An eclipse of pea seed‐borne mosaic virus in vegetative tissue of pea following repeated transmission through the seed

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Abstract
Summary: Four isolates of pea seed‐borne mosaic virus (PSbMV) representing pathotypes P1 (isolates US and Q) and P4 (isolates S4 and S6), and groups III (US and Q) and V (S4 and S6) have been used in a study of the survival and partitioning of PSbMV under conditions of continuous seed transmission in the commercial pea cultivar Dundale.Assays suitable for detecting virus in small tissue samples were developed, and included dot‐immunobinding assay with antisera to both PSbMV and cytoplasmic inclusion body (CIB) protein, and dot hybridisation assay (DHA) with cDNA transcribed from virus RNA. Under the conditions of our experiments, seed transmission occurred at rates exceeding 90% for all virus isolates.Virus was detectable by serology and symptoms in inoculated plants, and in all vegetative tissue of second generation plants raised from seed of the inoculated plants. However, in the third, fourth and fifth sequential generations raised from seed, all plants were symptomless. Neither virus nor CIB were detectable in leaf, stem or roots by serology, but both were readily detectable in some floral parts, and in immature and mature seed. Mature seed contained virus and CIB antigen in the testa, cotyledon and embryo. Inoculum prepared from whole seeds was infectious. The testa was shown not to be involved in transmission between generations, thus implicating the embryo alone in vertical transmission. Virus antigen could not be detected in the emerging cotyledons of germinating seed and all true leaves by serology, but the leaves contained PSbMV RNA detectable by DHA.These results show that PSbMV infection can be transferred through the vegetative phase at a subliminal level, and reaches relatively high concentrations in floral parts and seeds. Thus PSbMV may be maintained at a high level of infection in seed in the absence of any apparent symptoms in the plant, and without a requirement for horizontal transmission between plants by vectors. Such a mechanism may explain the high levels of infection commonly reported in pea breeding lines.