Progress 09/30/10 to 10/01/15
Outputs
Target Audience: The target audience of this project is the research community, especially researchers working on mechanisms of plant resistance against insect pests and breeding insect pest-resistant plants, and researchers studying mechanisms of transmission of plant pathogens by insect vectors. Changes/Problems: I made two unsuccessful attempts to secure funding from the California Citrus Research Board for screening the UCR Citrus Variety Collection for resistance to the Asian citrus psyllid in order to identify resistant genotypes. This was the original objective 5 of the project. Without funding, this objective could not be met. In place of objective 5, I substituted a new objective related to an area of research that has developed since writing the original project proposal. This new objective is to enhance our understanding of the mechanisms of transmission of plant viruses by insect vectors. What opportunities for training and professional development has the project provided? Two postdoctoral scientists and one PhD graduate student have been involved in this project. The two postdoctoral scientists include Dr. Karla Medina-Ortega who recently moved from the project to a permanent position in the Crop Protection division of DuPont Corporation. Dr. Hsuan-Chieh Peng replaced Dr. Medina-Ortega last summer (2014) after she left for her current position at DuPont. Dr. Peng continues to be involved in the project. Dr. Jaclyn Zhou worked on the project for her dissertation research and completed her PhD in 2014. The project has trained all three in new research techniques for studying the interactions between plants and phloem-feeding insects, and has provided them with experience in designing experiments. How have the results been disseminated to communities of interest? Results have been disseminated to the community of interest (the research community) through publication in scientific journals and presentations at scientific meetings. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
Phloem is one of the two major tissues in plant veins and forms the tubular system through which nutrients are transported within the plant via phloem sap. Thus phloem is somewhat analogous to the human circulatory system and phloem sap analogous to blood. Several groups of insects, such as aphids, whiteflies, many leafhoppers and others, specialize by feeding on phloem sap with hypodermic needle-like mouthparts similar to the mouthparts of blood-feeding mosquitoes. Many of these phloem sap-feeding insects are major agricultural pests that severely damage plants by robbing plants of their nutrient-carrying phloem sap and by transmitting the majority of plant viruses. The major impact of this project has been a change in knowledge relating to our understanding of how plants can defend themselves against phloem sap-feeding insects and the details of the mechanisms by which these insects transmit plant viruses. The project has greatly expanded our knowledge of the role of phloem occlusion mechanisms (analogous to blood clotting) in defending plants against phloem sap-feeding insects. While phloem occlusion obviously is beneficial to the plant when the plant is damaged, it had long been hypothesized (but never tested due to technical difficulties) that phloem occlusion also can function as a plant defense against phloem sap-feeding insects. This project developed new techniques to test this hypothesis and demonstrated for the first time that phloem occlusion can be an effective defense against phloem sap-feeding insects. This information will be valuable to plant breeders for improving plant resistance against phloem sap-feeding insects. Plants have "inducible defenses" that are somewhat analogous to our immune system in that they are activated (induced) in response to attack by a pathogen or insect. The exact mechanism by which these defenses act against insects is poorly known. We examined the effect of these inducible defenses on phloem sap-feeding whiteflies and found little effect on feeding behavior. While this does not identify the mechanisms involved, it does rule out a potential mechanism and provides insights on what the mechanism is. This project played a major role in a study on transmission of criniviruses, a group of economically important plant viruses that are transmitted by whiteflies. Transmission of these plant viruses is a complex process that involves uptake of the virus when feeding on an infected plant, binding of the virus to specific sites in the insect, and subsequent release of the virus from the binding site when the insect later feeds on another plant. Interruption of any of these steps would stop the transmission process; thus elucidation of the mechanistic details of these steps is intended to identify vulnerable steps that might be exploited to interfere with virus transmission. This project developed a technique that allowed easy detection of the virus in its binding site in the insect's alimentary canal. We used the technique to identify the molecular component in the virus coat that binds to the insect's foregut, and to determine that this molecular component is essential for virus transmission. Additionally, we demonstrated that in another insect-virus system, thrips and a tospovirus, the virus manipulates the insect's feeding behavior in a way that enhances virus transmission. We also demonstrated that the insecticide imidacloprid reduces the amount of phloem feeding by the potato psyllid, the insect that transmits the zebra chip disease organism. This is significant because the pathogen is transmitted when the insect is feeding on phloem. Specific Objectives 1) Determine how phloem-feeding insects are able to circumvent or reverse the phloem sieve element sealing response when they puncture sieve elements and determine if plant resistance to phloem-feeding insects can be achieved by a sieve element sealing mechanism that is not susceptible to the insect's manipulations. Accomplishments: We developed a protocol using electrical penetration graphs and confocal laser scanning microscopy that, for the first time, provided a methodology for examining the role of phloem occlusion proteins in defense against phloem sap-feeding insects. We demonstrated for the first time that phloem occlusion proteins can be an effective plant defense against phloem sap feeding insects. We also demonstrated that aphids cannot reverse phloem occlusion with their saliva. This has changed our paradigm regarding the ability of phloem sap feeding insects to reverse phloem occlusion. 