Location: Vegetable Crops Research2011 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
Iris Yellow Spot Virus (IYSV) was mechanically inoculated onto diverse plants to determine host range. Datura stramonium had 25-30 small chlorotic local lesions 10-12 days post inoculation (DPI) and the virus did not spread. Nicotiana benthamiana had chlorotic symptoms 7 to 10 DPI which expanded to cause drying of leaves by 20-25 DPI. Infection by IYSV was confirmed by enzyme-linked immunosorbent assay (ELISA) and reverse transcription polymerase chain reaction (RT-PCR) both in inoculated and systematic leaves. Several IYSV isolates were screened on these plants and two biologically distinct strains of IYSV were characterized among isolates from California (CA), Idaho (ID), and Washington (WA). Virus movement was slower as compared to ID and WA isolates. Allium altaicum, A. galanthum, A. roylei, A. schoenoprasum, A. tuberosum, and A. vavilovii were evaluated under natural field conditions. Symptoms of IYSV were observed on leaves of these Allium species in experimental plots in Las Cruces, NM and IYSV was detected in symptomatic leaves of A. altaicum, A. vavilovii, A. tuberosum, A. schoenoprasum and A. roylei using ELISA and RT-PCR. Attempts were made to mechanically inoculate IYSV onto onion. IYSV inoculum from N. benthamiana was homogenized in 0.01 M sodium phosphate buffer (pH 7.0) containing 4% ß-mercaptoethanol and manually applied using cotton buds to leaves on 40-50 day old onions dusted with carborandum. Inoculated plants were maintained at 30/23 C (day/night). Chlorotic lesions with regular and irregular borders were observed 15-20 DPI. Samples were positive for IYSV as tested by ELISA. This is an important step toward developing efficient mechanical inoculation for IYSV in onion. So far 20% inoculated plants were infected by IYSV and we continue to improve the inoculation efficiency. IYSV genome consists of 3 ribonucleic acids (RNAs). While the genome structure and organization of the two smaller (middle and small) RNAs have been described, the L RNA remained uncharacterized. The L RNA of IYSV is 8880 nucleotides in length and contains a single open reading frame. The primary translation product shares many features of an RNA-dependent RNA polymerase coded by L RNAs of other tospoviruses. Project is monitored through e-mail and phone calls. Refereed Journal Articles: 1. Bag, S., and H.R. Pappu. 2009. Symptomatology of Iris yellow spot virus in selected indicator hosts. Plant Health Progress. doi:10.1094/PHP-2009-0824-01-BR. 2. Bag, S., K.L. Druffel and H.R. Pappu. 2010. Structure and genome organization of the large RNA of Iris yellow spot virus (genus Tospovirus, family Bunyaviridae). Archives of Virology 155:275–279 (DOI 10.1007/s00705-009-0568-5). 3. Cramer, C.S, S. Bag, H.F. Schwartz, and H.R. Pappu. 2011. Susceptibility of onion (Allium spp) relatives to Iris yellow spot virus. Plant Disease DOI: 10.1094/PDIS-11-10-0819.