Pathogenesis-related proteins in lima bean leaves infected with tobacco ringspot virus and their distribution within and around local lesions

Abstract

Tobacco ringspot virus (TRSV) induces circular, darkbrown local lesions on primary leaves of lima bean (Phaseolus lunatus cv Nemagreen) with a concomitant production of three basic and three acidic pathogenesisrelated (PR) proteins. The three basic proteins are: a 21 kDa protein related serologically to Pinto bean PR-4d and tobacco PR-5 proteins; a 36 kDa glucanase that is related to tobacco PR-2; and, a 31 kDa chitinase related serologically to ethylene-induced bean chitinase. The three acidic 18 kDa lima bean PR proteins are serologically similar and probably are charged isomers of the same protein. The 21 kDa basic protein and the 18 kDa acidic protein accumulated preferentially at the lesion center while the 31 kDa chitinase and TRSV were distributed evenly throughout the necrotic area. In green tissue immediately surrounding a lesion, the amounts of PR proteins were comparable to or lower than those in the necrotic area, and virions were not detected. This mode of spatial distribution indicates that lima bean PR proteins are not involved in TRSV localization, and is consistent with other observations that PR proteins play no direct role in restricting viral spread.