2) Determine how beet leafhopper, Circulifer tenellus, is able to successfully feed on phloem sap despite its stylets being apparently too large to pierce sieve elements without killing them; and determine how inoculation of beet curly top virus by beet leafhopper is blocked on virus-resistant tomato genotypes. Accomplishments: We did not make any progress on this objective. 3) Determine how the jasmonic acid and salicylic acid induced defense pathways affect feeding of the silverleaf whitefly, Bemisia argentifolii, and determine the specific phases in whitefly feeding that trigger the many transcriptional changes in the plant in response to whitefly feeding. Accomplishments: Insect feeding on plants triggers several inducible plant defenses that, through mostly undetermined mechanisms, negatively impact the insect. Previous work indicated that the salicylic acid defense pathway in the model plant Arabidopsis inhibited whitefly growth and development. One potential way in which this defense pathway could affect the whiteflies is through inhibition of feeding behavior. Consequently, we tested this hypothesis and found, contrary to expectation, feeding behavior differed little among the Arabidopsis mutants and wild type control. 4) Identify the mechanism of whitefly resistance in Gossypium thurberi, identify resistant plants in a G. thurberi/arboreum hybrid, cross this hybrid with G. hirsutum and identify resistant plants in the G. thurberi/arboreum/hirsutum triple hybrid. Accomplishments: We ran only one experiment looking at whitefly oviposition preference between commercial cotton and Gossypium thurberi. We did not detect a significant difference. 5) Enhance our understanding of the mechanisms of transmission of plant viruses by insect vectors. (Note: this objective replaced the original objective 5 on plant resistance against Asian citrus psyllid due to lack of funding to support the original objective 5). Accomplishments: We developed a new technique for labeling criniviruses in their whitefly vectors ("vectors" are insects that transmit pathogens). Our technique requires less than 2% of the time it would take using the previous method. This has greatly facilitated experiments that would be too tedious and time consuming using the previous technique. Using this technique, we: 1) identified the whitefly foregut as the site where the virus binds to the vector; 2) determined that the virus minor coat protein was responsible for binding the virus to the whitefly's foregut; and 3) demonstrated that binding of the minor coat protein to the whitefly foregut is essential for virus transmission; if it cannot bind, the virus cannot get transmitted. We also found that when thrips acquire the tospovirus that causes tomato spotted wilt disease, the virus directly affects the thrips feeding behavior in a way that makes the thrips more efficient at transmitting the virus. This was the first documented case of a plant pathogen having a direct effect on its insect vector (as opposed to an indirect effect mediated through the plant). We also found that application of the insecticide imidacloprid to potato plants at recommended field rates significantly reduced potato psyllid salivation into phloem and ingestion of phloem sap, the behaviors associated with inoculation and acquisition, respectively, of the zebra chip disease pathogen.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2015
Citation:
Medina-Ortega KJ & Walker GP 2015. Faba bean forisomes can function in defense against generalist aphids. Plant Cell & Environment online version (print version due in early 2015)
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Progress 01/01/13 to 09/30/13
Outputs
Target Audience: The target audienec of this project is the scientific community, specifically entomologists and plant biologists studying insect-plant interactions and crop resistance against insect pests. Changes/Problems: I have been unsuccessful at obtaining funding to address Objective 5 (Screen the UCR Citrus variety collection for resistance to the Asian citrus psyllid and identify resistant genotypes). Consequently this objective has been dropped. What opportunities for training and professional development has the project provided? I have been an active mentor of a postdoc, Dr. Karla Medina-Ortega. I have trained her in a variety of experimental techniques and I am mentoring her in professional development and working closely with her to improve her scientiffic writing skills. How have the results been disseminated to communities of interest? Results of the project have been disseminated to the primary community of interest, the scientific community, by publication in journals and books (see products) and by presentations at scientific meetings (e.g., national meeting of the Entomological Society of America). What do you plan to do during the next reporting period to accomplish the goals? We will continue to examine the interaction between phloem sap-feeding insects and plant phloem occlusion mechanisms (Objective 1). This coming year, we will focus on a different aphid-plant combination: the melon aphid and two melon genotypes, one resistant and the other susceptible to the aphid. We also will be examining the interaction between phloem occlusion mechanisms and another phloem sap-feeding insect family: the Aleyrodidae (whiteflies). We also will begin and hopefully complete our experiments on Objective 2 (Determine how beet leafhopper, Circulifer tenellus, is able to successfully feed on phloem sap despite its stylets being apparently too large to pierce sieve elements without killing them; and determine how inoculation of beet curly top virus by beet leafhopper is blocked on virus-resistant tomato genotypes). We will finish up and publish results from the main part of Objective 3 (Determine how the jasmonic acid and salicylic acid induced defense pathways affect feeding of the silverleaf whitefly, Bemisia argentifolii) Finally, we will conduct additional experiments for Objective 4 (Identify the mechanism of whitefly resistance in Gossypium thurberi).
