Accumulation of Pathogenesis-Related Proteins in Tobacco Leaves Irradiated with UV-B

Nicotiana tabacum

L. (cv. Petit Havana SR1) were grown under ultraviolet-B (UV-B, 290–320 nm) irradiation, and soluble proteins were extracted from the leaves. Two-dimensional electrophoresis revealed that a minimum of 12 polypeptides were induced by UV-B. Polypeptides which were so abundant as to be detectable by Coomassie brilliant blue staining were then subjected to N-terminal amino acid sequence analyses. Two of the polypeptides were identified as a 23 kDa protein of PS II and 6 as a pathogenesis-related protein 5 (PR-5). Immunoblotting demonstrated that other PR proteins, PR-1 and PR-3 were also induced by UV-B. Salicylic acid (SA), which is an important component of signal transduction that leads to the expression of PR proteins and exhibition of acquired resistance to pathogens, increased in response to exposure to UV-B. In addition, the activity of phenylalanine ammonialyase, which catalyzes the synthesis from phenylalanine of trans-cinnamic acid, the endogenous precursor of SA, was transiently increased by UV-B irradiation. These results suggest that UV-B activates the signal transduction pathway, which is a common step in pathogen infection.