.g., Illumina), is being used: 1) to elucidate the midgut transcriptomes of various insect species, and 2) to identify insect genes that are differentially expressed as a result of host plant chemistry. Results obtained from these experiments are providing new insight into the mechanisms by which lepidopteran insects actively modify their digestive physiology to adapt to host plant defenses.

Current Genetics, 1988

The purpose of this work is to identify and quantitate in vivo 2/~ plasmid FLP-independent recomb... more The purpose of this work is to identify and quantitate in vivo 2/~ plasmid FLP-independent recombination in yeast, using a nonselective assay system for rapid detection of phenotypic expression of the recombination events. A tester plasmid was constructed such that in vivo recombination between 2/l direct repeat sequences produces the resolution of the plasmid into two circular DNA molecules. This recombinational event is detected as a phenotypic shift from red to white colonies, due to the mitotic loss of the plasmid portion containing the yeast ADE8 gene in a recipient adel ade2 ade8 genetic background. In the absence of the 2/~ FLP recombinase and/or its target DNA sequence, recombination is not abolished but rather continues at a high frequency of about 17%. This suggests that the FLP-independent events are mediated by the chromosomally-encoded general homologous recombination system. We therefore conclude that the totality of 2/a DNA recombination events occurring in FLP + cells is the contribution of both FLPmediated and FLP-independent events.

Phytochemistry, Sep 30, 2009

The phytohormone jasmonate (JA) regulates a wide range of growth, developmental, and defense-rela... more The phytohormone jasmonate (JA) regulates a wide range of growth, developmental, and defense-related processes during the plant life cycle. Identification of the JAZ family of proteins that repress JA responses has facilitated rapid progress in understanding how this lipid-derived hormone controls gene expression. Recent analysis of JAZ proteins has provided insight into the nature of the JA receptor, the chemical specificity of signal perception, and cross-talk between JA and other hormone response pathways. Functional diversification of JAZ proteins by alternative splicing, together with the ability of JAZ proteins to homoand heterodimerize, provide mechanisms to enhance combinatorial diversity and versatility in gene regulation by JA.

Plants, 2016

Plant growth is often constrained by the limited availability of resources in the microenvironmen... more Plant growth is often constrained by the limited availability of resources in the microenvironment. Despite the continuous threat of attack from insect herbivores and pathogens, investment in defense represents a lost opportunity to expand photosynthetic capacity in leaves and absorption of nutrients and water by roots. To mitigate the metabolic expenditure on defense, plants have evolved inducible defense strategies. The plant hormone jasmonate (JA) is a key regulator of many inducible defenses. Synthesis of JA in response to perceived danger leads to the deployment of a variety of defensive structures and compounds, along with a potent inhibition of growth. Genetic studies have established an important role for JA in mediating tradeoffs between growth and defense. However, several gaps remain in understanding of how JA signaling inhibits growth, either through direct transcriptional control of JA-response genes or crosstalk with other signaling pathways. Here, we highlight recent progress in uncovering the role of JA in controlling growth-defense balance and its relationship to resource acquisition and allocation. We also discuss tradeoffs in the context of the ability of JA to promote increased leaf mass per area (LMA), which is a key indicator of leaf construction costs and leaf life span.

The Plant Journal For Cell and Molecular Biology, Nov 1, 2002

12-Oxophytodienoate reductases (OPRs) belong to a family of¯avin-dependent oxidoreductases. With ... more 12-Oxophytodienoate reductases (OPRs) belong to a family of¯avin-dependent oxidoreductases. With two new tomato isoforms reported here, three OPRs have now been characterized in both tomato and Arabidopsis. Only one of these isoforms (OPR3) participates directly in the octadecanoid pathway for jasmonic acid biosynthesis, as only OPR3 reduces the 9S,13S-stereoisomer of 12-oxophytodienoic acid, the biological precursor of jasmonic acid. The subcellular localization of OPRs was analyzed in tomato and Arabidopsis. The OPR3 protein and activity were consistently found in peroxisomes where they co-localize with the enzymes of b-oxidation which catalyze the ®nal steps in the formation of jasmonic acid. The octadecanoid pathway is thus con®ned to plastids and peroxisomes and, in contrast to previous assumptions, does not involve the cytosolic compartment. The expression of tomato (Lycopersicon esculentum, Le) OPR3 was analyzed in the context of defense-related genes using a microarray comprising 233 cDNA probes. LeOPR3 was found to be up-regulated after wounding with induction kinetics resembling those of other octadecanoid pathway enzymes. In contrast to the induction of genes for wound response proteins (e.g. proteinase inhibitors), the accumulation of octadecanoid pathway transcripts was found to be more rapid and transient in wounded leaves, but hardly detectable in unwounded, systemic leaves. Consistent with the expression data, OPDA and JA were found to accumulate locally but not systemically in the leaves of wounded tomato plants. The transcriptional activation of the octadecanoid pathway and the accumulation of JA to high levels are, thus not required for the activation of defense gene expression in systemic tissues.