Impacts
What was accomplished under these goals?
Phloem sap-feeding insects such as aphids and whiteflies are major agricultural pests throughout the world, and most crop plants have at least one phloem sap-feeding insect pest. In addition to the direct damage that they inflict on crops, the great majority of plant viruses are transmitted phloem sap-feeding insects. Thus these insects are a "double threat" to agriculture. Host plant resistance against pests is an environmentally friendly mode of pest control and can preclude the need for spraying pesticides. Development of pest resistant crop varieties is facilitated by understanding the mechanism(s) of resistance, and this project is focused on understanding these mechanisms. Objectives 1 and 3 have been the focus of the current year. These two objectives focus on two potential mechanisms of plant resistance against phloem sap-feeding insects. Objective 1 examines "phloem occlusion mechanisms" as potential mechanisms of resistance against phloem-sap-feeding insects. Phloem is the plant tissue responsible for transporting nutrient-containing sap throughout the plant and is analogous to our circulatory system. Just as our circulatory system has clotting and coagulation mechanisms to prevent loss of blood when our veins or arteries get damaged, plants have analogous systems referred to as "phloem occlusion mechanisms" that prevent loss of sap when plant veins are damaged. Phloem sap-feeding insects such as aphids and whiteflies feed on phloem sap in a manner very similar to the way mosquitoes feed on blood: they have very thin, flexible hypodermic needle-like mouthparts that penetrate through the plant tissue until they reach a phloem tube which they then penetrate and begin sucking sap. It has long been hypothesized that phloem occlusion mechanisms may provide resistance against phloem sap-feeding insects by plugging (occluding) phloem transport tubes when penetrated by these insects, but due to technical hurdles, this hypothesis has never been unambiguously tested until recently in this project. We found that phloem occlusion can provide resistance to aphids, but the resistance is aphid species-specific. In our experimental plant system, phloem occlusion in faba bean provides resistance against the green peach aphid, but not the pea aphid. Objective 3 examines the mechanisms by which inducible plant defenses can slow down development of whiteflies. Although not generally known among the public, plants possess defense pathways (analogous to animal immune systems) that turn on in response to challenges such as attack by pathogens or insects. Previous work has shown that one of these defense pathways can slow down development of whiteflies. While this would not kill whiteflies directly, slower development would make them more vulnerable to natural enemies and thus facilitate biological control of whiteflies. We found that, contrary to expectation, the active defense pathway did not affect whitefly feeding behavior: they fed equally as well on plants where the active defense pathway was elevated or suppressed. Specific goals, activities and outcomes Objective 1. The hypothesis that phloem plugs that block sap flow can be dismantled by aphid saliva was tested. A recent high-profile publication indicated that aphids can disrupt phloem plugs and thus overcome this potential source of resistance. These results were based primarily on in vitro experiments (i.e., "test tube" experiments), so we tested them in vivo (in the living plant) using three aphid species, including the species that was the focus of the high-profile paper. We found that while all three aphid species salivate into phloem tubes when the phloem is plugged, none of them were able to dismantle the plug. This study was published this past year. In another study, we found that faba bean is resistant to the green peach aphid and phloem occlusion is the mechanism. When green peach aphid mouthparts penetrated the phloem, an occlusion response was triggered preventing the aphid from ingesting phloem sap. When we treated the plant