SALICYLIC-ACID, ETHYLENE, AND PATHOGEN RESISTANCE IN TOBACCO

Salicylic acid (SA) plays an important regulatory role in the resistan ce response of N-gene tobacco to tobacco mosaic virus (TMV). To determ ine whether SA accumulation following inoculation with a necrotizing p athogen is a generalized phenomenon, endogenous SA levels were quantif ied following inoculation of two species of Nicotiana with viral, bact erial, and fungal pathogens. In Xanthi-nc (NN) and Xanthi (nn) cultiva rs of Nicotiana tabacum, tobacco necrosis virus produced necrotic lesi ons and a more than 28-fold increase in total SA (the sum of free and beta-O-D-glucosyl SA) within 96 hr. Significant increases in SA were a lso observed in Nicotiana sylvestris inoculated with a mutant TMV stra in capable of producing necrotic lesions in this tobacco species. Infi ltration of Xanthi-nc and Xanthi tobacco leaves with Pseudomonas syrin gae pv. tomato also produced a 100-fold increase in total SA within 72 hr. Stem injection with blue mold (Peronospora tabacina) sporangia pr oduced 3.6- and 18.8-fold increases of free and total SA, respectively , in previously uninfected leaves, which coincided with an increase in resistance. Exposure of TMV-inoculated tobacco leaf disks to ethylene (10 mu l/L) resulted in a reduction in SA accumulation. However, an i nhibitor of ethylene action, 2,5-norbornadiene, did not produce a sign ificant change in SA accumulation in TMV-inoculated leaf tissues. The relatively minor negative effect of ethylene on SA production suggests that ethylene is not directly involved in the signal transduction pat hway that leads to SA accumulation and export from the tissues infecte d with necrotizing pathogens.