Belowground insect herbivory induces systemic volatile emissions that strengthen neighbouring plant resistance aboveground.

Plants transmit ecologically relevant messages to neighbouring plants through chemical cues. For instance, insect herbivory triggers the production of herbivore-induced plant volatiles (HIPVs), which can enhance neighbouring plant defences. HIPVs are emitted from directly damaged plant tissues and from systemic, nondamaged tissues. Although volatile-mediated interplant interactions have been observed both above- and belowground, it remains unknown whether belowground herbivory induces systemic HIPVs aboveground that influence neighbouring plants. To explore how belowground herbivory affects interplant interactions aboveground, we characterised systemic HIPVs from squash induced by belowground striped cucumber beetle (Acalymma vittatum) larval herbivory. We exposed squash 'receiver plants' to systemic HIPVs or volatiles from nondamaged plants. We then measured herbivore resistance by challenging 'receiver plants' with aboveground-feeding herbivores: adult beetles (A. vittatum) or squash bugs (Anasa tristis). We discovered belowground-damaged plants emitted more (E)-ß-ocimene, a key volatile from the systemic HIPV blend, than nondamaged controls, and that exposure to systemic HIPVs enhanced neighbouring plant resistance to aboveground squash bugs, but not adult beetles. Further investigations into the mechanism of interplant interaction revealed ß-ocimene alone can elicit plant resistance against squash bugs. Overall, our findings reveal a novel form of volatile-mediated interactions between plants spanning across aboveground-belowground plant systems.

Improving Natural Enemy Selection in Biological Control through Greater Attention to Chemical Ecology and Host-Associated Differentiation of Target Arthropod Pests.

Host-associated differentiation (HAD) refers to cases in which genetically distinct populations of a species (e.g., herbivores or natural enemies) preferentially reproduce or feed on different host species. In agroecosystems, HAD often results in unique strains or biotypes of pest species, each attacking different species of crops. However, HAD is not restricted to pest populations, and may cascade to the third trophic level, affecting host selection by natural enemies, and ultimately leading to HAD within natural enemy species. Natural enemy HAD may affect the outcomes of biological control efforts, whether classical, conservation, or augmentative. Here, we explore the potential effects of pest and natural enemy HAD on biological control in agroecosystems, with emphases on current knowledge gaps and implications of HAD for selection of biological control agents. Additionally, given the importance of semiochemicals in mediating interactions between trophic levels, we emphasize the role of chemical ecology in interactions between pests and natural enemies, and suggest areas of consideration for biological control. Overall, we aim to jump-start a conversation concerning the relevance of HAD in biological control by reviewing currently available information on natural enemy HAD, identifying challenges to incorporating HAD considerations into biological control efforts, and proposing future research directions on natural enemy selection and HAD.

Defense Suppression through Interplant Communication Depends on the Attacking Herbivore Species.

In response to herbivory, plants emit volatile compounds that play important roles in plant defense. Herbivore-induced plant volatiles (HIPVs) can deter herbivores, recruit natural enemies, and warn other plants of possible herbivore attack. Following HIPV detection, neighboring plants often respond by enhancing their anti-herbivore defenses, but a recent study found that herbivores can manipulate HIPV-interplant communication for their own benefit and suppress defenses in neighboring plants. Herbivores induce species-specific blends of HIPVs and how these different blends affect the specificity of plant defense responses remains unclear. Here we assessed how HIPVs from zucchini plants (Cucurbita pepo) challenged with different herbivore species affect resistance in neighboring plants. Volatile "emitter" plants were damaged by one of three herbivore species: saltmarsh caterpillars (Estigmene acrea), squash bugs (Anasa tristis), or striped cucumber beetles (Acalymma vittatum), or were left as undamaged controls. Neighboring "receiver" plants were exposed to HIPVs or control volatiles and then challenged by the associated herbivore species. As measures of plant resistance, we quantified herbivore feeding damage and defense-related phytohormones in receivers. We found that the three herbivore species induced different HIPV blends from squash plants. HIPVs induced by saltmarsh caterpillars suppressed defenses in receivers, leading to greater herbivory and lower defense induction compared to controls. In contrast, HIPVs induced by cucumber beetles and squash bugs did not affect plant resistance to subsequent herbivory in receivers. Our study shows that herbivore species identity affects volatile-mediated interplant communication in zucchini, revealing a new example of herbivore defense suppression through volatile cues.