veins with a calcium channel blocker which prevents phloem occlusion, green peach aphid was able to feed on faba bean without difficulty. This was a very exciting result as it was the first time it has been demonstrated that phloem occlusion can result in aphid resistance. Objective 2. We did not work on objective 2 this past year. Objective 3. We used the electrical penetration graph (EPG) technique to examine the feeding behavior of whitefly adults and nymphs on four mutant lines of the plant Arabidopsis: mutant 1) the jasmonic acid (JA) defense pathway was enhanced; mutant 2) the JA defense pathway was inhibited; mutant 3) the salicylic acid (SA) defense pathway was enhanced; mutant 4) the SA defense pathway was inhibited. Wildtype Arabidopsis were used as a control. In past work, whitefly development was significantly slower on mutants 1 and 4 and significantly faster on mutants 2 and 3. The EPG technique provides great detail on the feeding behavior of phloem sap-feeding insects, almost all of which takes place inside the plant tissue and thus is not directly observable. Contrary to expectation, feeding behavior differed little among the Arabidopsis mutants and wildtype control. In terms of the most important feeding behaviors, those that indicate how much phloem sap is ingested, there was little or no difference among the plants; consequently, the previous observations that development was slow on mutants 1 and 4 cannot be explained by an effect on feeding behavior. Therefore, if whiteflies feed equally well on all 4 mutants and the wildtype, it implies that differences in phloem sap quality is responsible for the slow development on mutants 1 and 4. Likely candidates are nutritional quality of the sap, digestion inhibitors, or components in the phloem sap that interfere with the whiteflies' endosymbionts upon which they depend for proper nutrition. Objective 4. We did not devote a lot of time to objective 4 this past year. We ran one experiment looking at whitefly oviposition preference between commercial cotton and Gossypium thurberi. We did not detect a significant difference. Objective 5. I have been unable to acquire funding for addressing objective 5. Consequently, Objective 5 has been dropped from the project. Change in knowledge After publication of the high profile paper alluded to previously, it was widely accepted that aphids are able to dismantle phloem occlusion plugs by salivating into the affected phloem tube. As noted previously, that conclusion was based on in vitro experiments, but when we examined this in the real world in vivo condition, we found no evidence to support this. A very recent review on phloem physiology notes that our understanding of the interaction between aphids and phloem occlusion mechanisms must be reconsidered based on our newly published findings from this project. Experiments completed during the current reporting period demonstrated for the first time that phloem occlusion mechanisms can provide resistance against phloem sap-feeding insects. This had been hypothesized at least since the 1990's but had never been verified largely due to lack of suitable techniques to test the hypothesis (see Change in action, below). Our studies examining the effect of Arabidopsis defense pathways against whiteflies have increased our understanding of the mechanism by which these defense pathways affect whitefly development. Change in action The study of interactions between phloem sap-feeding insects and phloem occlusion has been hampered by lack of good techniques to address critical questions. The techniques that we developed that led to the changes in knowledge, mentioned above, will allow other researchers to use our techniques to further our understanding of this critical interaction that can determine whether a plant is resistant or susceptible to a phloem sap-feeding insect.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2013
Citation:
Medina-Ortega, K. J. and G. P. Walker. 2013. Does aphid salivation affect phloem sieve element occlusion in vivo? Journal of Experimental Botany 64: 5525-5535.
