1.11.82
INFECTION OF BROME MOSAIC VIRUS IN BARLEY LEAVES IS RESTRICTED PREDOMINANTLY TO CELLS IN OR ASSOCIATED WITH VEINS

XS DING and RS NELSON

Plant Biology Division, Samuel Roberts Noble Foundation, PO Box 2180, 2510 Sam Noble Parkway, Ardmore, OK 73401, USA

Background and objectives
Previous studies have shown that brome mosaic virus (BMV), a tripartite positive-sense RNA virus, is present in many parts of the world infecting barley and several other monocotyledonous plants in the family Gramineae. Although the virus does not cause major economic losses in the agricultural industry, it has been used extensively as a model system to study virus replication and gene expression and virus-plant interactions. We recently investigated the accumulation and spread of BMV in infected barley plants and found, for the first time, that the virus infects predominantly cells in or associated with the leaf.

Materials and methods
Barley cv. Morex was grown under controlled temperature and light conditions. They were inoculated with BMV at the third leaf stage. At various days post-inoculation, the inoculated and systemically-infected leaves of the plants were harvested and analyzed for virus infection by ELISA, immunocytochemistry and in situ hybridization techniques as previously described [1].

Results and conclusions
Numerous reports have demonstrated that BMV can replicate efficiently in barley protoplasts, and its RNA components can be detected in infected barley protoplasts as early as 6 h post-inoculation. By electron microscopy, Paliwal [2] studied the cytological effects of BMV in infected barley leaves and observed many virions in infected mesophyll, phloem and xylem cells. He suggested that mesophyll cells are the main site of BMV replication in barley leaves.

Results from our work show, however, that BMV infects and accumulates predominantly in bundle sheath, phloem and xylem cells, of the vascular tissue, and only a few mesophyll cells adjacent to veins in both inoculated and systemically infected cv. Morex leaves. This finding indicates that BMV is a vascular-associated virus in barley and possibly other monocotyledonous plants. Our results also show that, although BMV can spread rapidly in longitudinal veins, its movement between longitudinal veins via transverse veins is very inefficient. The mechanism by which BMV spreads within and between veins in barley leaves will be further discussed.

References
1. Ding XS, Carter SA, Nelson RS, 1996. BioTechniques 20, 111-15.
2. Paliwal YC, 1970. Ultrastructural Research 30, 491-502.