Chemical Cues from Entomopathogenic Nematodes Vary Across Three Species with Different Foraging Strategies, Triggering Different Behavioral Responses in Prey and Competitors.

Chemical cues play important roles in predator-prey interactions. Semiochemicals can aid predator foraging and alert prey organisms to the presence of predators. Previous work suggests that predator traits differentially influence prey behavior, however, empirical data on how prey organisms respond to chemical cues from predator species with different hunting strategies, and how foraging predators react to cues from potential competitors, is lacking. Furthermore, most research in this area has focused on aquatic and aboveground terrestrial systems, while interactions among belowground, soiling-dwelling organisms have received relatively little attention. Here, we assessed how chemical cues from three species of entomopathogenic nematodes (EPNs), each with a different foraging strategy, influenced herbivore (cucumber beetle) and natural enemy (EPN) foraging behavior. We predicted these cues could serve as chemical indicators of increased predation risk, prey availability, or competition. Our findings revealed that foraging cucumber beetle larvae avoided chemical cues from Heterorhabditis bacteriophora (active-foraging cruiser EPNs), but not Steinernema carpocapsae (ambusher EPNs) or Steinernema riobrave (intermediate-foraging EPNs). In contrast, foraging H. bacteriophora EPNs were attracted to cues produced by the two Steinernema species but not conspecific cues. Notably, the three EPN species produced distinct blends of olfactory cues, with only a few semi-conserved compounds across species. These results indicate that a belowground insect herbivore responds differently to chemical cues from different EPN species, with some EPN species avoiding prey detection. Moreover, the active-hunting EPNs were attracted to heterospecific cues, suggesting these cues indicate a greater probability of available prey, rather than strong interspecific competition.

Divide and conquer: Multicolonial structure, nestmate recognition, and antagonistic behaviors in dense populations of the invasive ant Brachymyrmex patagonicus.

The ecological success of ants has made them abundant in most environments, yet inter- and intraspecific competition usually limit nest density for a given population. Most invasive ant populations circumvent this limitation through a supercolonial structure, eliminating intraspecific competition through a loss of nestmate recognition and lack of aggression toward non-nestmates. Native to South America, Brachymyrmex patagonicus has recently invaded many locations worldwide, with invasive populations described as extremely large and dense. Yet, in contrast with most invasive ants, this species exhibits a multicolonial structure, whereby each colony occupies a single nest. Here, we investigated the interplay between genetic diversity, chemical recognition, and aggressive behaviors in an invasive population of B. patagonicus. We found that, in its invasive range, this species reaches a high nest density with individual colonies located every 2.5 m and that colony boundaries are maintained through aggression toward non-nestmates. This recognition and antagonism toward non-nestmates is mediated by chemical differentiation between colonies, as different colonies exhibit distinct chemical profiles. We highlighted that the level of aggression between colonies is correlated with their degree of genetic difference, but not their overall chemical differentiation. This may suggest that only a few chemical compounds influence nestmate recognition in this species or that weak chemical differences are sufficient to elicit aggression. Overall, this study demonstrates that invasive ant populations can reach high densities despite a multicolonial structure with strong aggression between colonies, raising questions about the factors underlying their ecological success and mitigating negative consequences of competitive interactions.

The Role of Plant-Associated Microbes in Mediating Host-Plant Selection by Insect Herbivores.

There is increasing evidence that plant-associated microorganisms play important roles in shaping interactions between plants and insect herbivores. Studies of both pathogenic and beneficial plant microbes have documented wide-ranging effects on herbivore behavior and performance. Some studies, for example, have reported enhanced insect-repellent traits or reduced performance of herbivores on microbe-associated plants, while others have documented increased herbivore attraction or performance. Insect herbivores frequently rely on plant cues during foraging and oviposition, suggesting that plant-associated microbes affecting these cues can indirectly influence herbivore preference. We review and synthesize recent literature to provide new insights into the ways pathogenic and beneficial plant-associated microbes alter visual, olfactory, and gustatory cues of plants that affect host-plant selection by insect herbivores. We discuss the underlying mechanisms, ecological implications, and future directions for studies of plant-microbial symbionts that indirectly influence herbivore behavior by altering plant traits.