- Type:
Book Chapters
Status:
Accepted
Year Published:
2014
Citation:
Ng, J. and G. P. Walker. 2014. Whitefly feeding behavior and transmission of non-circulative plant viruses. In: Vector-Mediated Transmission of Plant Pathogens. (J. K. Brown, ed.). American Phytopathological Society Press. in press
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Progress 01/01/12 to 12/31/12
Outputs
OUTPUTS: The outputs of the project during the past year are related to the project's objectives 1, 3 and 5. Objective 1 is to determine how phloem-feeding insects are able to circumvent or reverse the phloem sieve element sealing response when they puncture sieve elements and determine if plant resistance to phloem-feeding insects can be achieved by a sieve element sealing mechanism that is not susceptible to the insect's manipulations. We completed and published a study to determine whether or not stylet penetration of phloem sieve elements by a specialist aphid feeding on its host plant (Acyrthosiphon pisum and faba bean) triggers protein occlusion of the sieve elements. We also completed a parallel study (not yet published) to determine whether or not stylet penetration of phloem sieve elements by a generalist aphid (Macrosiphum euphorbiae) feeding on faba bean triggers protein occlusion of the sieve elements. Another study was completed (manuscript in preparation) to determine whether or not aphid salivation in vivo reverses protein occlusions of sieve elements. Objective 3 is to determine how the jasmonic acid (JA) and salicylic acid (SA) induced defense pathways affect feeding of the silverleaf whitefly, Bemisia argentifolii, and determine the specific phases in whitefly feeding that trigger the many transcriptional changes in the plant in response to whitefly feeding. Feeding behavior studies were completed for the adult and nymphal stages of B. argentifolii on 5 Arabidopsis genotypes: Wild type Columbia and four mutants: cim10 (constituitively active SA defense pathway); cev1 (constituitively active JA defense pathway); npr1 (impaired SA defense pathway); coi1 (impaired JA defense pathway). Analysis of the feeding behavior data currently is underway. Objective 5 is to enhance our understanding of the mechanisms of transmission of plant viruses by insect vectors. During the past year, I contributed to a review article on insect transmission of plant viruses. PARTICIPANTS: Dr. Karla Medina-Ortega is a postdoc in my laboratory and is working on Objective 1 of the project. Jaclyn Zhou is a PhD student in the Cell, Molecular and Developmental Biology interdepartmental program at the University of California, Riverside (UCR). She is working on Objective 3 of the project. Dr. Linda Walling is a Professor in the Department of Botany and Plant Science at UCR. She and I are co-advisors for Jaclyn Zhou and Dr. Walling is involved with Objective 3 of the project. Dr. James Ng is an Associate Professor in the Department of Plant Pathology and Microbiology at UCR. He is a collaborator on a new objective that was substituted for the original objective 5 (see Project Modifications). TARGET AUDIENCES: The target audience of this project is the research community, especially researchers working on mechanisms of plant resistance against insect pests and breeding insect pest-resistant plants, and researchers studying mechanisms of transmission of plant viruses by insect vectors. PROJECT MODIFICATIONS: I made two unsuccessful attempts to secure funding from the California Citrus Research Board for screening the UCR Citrus Variety Collection for resistance to the Asian citrus psyllid in order to identify resistant genotypes. This was the original objective 5 of the project. Without funding, this objective cannot be met. In place of objective 5, I substituted a new objective related to an area of research that has developed since writing the original project proposal. This new objective is to enhance our understanding of the mechanisms of transmission of plant viruses by insect vectors.
Impacts
Regarding Objective 1, the study that was completed and published this year found that in one system of a specialist aphid and its host plant (Acyrthosiphon pisum and faba bean), stylet penetration of sieve elements usually does not trigger protein occlusion of the sieve elements. In contrast, protein occlusion of sieve elements is triggered by stylet penetration of faba bean sieve elements by a generalist aphid, Macrosiphum euphorbiae. This results in greatly reduced feeding success by M. euphorbiae on faba bean; they experience great difficulty initiating phloem sap ingestion after penetrating a sieve element. In response, they usually give up, withdraw their stylets from the sieve element, and then try another sieve element. This feeding difficulty is the likely reason for the stunted body size when this aphid is reared on faba bean. This is a very exciting result as it shows for the first time that the ability of an aphid to avoid triggering protein occlusion of sieve elements varies among aphid species and can determine the relative susceptibility of a plant to different aphid species. In another study, we found (as expected from research in other labs) that when protein occlusion of sieve elements is experimentally induced, aphids switch behavior from sap ingestion to pumping saliva into the sieve element. However, we refuted the hypothesis formulated in these previous studies that the saliva would break up the protein occlusion and allow sap to flow through the sieve element. At least in the A. pisum / faba bean system, salivation into an occluded sieve element in vivo did not affect the occlusion. Regarding Objective 3, earlier studies detected retarded nymphal development on Arabidopsis mutant plants that either had the jasmonic acid (JA) defense pathway constituitively active or the salicylic acid (SA) defense pathway impaired; and conversely found accelerated nymphal development on mutant plants that either had the SA defense pathway constituitively active or the JA defense pathway impaired. The effect of the same JA and SA mutants on adult feeding behavior did not follow this pattern, and in some cases was opposite of expectations. For example, mean duration of phloem sap ingestion is expected to be higher on favorable hosts; however, it was highest on cev1, the genotype with the JA defense pathway constituitively active, and where whitefly nymphal development was retarded. Analysis of the effect of these mutants on feeding behavior of B. argentifolii nymphs is underway.
