Seed Transmission of Viruses in Cowpea and Azuki Bean Plants

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Relations between seed transmission and gamete infection were studied on cowpea and azuki bean plants inoculated with seed-borne viruses. Virus-host combinations tested were, (1) azuki bean mosaic virus (AzMV)-azuki bean, (2) cowpea aphid-borne mosaic virus (CAMV)-cowpea, (3) cucumber mosaic virus (CMV)-azuki bean, (4) subclover mottle virus (SMV, a strain of broad bean wilt virus)-cowpea, and (5) CAMV-azuki bean. Seed transmission occurs in the former two combinations, but does not occur in the remaining three combinations, as described in a previous paper (Tsuchizaki et al., 1970). Seed transmission through either pollen or ovule of infected plants occurred only in the case of virus-host combinations that were seed transmitted. CAMV was recovered from pollen, anthers, and ovaries, of CAMV-infected cowpea plants, whereas SMV was recovered from anthers and ovaries, but not from pollen, of SMV-infected cowpea plants. No correlation between virus concentration in floral parts and seed transmission was found in virus infected cowpea or azuki bean plants, although the virus concentration was generally lower in pistils than in petals or leaves. Virus concentration was higher in anthers, but lower in ovaries, in CAMV-infected cowpea plants than in CAMV-infected azuki bean plants. Cowpea plants were inoculated with CAMV in their flowering time. The mature seeds producted on them were collected separately according to their flowering days, and transmission of CAMV through the seeds were examined. The result showed that seed transmission occurred only when the mother plants were inoculated with CAMV at least 20 days before the flowering day, while CAMV was recovered from pollen only when the mother plants were inoculated at least 17 days before the flowering day. From these results it was concluded that seed transmission of a virus depended largely on gamete infection by the virus.