Multiple viral determinants affect seed transmission of pea seedborne mosaic virus in Pisum sativum

1 Biotechnology Group, The Danish Institute of Plant and Soil Science, Lyngby, DK-2800, Denmark 2 Department of Microbiology and Center for Gene Research and Biotechnology, Oregon State University, Corvallis 97331-3804, USA 3 Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331-2902, USA 4 Department of Virus Research, John Innes Centre, Norwich NR4 7UH, UK Two pea seedborne mosaic potyvirus (PSbMV) isolates, P-1 DPD1 (P-1), which is highly seed-transmitted, and P-4 NY (P-4), which is rarely seed-transmitted, and chimeras between P-1 and P-4 were analysed to map the viral genetic determinants of seed transmission. Infectivity of chimeric viruses was evaluated by inoculating Pisum sativum with RNA transcribed in vitro from recombinant full-length cDNA clones. The chimeric viruses that were used demonstrated that a genomic segment encoding the 49 kDa protease and putative RNA polymerase was responsible for symptom induction. Attempts to determine transmission of the chimeric viruses in P. sativum cultivars known to transmit P-1 at high frequencies showed that seed transmission is a quantitative character influenced by multiple viral determinants. Seed transmission frequency did not correlate with accumulation of virus in vegetative tissue. The 5' 2·5 kb of the 10 kb PSbMV genome had a major influence on the seed transmission frequency and was analysed further. This showed that, while the helper-component protease was a major determinant of seed transmission, the potyviral P1-protease exerted no measurable influence. Received 13 June 1996; accepted 22 August 1996.