The phenylalanine pathway is the main route of salicylic acid biosynthesis in Tobacco mosaic virus-infected tobacco leaves

Abstract

In plants, salicylic acid (SA), a molecule important for resistance to pathogens, is synthesized from phenylalanine or isochorismate. Although SA is reportedly synthesized via the isochorismate pathway in pathogen-infected Arabidopsis, the predominant pathway in pathogen-infected tobacco has not been known. To determine the pathway in tobacco, we studied the gene expression and enzyme activity of phenylalanine ammonia-lyase (PAL) and isochorismate synthase (ICS) in tobacco leaves infected with Tobacco mosaic virus (TMV). Two days after TMV inoculation, necrotic lesions had appeared, and the levels of SA, PAL activity, and transcripts of PAL A and PAL B had increased substantially. In contrast, no ICS activity was detected, and the levels of ICS transcript did not increase, after the formation of necrotic lesions caused by TMV infection. These results suggest that SA is synthesized mainly by the phenylalanine pathway in TMV-infected and necrotic-lesion-bearing tobacco leaves.