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United States Department of Agriculture

Agricultural Research Service


Location: Vegetable Crops Research Unit

2013 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:

This project was renumbered from 3655-21000-048-14A to 3655-21000-062-04A. This is the final report, project terminated 05/31/2013. Iris yellow spot virus (IYSV) is an economically important virus and a threat to onion seed and bulb crops in many onion growing regions of the world. In recent years, IYSV has spread rapidly in many states in the United States (US) and in several European and Asian countries. The disease is most damaging to onion seed crops where losses may approach 100%. IYSV is transmitted by the insect onion thrips and with lesser efficiency by tobacco thrips.

Experimental hosts were evaluated to study response to mechanical inoculation under controlled greenhouse conditions, including Arabidopsis thaliana COL 1, Capsicum annuum (Serrano pepper), Cerastium glomeratum (mouse ear chickweed), Chenopodium quinoa, Datura innooxia, D. ferox, D. stramonium, Nicotiana benthamiana, N. tabacum, Solanum melongena (egg plant), and Vigna unguiculata (Heirloom variety). Based on the symptom and disease development and subsequent verification of IYSV infection in these plant species by laboratory tests, C. quinoa, D. stramonium, N. benthamiana, and V. unguiculata were found as suitable experimental hosts for biological studies of IYSV. Knowledge about the existence of IYSV strains that differ in their virulence is essential for developing virus resistant varieties. While considerable information on the genetic diversity of IYSV 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 commercial onion fields were evaluated using the following criteria: ability to establish infection and become systemic, and the severity of the disease in inoculated plants. Additionally, the nucleocapsid gene of these biologically distinct isolates of IYSY was characterized at the molecular level. Results showed that the virus exists as distinct strains that differ in their ability to cause damage - information that will be useful for evaluating and developing resistant cultivars.

Since the virus is efficiently spread by Thrips tabaci, the ability to rapidly detect the virus in thrips vectors would facilitate studies on the role of thrips in the virus epidemiology and formulate better vector management strategies. Using a polyclonal antiserum produced against the recombinant, E. coli-expressed nonstructural protein (NSs) coded by the small (S) Ribonucleic acid (RNA) of IYSV, an enzyme-linked immunosorbent assay (ELISA) test was developed for detecting IYSV in single adult thrips. The approach enabled estimating the proportion of thrips transmitters among a large number of field-collected thrips. Availability of a practical and inexpensive test to identify thrips vectors is useful in epidemiological studies to better understand the role of thrips vectors in IYSV outbreaks in onion.

Efforts continued to develop a protocol for mechanical inoculation of onion plants with IYSV with the goal to develop a screening protocol for disease resistance under controlled conditions. 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), inter-virus interactions were investigated at the molecular level in dually-infected plants. In our previous studies, under this project, we found that D. Stramonium is a permissive host for TSWV, while it restricts the movement of IYSV to inoculated leaves. In plants infected with both viruses, 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. 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.

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 and Objective 2, Utilize current biotechnology to discover and evaluate genetic variation and to map agriculturally important traits in Allium, Cucurbit, and Daucus germplasm, and to develop genetic and breeding stocks.

Last Modified: 7/25/2014
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