Publications
- Stafford, C. A., G. P. Walker & D. E. Ullman. 2012. Hitching a ride: vector feeding and virus transmission. Communicative & Integrative Biology 5: 43-49.
- Walker, G. P. & K. J. Medina-Ortega. 2012. Penetration of faba bean sieve elements by pea aphid does not trigger forisome dispersal. Entomologia Experimentalis et Applicata 144: 326-335.
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Progress 01/01/11 to 12/31/11
Outputs
OUTPUTS: The main activities in the past year have been conducting and analyzing various experiments. The major experiments conducted and analyzed were: 1) experiments to determine if penetration of phloem sieve tubes by aphids triggers the primary phloem defense response (formation of protein plugs that clog sieve tubes); 2) experiments to determine if aphid saliva is capable of reversing a proteinaceous sieve tube plug; 3) experiments to determine the effect of the insecticide imidacloprid on potato psyllid feeding behavior, specifically the phloem-related behaviors associated with transmission of the Zebra Chip Disease pathogen which causes major economic loss in potato production. In addition, experiments were conducted to examine the effect of four defense mutants of Arabidopsis thaliana on feeding behavior of the whitefly Bemisia argentifolii; analysis is pending. Other activities include mentoring and training a new PhD whom I hired as a postdoc on a recent NSF grant that I received, and mentoring an undergraduate student who is conducting an original research project in my lab. Events engaged in during the past year include 1) organizing and securing funding for an international symposium on Hemiptera-Plant Interactions in Piracicaba, Brazil (I was one of 4 organizers and I secured travel support from NSF for students and early career scientists); 2) I made a presentation at the Brazilian conference as well as at the annual meeting of the Entomological Society of America; 3) research from my lab was presented by a student at the 2011 SCRI Zebra Chip Annual Meeting in San Antonio, Texas; 4) I trained a postdoctoral scientist from the University of Florida who came to my lab for a week to learn some of the techniques developed in my lab so that he can use them in his research in Florida. PARTICIPANTS: Dr. Karla Medina-Ortega is a postdoc in my laboratory and is working primarily on Objective 1 of the project. Jaclyn Zhou is a PhD student in the Cell, Molecular, and Developmental Biology interdepartmental program at UCR. She working on objective 3 of the project. Dr. Linda Walling is a professor in the Department of Botany and Plant Science at UCR. She and I are co-advisors for Jaclyn Zhou and is involved with objective 3 of the project. Dr. James Ng is an assistant professor in the Department of Plant Pathology and Microbiology at UCR. He is a collaborator on a new objective in the project (see Project Modifications) where we are studying mechanisms of transmission of plant closteroviruses by insect vectors. His postdoc, Dr. Angel Chen, also has worked on this project. Dr. Diane Ullman is a professor in the Department of Entomology at the University of California, Davis. She also is a collaborator on a new objective in the project (see Project Modifications). We have collaborated on a study examining how a plant virus directly affects its insect vector in a way that enhances virus transmission. Her graduate student, Candice Stafford, conducted that work in my laboratory. As part of the project, I have been training and mentoring the postdoc on the project, Dr. Karla Medina-Ortega, and I have been training the graduate student, Jaclyn Zhou. I also have been guiding an undergraduate student, Lauren Chun, who is working on an original research project in my lab. Additionally, I have been training and guiding a recent UCR graduate, Jeffrey Lukito, who is planning on attending graduate school in Entomology. Mr. Lukito is working on an original research project under my guidance. I also trained a postdoctoral scientist, Dr. Felix Cervantes, from the University of Florida who came to my lab for a week to learn some of the techniques developed in my lab so that he can use them in his research in Florida. TARGET AUDIENCES: The primary target is the scientific community, and as such, results have been published in peer-reviewed scientific journals and presentations made at scientific meetings. PROJECT MODIFICATIONS: I have made two unsuccessful attempts at securing funding from the California Citrus Research Board for screening the UCR Citrus Variety Collection for resistance to the Asian citrus psyllid and identifying resistant genotypes. This was objective 5 of the project. Without funding, this objective cannot be met. In place of objective 5, I am pursuing an area of research I have been involved in that has panned out since I wrote the original objectives. This new objective is to enhance our understanding of the mechanisms of transmission of plant viruses by insect vectors (mostly phloem-feeders).
