Seed Transmission of Viruses in Cowpea and Azuki Bean Plants

Abstract

Ultra-thin sections of apical meristem from flower buds of cowpea and azuki bean plants, infected with cowpea aphid-borne mosaic virus (CAMV) or with azuki bean mosaic virus (AzMV), were examined under electron microscope for the distribution of virus particles and cytoplasmic inclusions. In a cowpea variety “Zairai-Tsurunashi-Kintoki” infected with CAMV, both virus particles and cytoplasmic inclusions were seen in the preparation of meristematic tissues at a distance of 0.1-0.2mm from apical end, but these were found more frequently in plants infected with a strain (CAMV-3) known to be transmitted through seeds at a moderately high percentage (approx. 28%) than in those infected with another strain (CAMV-1) of a lower percentage of seed transmission (approx. 6%). No virus particles and inclusions were detected in the cell layer of 0.1mm from apical end. Similar results were obtained with another cowpea variety infected by CAMV-3 and -1, although in meristem infected by CAMV-1 the virus particles appeared to be located farther than 0.2mm from apical end.In azuki bean plant, virus particles and inclusions were detected in meristematic tissues at a distance of 0.1-0.2mm from apical end, infected by AzMV that is seed transmissible, but none in the comparable region, down to 0.2-0.3mm, of tissue infected by CAMV that is not transmitted through seed of azuki bean.Electron microscopic examination of individual pollens for virus particles revealed that 6.8 and 18.6% of pollens from cowpea plants infected with CAMV-1 and CAMV-3, respectively, contained virus particles.A hypothesis is proposed to explain seed transmission of virus or lack of it in cowpea and azuki bean as follows. In seed transmitted virus-host combinations: virus is distributed in meristematic tissues of flower buds except a layers of 0.1mm from apical end, and can move into male or female gametes, resulting in infection of some embryos with virus in concentration high enough to produce infected seeds. Developing immature embryos may occasionally be infected through direct invasion by virus from mother plant, but virus concentration is not sufficient to cause seed transmission. In non-seedtransmitted virus-host combinations: virus is not distributed in meristematic tissues of 0.2 or 0.3mm from apical end of flower buds, and infection of both gamete and embryo does not occur. In addition, developing immature embyos are not liable to infection through direct invasion by virus from mother plant.