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Potyviral 6K2 Protein Long-Distance Movement and Symptom-Induction Functions Are Independent and Host-Specific

May 2004 , Volume 17 , Number  5
Pages  502 - 510

Carl Spetz 1 and Jari P. T. Valkonen 1 , 2

1Department of Plant Biology and Forest Genetics, Genetics Centre, SLU, PO Box 7080, SE-750 07 Uppsala, Sweden; 2Department of Applied Biology, PO Box 27, FIN-00014 University of Helsinki, Finland

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Accepted 22 December 2003.

Deletion of various portions, or insertion of six histidine residues (6×His) into various positions of the membrane-bound 6K2 protein (53 amino acids) of Potato virus A (PVA, genus Potyvirus), inhibited systemic infection in Nicotiana tabacum and N. benthamiana plants. However, a spontaneous mutation (Gly2Cys) that occurred in 6K2 adjacent to the 6×His insert placed between Ser1 and Gly2 enabled systemic infection in a single N. benthamiana plant. No symptoms were observed, but virus titers were similar to the symptom-inducing wild-type (wt) PVA. N. tabacum plants were not systemically infected, albeit virus propagation was observed in inoculated protoplasts. The 6×His/Gly2Cys mutant was reconstructed in vitro and serially propagated by mechanical inoculation in N. benthamiana. Following the third passage, a novel viral mutant appeared, lacking the last four His residues of the insert, as well as the Gly2 and Thr3 of 6K2. It infected N. tabacum plants systemically, and in the systemically infected N. benthamiana leaves, vein chlorosis and mild yellowing symptoms were observed, typical of wt PVA infection. The mutant virus accumulated to titers similar to wt PVA in both hosts. These results show that the PVA 6K2 protein affects viral long-distance movement and symptom induction independently and in a host-specific manner.

© 2004 The American Phytopathological Society