Huntingtin Supplies a csaA-Independent Function Essential for EDTA-Resistant Homotypic Cell Adhesion in Dictyostelium discoideum.

BACKGROUND: The CAG triplet repeat expansion mutation in the HTT locus, which results in neurodegeneration in Huntington's disease, elongates a polyglutamine tract in huntingtin, a HEAT/HEAT-like protein that has been highly structurally conserved through evolution. In several organisms, huntingtin is necessary for proper cell-cell adhesion and normal development. OBJECTIVE: Dictyostelium discoideum huntingtin null (htt-) cells display a variety of developmental abnormalities and completely fail to acquire EDTA-resistant homotypic cell adhesion during starvation in suspension culture. METHODS: Here, we have assessed the hypothesis that htt may be a genetic interactor of csaA, a major regulator of EDTA-resistant homotypic cell adhesion in D. discoideum. Immunoblot analysis demonstrated that csaA protein expression is dysregulated in htt- cells. RESULTS: Unexpectedly, csaA overexpression, previously shown to rescue csaA- cell adhesion, failed to rescue the htt- adhesion defect. Thus, while htt was required for proper expression of the csaA protein, csaA overexpression was not sufficient to confer EDTA-resistant adhesion in the context of the htt- genetic background in contrast to parental cells. This implies a novel role for htt in conferring csaA-dependent, EDTA-resistant cell adhesion that warrants further investigation. Calcium supplementation restored both endogenous csaA protein levels and EDTA-resistant adhesion in htt- cells. CONCLUSIONS: Our data suggests the existence of an additional mechanism that overcomes the EDTA-resistant adhesion defect of htt- cells in the early development of D. discoideum.

Cardiolipin and mitochondrial phosphatidylethanolamine have overlapping functions in mitochondrial fusion in Saccharomyces cerevisiae.

The two non-bilayer forming mitochondrial phospholipids cardiolipin (CL) and phosphatidylethanolamine (PE) play crucial roles in maintaining mitochondrial morphology. We have shown previously that CL and PE have overlapping functions, and the loss of both is synthetically lethal. Because the lack of CL does not lead to defects in the mitochondrial network in Saccharomyces cerevisiae, we hypothesized that PE may compensate for CL in the maintenance of mitochondrial tubular morphology and fusion. To test this hypothesis, we constructed a conditional mutant crd1Δpsd1Δ containing null alleles of CRD1 (CL synthase) and PSD1 (mitochondrial phosphatidylserine decarboxylase), in which the wild type CRD1 gene is expressed on a plasmid under control of the TET(OFF) promoter. In the presence of tetracycline, the mutant exhibited highly fragmented mitochondria, loss of mitochondrial DNA, and reduced membrane potential, characteristic of fusion mutants. Deletion of DNM1, required for mitochondrial fission, restored the tubular mitochondrial morphology. Loss of CL and mitochondrial PE led to reduced levels of small and large isoforms of the fusion protein Mgm1p, possibly accounting for the fusion defect. Taken together, these data demonstrate for the first time in vivo that CL and mitochondrial PE are required to maintain tubular mitochondrial morphology and have overlapping functions in mitochondrial fusion.

HD CAG-correlated gene expression changes support a simple dominant gain of function.