Proceedings of the National Academy of Sciences, 2015

In the past decade, characterization of the host targets of pathogen virulence factors took a cen... more In the past decade, characterization of the host targets of pathogen virulence factors took a center stage in the study of pathogenesis and disease susceptibility in plants and humans. However, the impressive knowledge of host targets has not been broadly exploited to inhibit pathogen infection. Here, we show that host target modification could be a promising new approach to “protect” the disease-vulnerable components of plants. In particular, recent studies have identified the plant hormone jasmonate (JA) receptor as one of the common targets of virulence factors from highly evolved biotrophic/hemibiotrophic pathogens. Strains of the bacterial pathogen Pseudomonas syringae, for example, produce proteinaceous effectors, as well as a JA-mimicking toxin, coronatine (COR), to activate JA signaling as a mechanism to promote disease susceptibility. Guided by the crystal structure of the JA receptor and evolutionary clues, we succeeded in modifying the JA receptor to allow for sufficient ...

The biosynthesis of jasmonic acid (JA) in plant peroxisomes requires the action of acyl-coenzyme ... more The biosynthesis of jasmonic acid (JA) in plant peroxisomes requires the action of acyl-coenzyme A oxidase (ACX). Among the five expressed members (ACX1-5) of the ACX gene family in Arabidopsis (Arabidopsis thaliana), only ACX1 is known to serve a role in JA production. Here, we used transgenic promoter-reporter lines to show that ACX1 is highly expressed in mature and germinating pollen, stem epidermal cells, and other tissues in which jasmonate-signaled processes occur. Wound-induced JA accumulation was reduced in a mutant that is defective in ACX1 and was abolished in a mutant that is impaired in both ACX1 and its closely related paralog, ACX5. The severe JA deficiency in acx1/5 double mutants was accompanied by decreased resistance to the leaf-eating insect Trichoplusia ni. The double mutant also showed reduced pollen viability and fecundity. Treatment of acx1/5 plants with JA restored both protection against T. ni larvae and normal seed set. Unexpectedly, acx1/5 plants accumulated JA in response to infection by the necrotrophic fungal pathogen Alternaria brassicicola. In contrast to mutants that are impaired in jasmonate perception or early steps of the JA biosynthetic pathway, acx1/5 plants maintained resistance to A. brassicicola infection. These results indicate that ACX1/5-mediated JA synthesis is essential for resistance to chewing insects and male reproductive function and further suggest that other ACX isozymes contribute to JA production in response to A. brassicicola challenge. Thus, different types of biotic stress may induce JA synthesis via distinct enzymatic routes.

Genome research, 2015

Nucleosome positioning influences the access of transcription factors (TFs) to their binding site... more Nucleosome positioning influences the access of transcription factors (TFs) to their binding sites and gene expression. Studies in plant, animal, and fungal models demonstrate similar nucleosome positioning patterns along genes and correlations between occupancy and expression. However, the relationships among nucleosome positioning, cis-regulatory element accessibility, and gene expression in plants remain undefined. Here we showed that plant nucleosome depletion occurs on specific 6-mer motifs and this sequence-specific nucleosome depletion is predictive of expression levels. Nucleosome-depleted regions in Arabidopsis thaliana tend to have higher G/C content, unlike yeast, and are centered on specific G/C-rich 6-mers, suggesting that intrinsic sequence properties, such as G/C content, cannot fully explain plant nucleosome positioning. These 6-mer motif sites showed higher DNase I hypersensitivity and are flanked by strongly phased nucleosomes, consistent with known TF binding site...

