RHABDOVIRUS-LIKE AND CLOSTEROVIRUS-LIKE PARTICLES IN ULTRATHIN SECTIONS OF RIBES SPECIES WITH SYMPTOMS OF BLACK-CURRANT REVERSION AND GOOSEBERRY VEINBANDING DISEASES

In attempts to determine the causal agents of blackcurrant reversion ( BCRD) and gooseberry veinbanding (GVBD) diseases of Ribes species, det ails of the ultrastructure of different kinds of tissue from plants af fected with these different diseases were studied. In three of 12 blac kcurrant plants affected with BCRD, leaves and flowers of plants showi ng symptoms typical of the severe (R) form of the disease, contained r habdovirus-like particles c. 65-80 nm x 215-485 nm. They were seen mos t often in the nucleus of cells as single particles but were also foun d in clusters or rafts. In leaves, these virus-like particles (VLPs) w ere present only in cells associated with the xylem parenchyma where t hey occurred as membrane-bound clusters within the nucleus. In flowers , they were also found in phloem parenchyma cells in the peripheral cy toplasm and very occasionally in the cytoplasm of epidermal cells. All non-nuclear VLPs were membrane-bound, either singly or in groups and the membrane seemed to be part of the endoplasmic reticulum. The propo rtion of vascular cells containing these VLPs was very low (< 1%). In a few cells, smaller bacilliform particles, c. 40-50 nm x 200-250 nm, were found in the nucleus together with the larger particles. Double-m embrane bodies, detected in fig leaves affected with fig mosaic (the a gent of which is also mite-transmitted), were not detected in any BCRD -affected plants. In leaf tissue of one of three gooseberry and one of two blackcurrant planes affected with GVBD, two kinds of VLPs were fo und. Rhabdovirus-like particles, similar to those in BCRD-affected mat erial, were present in the nuclei, perinuclear space and cytoplasm of xylem parenchyma cells. They were c. 60-72 nm x 155-230 nm but there w as no evidence of the smaller rhabdovirus-like particles detected in a few cells of BCRD-affected tissues. The second kind of VLP was found in noncrystalline masses, with a mean centre-centre spacing of c. 10 n m, in the cytoplasm of phloem cells. These particles, together with ot her ultrastructural changes, were typical of those reported for aphid- transmitted closteroviruses. No badnavirus-like particles, reported pr eviously from GVBD-affected plants, were observed in any of the plants studied. The significance of these findings in relation to these two important diseases of commercial Ribes species is discussed.