Huntington's disease is initiated by the expression of a CAG repeat-encoded polyglutamine region in full-length huntingtin, with dominant effects that vary continuously with CAG size. The mechanism could involve a simple gain of function or a more complex gain of function coupled to a loss of function (e.g. dominant negative-graded loss of function). To distinguish these alternatives, we compared genome-wide gene expression changes correlated with CAG size across an allelic series of heterozygous CAG knock-in mouse embryonic stem (ES) cell lines (Hdh(Q20/7), Hdh(Q50/7), Hdh(Q91/7), Hdh(Q111/7)), to genes differentially expressed between Hdh(ex4/5/ex4/5) huntingtin null and wild-type (Hdh(Q7/7)) parental ES cells. The set of 73 genes whose expression varied continuously with CAG length had minimal overlap with the 754-member huntingtin-null gene set but the two were not completely unconnected. Rather, the 172 CAG length-correlated pathways and 238 huntingtin-null significant pathways clustered into 13 shared categories at the network level. A closer examination of the energy metabolism and the lipid/sterol/lipoprotein metabolism categories revealed that CAG length-correlated genes and huntingtin-null-altered genes either were different members of the same pathways or were in unique, but interconnected pathways. Thus, varying the polyglutamine size in full-length huntingtin produced gene expression changes that were distinct from, but related to, the effects of lack of huntingtin. These findings support a simple gain-of-function mechanism acting through a property of the full-length huntingtin protein and point to CAG-correlative approaches to discover its effects. Moreover, for therapeutic strategies based on huntingtin suppression, our data highlight processes that may be more sensitive to the disease trigger than to decreased huntingtin levels.

Deficiency of huntingtin has pleiotropic effects in the social amoeba Dictyostelium discoideum.

Huntingtin is a large HEAT repeat protein first identified in humans, where a polyglutamine tract expansion near the amino terminus causes a gain-of-function mechanism that leads to selective neuronal loss in Huntington's disease (HD). Genetic evidence in humans and knock-in mouse models suggests that this gain-of-function involves an increase or deregulation of some aspect of huntingtin's normal function(s), which remains poorly understood. As huntingtin shows evolutionary conservation, a powerful approach to discovering its normal biochemical role(s) is to study the effects caused by its deficiency in a model organism with a short life-cycle that comprises both cellular and multicellular developmental stages. To facilitate studies aimed at detailed knowledge of huntingtin's normal function(s), we generated a null mutant of hd, the HD ortholog in Dictyostelium discoideum. Dictyostelium cells lacking endogenous huntingtin were viable but during development did not exhibit the typical polarized morphology of Dictyostelium cells, streamed poorly to form aggregates by accretion rather than chemotaxis, showed disorganized F-actin staining, exhibited extreme sensitivity to hypoosmotic stress, and failed to form EDTA-resistant cell-cell contacts. Surprisingly, chemotactic streaming could be rescued in the presence of the bivalent cations Ca(2+) or Mg(2+) but not pulses of cAMP. Although hd(-) cells completed development, it was delayed and proceeded asynchronously, producing small fruiting bodies with round, defective spores that germinated spontaneously within a glassy sorus. When developed as chimeras with wild-type cells, hd(-) cells failed to populate the pre-spore region of the slug. In Dictyostelium, huntingtin deficiency is compatible with survival of the organism but renders cells sensitive to low osmolarity, which produces pleiotropic cell autonomous defects that affect cAMP signaling and as a consequence development. Thus, Dictyostelium provides a novel haploid organism model for genetic, cell biological, and biochemical studies to delineate the functions of the HD protein.

Striatal neurons expressing full-length mutant huntingtin exhibit decreased N-cadherin and altered neuritogenesis.

The expanded CAG repeat that causes striatal cell vulnerability in Huntington's disease (HD) encodes a polyglutamine tract in full-length huntingtin that is correlated with cellular [ATP] and [ATP/ADP]. Since striatal neurons are vulnerable to energy deficit, we have investigated, in Hdh CAG knock-in mice and striatal cells, the hypothesis that decreased energetics may affect neuronal (N)-cadherin, a candidate energy-sensitive adhesion protein that may contribute to HD striatal cell sensitivity. In vivo, N-cadherin was sensitive to ischemia and to the effects of full-length mutant huntingtin, progressively decreasing in Hdh(Q111) striatum with age. In cultured striatal cells, N-cadherin was decreased by ATP depletion and STHdh(Q111) striatal cells exhibited dramatically decreased N-cadherin, due to decreased Cdh2 mRNA and enhanced N-cadherin turnover, which was partially normalized by adenine supplementation to increase [ATP] and [ATP/ADP]. Consistent with decreased N-cadherin function, STHdh(Q111) striatal cells displayed profound deficits in calcium-dependent N-cadherin-mediated cell clustering and cell-substratum adhesion, and primary Hdh(Q111) striatal neuronal cells exhibited decreased N-cadherin and an abundance of immature neurites, featuring diffuse, rather than clustered, staining for N-cadherin and synaptic vesicle markers, which was partially rescued by adenine treatment. Thus, mutant full-length huntingtin, via energetic deficit, contributes to decreased N-cadherin levels in striatal neurons, with detrimental effects on neurite maturation, strongly suggesting that N-cadherin-mediated signaling merits investigation early in the HD pathogenic disease process.

