SEED TRANSMISSION OF CUCUMBER MOSAIC-VIRUS IN SPINACH

Spinach (Spinacia oleracea) seed from a commercial breeding line suspe cted of harboring cucumber mosaic virus (CMV) was analyzed for seed tr ansmission of the virus. Initial seed grow-out tests and enzyme-linked immunosorbent assay studies indicated that CMV was present in this se ed lot at a level of nearly 15%. To verify these results and gain insi ght into the mechanism of seed transmission, four combinations of cros ses between healthy and/or infected parent plants were conducted. None of the spinach seedlings derived from crossing healthy male and healt hy female plants contained CMV, whereas a portion of seedlings derived from all of the other three crosses, i.e., healthy male and infected female, infected male and healthy female, and infected male and infect ed female plants, were infected with CMV. The results demonstrate that CMV is seed transmitted in spinach and indicate that both male and fe male parent plants can serve as infection sources. Ultrastructural stu dies, including immunogold labeling, revealed the presence of virus pa rticles in the cytoplasm of ovary wall cells, ovule integuments and nu cellus, anther, and seed-coat cells, as well as fine fibril-containing vesicles and electron-dense inclusions of amorphous aggregates in the central vacuoles of these cells. In addition, reverse transcription-p olymerase chain reaction (RT-PCR) was used to amplify 860-bp cDNA frag ments containing the CMV coat protein (CP) gene from the embryo, endos perm, and pollen tissues of CMV-infected plants. Taken together, these studies indicate that CMV occurs in virtually all spinach reproductiv e tissues. Analysis of several RT-PCR amplified and cloned CP genes an d flanking sequences from parent and progeny plants revealed that the spinach-infecting CMV was a member of subgroup II. Furthermore, cDNA s equencing and restriction endonuclease mapping consistently revealed t wo sequence variants, designated SP103 and SP104, in most plants analy zed; These data suggest that there may have been mixed infections of t wo distinct, seed-transmitted CMV variants in spinach.