It estimated that over one million insect species feed on plants. During the coevolution of plant... more It estimated that over one million insect species feed on plants. During the coevolution of plant-insect relationships, plants developed defensive strategies to minimize insect herbivory, which in turn led to the development of counter adaptations by herbivores. Anti-nutritional and toxic host defenses that act post-ingestively often involve plant proteins that are adapted to the harsh environment of the insect gut. The plant hormone jasmonate (JA) plays a central role in regulating the induced expression of many of these defensive proteins, most notably proteinase inhibitors. We are employing a shotgun proteomic approach to identify JA-regulated proteins that are stable during passage through the digestive tract of various insect herbivores. Application of this approach to Manduca sexta larvae reared on tomato led to the identification of two JA-regulated enzymes, arginase and threonine deaminase, that retard the growth of lepidopteran insects by degrading essential amino acids (ar...

Jasmonic acid (JA) and its amino acid conjugate, jasmonoyl-isoleucine (JA-Ile), play important ro... more Jasmonic acid (JA) and its amino acid conjugate, jasmonoyl-isoleucine (JA-Ile), play important roles in regulating plant defense responses to insect herbivores. Recent studies indicate that JA-Ile promotes the degradation of JAsmonate ZIM-domain (JAZ) transcriptional repressors through the activity of the E 3 ubiquitin-ligase SCF COI1. Here, we investigated the regulation and function of JAZ genes during the interaction of Arabidopsis with the generalist herbivore Spodoptera exigua. Most members of the JAZ gene family were highly expressed in response to S. exigua feeding and mechanical wounding. JAZ transcript levels increased within 5 min of mechanical tissue damage, coincident with a large (~25-fold) rise in JA and JA-Ile levels. Wound-induced expression of JAZ and other COI1-dependent genes was not impaired in the jar1-1 mutant that is partially deficient in the conversion of JA to JA-Ile. Experiments performed with the protein synthesis inhibitor cycloheximide provided evidence that JAZs, MYC2, and genes encoding several JA biosynthetic enzymes are primary response genes whose expression is de-repressed upon COI1-dependent turnover of a labile repressor protein(s). We also show that overexpression of a modified form of JAZ1 (JAZ1∆3A) that is stable in the presence of JA compromises host resistance to feeding by S. exigua larvae. These findings establish a role for JAZ proteins in the regulation of plant anti-insect defense, and support the hypothesis that JA-Ile and perhaps other JA derivatives activate COI1-dependent wound responses in Arabidopsis. Our results also indicate that the timing of JA-induced transcription in response to wounding is more rapid than previously realized.

Current Opinion in Insect Science, 2015

The co-evolutionary conflict between insect herbivores and their host plants is profoundly influe... more The co-evolutionary conflict between insect herbivores and their host plants is profoundly influenced by biochemical reactions associated with passage of toxin-laden plant material through the herbivore digestive canal. Insect herbivores provide excellent models in which to understand the mechanistic interplay between nutrition and detoxification, how plant defense compounds hijack these processes, and how insects adapt to host defense chemistry. Expanding genome sequence information and genetic approaches to manipulate gene function in both interacting partners are providing new insights into the genetic underpinnings of host preference and plasticity in gut physiology. Fundamental knowledge gained from these studies has practical application in understanding how insects evolve resistance to pesticides, and may also inform efforts to better understand how plant chemicals impact human health.

Induced Plant Resistance to Herbivory, 2008

Resistance factors for direct plant defense against herbivorous insects comprise plant traits tha... more Resistance factors for direct plant defense against herbivorous insects comprise plant traits that negatively affect insect preference (host plant selection, oviposition, feeding behavior) or performance (growth rate, development, reproductive success) resulting in increased plant fitness in a hostile environment. Such traits include morphological features for physical defense, like thorns, spines, and trichomes, epicuticular wax films and wax crystals, tissue toughness, as well as secretory structures and conduits for latices or resins. They also include compounds for chemical defense, like secondary metabolites, digestibility reducing proteins, and antinutritive enzymes. All these traits may be expressed constitutively as preformed resistance factors, or they may be inducible and deployed only after attack by insect herbivores. The induction of defensive traits is not restricted to the site of attack but extends to non-infested healthy parts of the plants. The systemic nature of plant responses to herbivore attack necessitates a long-distance signaling system capable of generating, transporting, and interpreting alarm signals produced at the plant-herbivore interface. Much of the research on the signaling events triggered by herbivory has focused on tomato and other solanaceous plants. In this model system, the peptide systemin acts at or near the wound site to amplify the production of jasmonic acid. Jasmonic acid or its metabolites serve as phloem-mobile long-distance signals, and induce the expression of defense genes in distal parts of the plant. In this chapter, we will provide an overview of physical and chemical defense traits, and review the signaling mechanisms that account for their inducible expression after insect attack.