Impacts
Results of the experiments on phloem defense responses to feeding by aphids do not support the current paradigm that aphid feeding triggers a phloem defense response and that salivary components injected by the aphids are necessary to reverse the defense response. The primary defense response examined is the formation of protein plugs that clog phloem sieve tubes which are the conduits for sap that transports nutrients from one part of the plant to another. In order for aphids to successfully feed, these conduits must remain open and flowing. Contrary to the current paradigm, penetration of sieve tubes by aphid mouthparts does not trigger a proteinaceous plug, and sap flows freely despite the sieve tube being penetrated. Also when proteinaceous plugs are experimentally triggered and the aphid begins to inject large amounts of saliva in response to the plug, results so far indicate that the saliva does not reverse the plug, again contrary to the current paradigm. In total, the results indicate that our current understanding of the role of sieve tube plugging in aphid defense and how aphids can overcome this defense needs re-evaluation. In order to conduct the research in the phloem-defense project, I had to develop a new technique for examining phloem sieve tube response to insect feeding. All previously published studies, of which I am aware, that examined sieve tubes as they were fed upon by aphids or whiteflies used chemical fixation techniques. Chemical fixation is a relatively slow process and cannot be used to examine sieve tubes at precise moments during insect feeding behavior. While cryofixation, freeze substitution, electrical penetration graphs (EPGs), and confocal microscopy are not new techniques, nobody to my knowledge had put them all together to study the interactions between these insects and the sieve tubes on which they feed. Integrating these techniques as I have done in this project provides data that was not possible by the earlier techniques. Consequently, I expect the techniques that I developed in this project to be widely adopted by other labs interested in phloem-insect interactions. Application of imidacloprid to potato plants at recommended field rates significantly reduced potato psyllid salivation into phloem and ingestion of phloem sap, the behaviors associated with inoculation and acquisition, respectively, of the Zebra Chip Disease pathogen. This effect lasted at least 4 weeks post-application.
Publications
- Stafford, C. A., G. P. Walker & D. E. Ullman. 2011. Infection with a plant virus modifies vector feeding behavior. Proceedings of the National Academy of Sciences 108: 9350-9355.
- Chen, A. Y. S., G. P. Walker, D. Carter & J. C. K. Ng 2011. A virus capsid component mediates virion retention and transmission by its insect vector. Proceedings of the National Academy of Sciences 108: 16777-16782.
- Butler, C. D., G. P. Walker & J. T. Trumble 2012. Feeding disruption of potato psyllid, Bactericera cockerelli, by imidacloprid as measured by electrical penetration graphs. Entomologia Experimentalis et Applicata 142: 247-257.
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Progress 10/01/10 to 12/31/10
Outputs
OUTPUTS: This is a new project that started October 1, 2010, so this report covers only a three month period. The only output so far is in the category "Activity": experiments on the effect of aphid feeding on sieve element clogging have been initiated and are continuing. Current findings and those through June 2011 will be disseminated at an international symposium on Hemiptera-Plant Interactions in Piracicaba, Brazil, July 2011. PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: The primary target audience is the scientific community and plant breeders. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
This is a new project that started October 1, 2010, so this report covers only a three month period. Findings so far indicate that contrary to expectations, initial penetration of sieve elements by pea aphid does not trigger forisome dispersal and sieve element clogging in legumes. This suggests that the seal between the aphid stylet tips and the sieve element plasmalemma is very tight and does not permit escape of phloem sap through the penetration site; otherwise loss of turgor pressure in the sieve element would trigger forisome dispersal and sieve element clogging. This also brings to question the role of aphid saliva which is secreted into sieve elements immediately following initial penetration; this initial salivary secretion had been hypothesized to reverse sieve element clogging that results from initial puncture, but results so far indicate that such clogging does not occur.
Publications
- Walker, G. P., T. M. Perring & T. P. Freeman. 2010. Life history, functional anatomy, feeding and mating behavior. In Stansly, P. A. & S. Naranjo (eds.) Bemisia: Bionomics and Management of a Global Pest. Springer. pp. 109- 160.
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