2011 Annual Report
1a.Objectives (from AD-416)
Specific experiments to be completed at Penn State by Gildow and Cox-Foster include: l. Comparison of RPV subcellular localization in vector and non-vector genotypes utilizing immunofluorescent microscopy combined with immunogold labeling for ultrastructural identification of virus and virus-tissue interactions associated with transmission barriers.
2. Characterization of RPV trafficking pathways utilizing injection recovery bioassays from vector and nonvector aphids to verify acquisition of infectious virus and to study virus viability and stability in the aphid hemocoel environment.
1b.Approach (from AD-416)
Identification of RPV localization in S. graminum will use a tandem immunofluorescence (IF)-transmission electron microscopy (TEM) approach to facilitate viral subcellular localization by TEM. Briefly, aphids either membrane fed or microinjected with purified virus will be fixed and paraffin embedded. Paraffin sections of gut and salivary tissues will be incubated in primary virus-specific polyclonal antibody, followed by secondary antibody linked to Alexa fluor 488. These will pinpoint the cells for EM sectioning. Then routine ultrastructural TEM studies will determine the cellular mechanisms regulating virus recognition and transport. Aphids of each nonvector genotype will be examined for pathways of virus acquisition through various sites along the anterior and posterior midgut and hindgut. An additional exciting finding that might emerge from this study would be the identification of a hindgut or midgut escape barrier by which virus is permitted to enter and traffic through the cell, but is prevented from exiting. These results when combined with ultrastructural observations of virus associations with cellular organelles should indicate whether virions are moving along the endocytotic-exocytotic transmission pathway. Failure to transmit may be caused by structural barriers to diffusion through the basal plasmalemma of the gut or ASG, missing cellular components preventing virus recognition and endocytosis at the cell membranes, or disruption of the transcytosis pathway preventing directional movement of virions through gut or ASG cells. Virions accumulating in HG or ASG as a result of failure to be transported through cells are expected to be visualized as aggregates of virions in lysosomes. Characterization of RPV trafficking pathways will use injection recovery bioassays to characterize RPV acquisition ability of all vector and nonvector F2 clones and determine whether non-vector aphids with an apparent strong ASG barrier have an immunological response that degrades virions in the hemolymph. Aphids with infectious virions in the hemolymph, but unable to transmit will identify genotypes with a major ASG transmission barriers. Efficiency of virus recovery should parallel efficiency of virus acquisition. Virus acquisition in hemolymph could be quantitated by immunospecifc EM analysis and real-time RT-PCR if appropriate. Real-time RT-PCR methods to quantitate RPV in hemolymph were developed during the previous grant cycle. If RPV virions are visualized escaping from gut cells into the hemocoel by IF and TEM, and verified by RT-PCR, but infectious virus cannot be recovered by hemolymph bioassay, this would suggest an aphid cellular immunological basis for a loss of transmission. Such a discovery would open a new area of study, as virus viability in the aphid hemocoel has not been extensively examined.
Last year we made a serendipitous discovery that plant host proteins were associated with purified preparations of the yellow dwarf virus, and that chemical additives used during the purification process could alter the composition of plant proteins associated with virus particles. Later it was discovered that changes in the virus-associated plant proteins were linked to changes in the aphid transmissibility of the virus. During the past year ultrastuctural studies were conducted that determined the non-transmissible virus preparations with an altered host protein contingent were ingested by the aphid and the virus was able to move into the gut tissue and be released into the insect blood. This suggests that the host proteins are likely involved in stabilizing the virus in the insect blood or helping to move the virus through the insect salivary system. Alternatively, the plant proteins may play a role in helping the virus establish an infection when the aphid deposits the virus into a new host plant. Using a highly targeted method of mass spectrometry called Selected Reaction Monitoring, peptides from six host plant proteins found to be associated with purified, transmissible virus were detected in aphids. Four of these proteins were more highly abundant in aphids that were fed on virus-infected tissue as compared to being fed on healthy tissue. These data prove that the host proteins we found associated with purified virions do associate with virus when it is circulating in the plant and the association is not a by-product of the purification. Furthermore, these four proteins are associated with the virus when they are ingested into the aphid. The role of these plant proteins in insect transmission of the virus is still unknown, but these proteins may be potential targets that could be modified in the plant to prevent transmission from plant to plant. Progress was monitored through regular lab meetings, email and phone conversations.