SYMPTOM SEVERITY OF BEET WESTERN YELLOWS VIRUS-STRAIN ST9 IS CONFERRED BY THE ST9-ASSOCIATED RNA AND IS NOT ASSOCIATED WITH VIRUS RELEASE FROM THE PHLOEM

The ST9 strain of beet western yellows virus (BWYV ST9) is unique amon g BWYV strains because it encapsidates not only its 5.6-kb genomic RNA but also a 2.8-kb RNA of distinct nucleotide sequence, designated as the STS-associated RNA. We obtained isolates of BWYV ST9 that are free of the associated RNA by transfecting Nicotiana tabacum protoplasts w ith transcripts of an ST9 genomic cDNA clone. Aphids were fed on extra cts of infected protoplasts and were transferred to young Shepherd's P urse (Capsella bursa-pastoris) plants. When the protoplast inoculum wa s ST9 genomic transcript or virion RNA of the L-1 strain of BWYV (free of the associated RNA), symptoms were mild and characteristic of BWYV L-1. When STS-associated RNA was included in the inoculum with genomi c RNA of either source, subsequently infected Shepherd's Purse plants showed the severe symptoms that are characteristic of BWYV ST9. Inclus ion of STS-associated RNA in the inoculum with ST9 genomic RNA increas ed the accumulation of capsid antigen and ST9 genomic RNA, relative to infections initiated with ST9 genomic RNA alone. Using gold-labeled a ntibody and electron microscopy, we assessed the distribution of virio ns in Shepherd's Purse plants. Regardless of whether the associated RN A was present, sites showing immunoreactivity above background levels were restricted to the phloem, suggesting that the increased BWYV ST9 titer and symptom severity that are correlated with the presence of th e STS-associated RNA are not due to escape of the infection from phloe m limitation. (C) 1994 Academic Press, Inc.