Zhongguo Xiong and Rodolfo Acosta-Leal
Department of Plant Pathology, University of Arizona
'Virgin A Mutant' (VAM) tobacco breeding line contains a single recessive gene conditioning resistance to potato virus Y (PVY) infection. NC745 is an anther-doubled line derived from a cross between VAM tobacco and Coker 86 and exhibits similar PVY resistance. Mechanical inoculation of PVY strain NN on VAM and NC745 plants produced 0/993 and 31/873 infected plants, respectively. Infectious virus and viral RNA were not detected from the asymptomatic plants by back inoculation to susceptible Burley tobaccos and by Northern hybridization. Virus progeny recovered from the infected NC745 plants were able to overcome the resistance in both VAM and NC745 plants and resulted in 100% infection on both plants when mechanically inoculated.
Genetic analysis of crosses between VAM and Burley tobacco confirmed the presence of a single recessive gene conditioning the PVY resistance. Replication of PVY-NN RNA differs in transfected Burley, NC745, and VAM protoplasts. Although PVY-NN replicates in all the protoplasts, protoplast printing experiments showed it replicates much faster in NC745 protoplasts. This increased rate of replication in NC745 may be partially responsible for the generation of resistance breaking isolates.
However, when intact leaves were inoculated a high concentration viruses, PVY-NN does not replicate very well in the inoculated VAM cells. Protoplast printing showed a localized ring of the capsid protein around the nucleus. We have not been able to explain the observed phenomenon. In inoculated NC745 intact leaves, PVY-NN apparently replicate quite well, but not as well as in Burley tobacco leaves. It appears to spread from cell-to-cell, judging from the increased number of infected cells in a time-course study. The intact leaf and protoplast studies suggest that the replication of PVY in the isolated protoplasts and in the intact leaves are not identical. Physiological changes experienced by the protoplasts during the isolation may have affected the outcome of the viral replication
Preliminary data so far indicate that the resistance mechanisms operate at two levels, replication and/or cell-to-cell movement. This resistance mechanism may also affect the replication fidelity of PVY-NN in NC745, leading to a high mutation rate and consequently generating resistance breaking isolates. Additional experiments are underway to further investigate this resistance and resistance interaction phenomenon.
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