New insights into the regulation of cardiolipin biosynthesis in yeast: implications for Barth syndrome.

Recent studies have revealed an array of novel regulatory mechanisms involved in the biosynthesis and metabolism of the phospholipid cardiolipin (CL), the signature lipid of mitochondria. CL plays an important role in cellular and mitochondrial function due in part to its association with a large number of mitochondrial proteins, including many which are unable to function optimally in the absence of CL. New insights into the complexity of regulation of CL provide further evidence of its importance in mitochondrial and cellular function. The biosynthesis of CL in yeast occurs via three enzymatic steps localized in the mitochondrial inner membrane. Regulation of this process by general phospholipid cross-pathway control and factors affecting mitochondrial development has been previously established. In this review, novel regulatory mechanisms that control CL biosynthesis are discussed. A unique form of inositol-mediated regulation has been identified in the CL biosynthetic pathway, independent of the INO2-INO4-OPI1 regulatory circuit that controls general phospholipid biosynthesis. Inositol leads to decreased activity of phosphatidylglycerolphosphate (PGP) synthase, which catalyzes the committed step of CL synthesis. Reduced enzymatic activity does not result from alteration of expression of the structural gene, but is instead due to increased phosphorylation of the enzyme. This is the first demonstration of phosphorylation in response to inositol and may have significant implications in understanding the role of inositol in other cellular regulatory pathways. Additionally, synthesis of CL has been shown to be dependent on mitochondrial pH, coordinately controlled with synthesis of mitochondrial phosphatidylethanolamine (PE), and may be regulated by mitochondrial DNA absence sensitive factor (MIDAS). Further characterization of these regulatory mechanisms holds great potential for the identification of novel functions of CL in mitochondrial and cellular processes.

Inositol phosphates and phosphoinositides in health and disease.

In the past two decades, considerable progress has been made toward understanding inositol phosphates and PI metabolism. However, there is still much to learn. The present challenge is to understand how inositol phosphates and PIs are compartmentalized, identify new targets of inositol phosphates and PIs, and elucidate the mechanisms underlying spatial and temporal regulation of the enzymes that metabolize inositol phosphates and PIs. Answers to these questions will help clarify the mechanisms of the diseases associated with these molecules and identify new possibilities for drug design.

Synthetic lethal interaction of the mitochondrial phosphatidylethanolamine and cardiolipin biosynthetic pathways in Saccharomyces cerevisiae.

Saccharomyces cerevisiae mitochondria contain enzymes required for synthesis of the phospholipids cardiolipin (CL) and phosphatidylethanolamine (PE), which are enriched in mitochondrial membranes. Previous studies indicated that PE may compensate for the lack of CL, and vice versa. These data suggest that PE and CL have overlapping functions and that the absence of both lipids may be lethal. To address this hypothesis, we determined whether the crd1delta mutant, which lacks CL, was viable in genetic backgrounds in which PE synthesis was genetically blocked. Deletion of the mitochondrial PE pathway gene PSD1 was synthetically lethal with the crd1delta mutant, whereas deletion of the Golgi and endoplasmic reticulum pathway genes PSD2 and DPL1 did not result in synthetic lethality. A 20-fold reduction in phosphatidylcholine did not affect the growth of crd1delta cells. Supplementation with ethanolamine, which led to increased PE synthesis, or with propanolamine, which led to synthesis of the novel phospholipid phosphatidylpropanolamine, failed to rescue the synthetic lethality of the crd1delta psd1delta cells. These results suggest that mitochondrial biosynthesis of PE is essential for the viability of yeast mutants lacking CL.