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United States Department of Agriculture

Agricultural Research Service


Location: Vegetable Crops Research

2012 Annual Report

1a. Objectives (from AD-416):
Workshops will be held at regional and national Allium meetings. We will develop a mechanical transmission protocol or Iris Yellow Spot Virus and validate sources of resistance or tolerance identified in field evaluations.

1b. Approach (from AD-416):
Sources of IYSV resistance or tolerance will be validated in controlled environments by mechanical inoculations. Deliver validated germplasms to private and public sector breeder. Develop workshops for public and private-sector researchers, students, and regional grower and consumer groups for onion to illustrate the usefulness of genomics to solve high-priority research goals.

3. Progress Report:
Efforts continued to develop a protocol for mechanical inoculation of onion plants with Iris yellow spot virus (IYSV) with the goal to develop a screening protocol. So far, efficiency of infection obtained by mechanical inoculation has been low in the range of 30 to 40%. Attempts to improve the efficiency are ongoing. The potential role of related tospoviruses in the infection process of IYSV was evaluated. Using two distinct tospoviruses, IYSV and Tomato spotted wilt virus (TSWV), we investigated the inter-virus interactions at the molecular level in dually-infected plants. Datura stramonium is a permissive host for TSWV, while it restricts the movement of IYSV to inoculated leaves. In plants infected with both viruses, however, TSWV facilitated the selective movement of the viral gene silencing suppressor (NSs) gene of IYSV to the younger, uninoculated leaves, thus turning a restrictive host into a more permissive one for IYSV. The small Ribonucleic acids (RNA) expression profiles of IYSV and TSWV in single- and dually-infected Datura plants showed that systemic leaves of dually-infected plants had reduced levels of TSWV N gene-specific small interfering RNAs (siRNAs). No TSWV NSs-specific siRNAs were detected neither in the inoculated nor systemic leaves of dually-infected datura plants indicating a more efficient suppression of host silencing machinery in the presence of NSs from both viruses as compared to the presence of only TSWV NSs. Moreover, siRNAs of neither IYSV N nor NSs genes in single- as well as dually-infected plants could be detected. Our study identified a new role for the viral gene silencing suppressor in potentially modulating the biology and host range of viruses and underscores the important role of virally-coded suppressors of gene silencing in virus infection of plants. This is the first experimental evidence of genetic complementation between two distinct tospoviruses in the Bunyaviridae family. To better understand the biological variability of IYSV, several field-collected isolates were evaluated on selected indicator hosts under controlled greenhouse conditions. While considerable information on the genetic diversity of the virus is available, little is known about the biological variability of the virus. Using two experimental hosts, Nicotiana benthamiana and Datura stramonium, IYSV from naturally infected onion fields was evaluated to determine the existence of biologically different isolates using the following criteria: ability to establish infection and become systemic, and the severity of the disease caused in inoculated plants. Additionally, the nucleocapsid gene of these biologically distinct isolates of IYSY was characterized at the molecular level. This research relates to Objective 1, Determine the genetic basis of and initiate selection for carrot, onion, cucumber, and melon quality attributes influencing human nutrition and health, disease resistances, and yield and quality components, and stress tolerance in cucurbits, and perform field performance and quality trials.

4. Accomplishments

Last Modified: 2/23/2016
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