Activation of a diverse set of genes during the tobacco resistance response to TMV is independent of salicylic acid; induction of a subset is also ethylene independent

Through differential screening of a cDNA library, we cloned six groups of g enes that are expressed relatively early in the inoculated leaves of tobacc o resisting infection by tobacco mosaic virus (TMV). Induction of all these genes was subsequently detected in the uninoculated leaves; thus, their ex pression is associated with the development of both local and systemic acqu ired resistance. Exogenously applied salicylic acid (SA) was observed to in duce these genes transiently. However, analyses with transgenic NahG plants , which are unable to accumulate SA, demonstrated that expression of these genes in TMV-inoculated leaves is mediated via an SA-independent pathway. B ecause the expression kinetics of these genes differ from those associated with the well-characterized pathogenesis-related protein (PR-1) and phenyla lanine ammonia-lyase (PAL) genes, we propose that they belong to a group wh ich we designate SIS, for SA-independent, systemically induced genes. Inter estingly, the expression of several SIS genes in the uninoculated leaves of TMV-infected NahG plants was delayed and/or reduced, raising the possibili ty that SA is involved in activating some of these genes in systemic tissue . Most of the SIS genes were induced by exogenous ethylene. However, analys es of infected NahG plants treated with ethylene action and/or synthesis in hibitors indicated that the TMV-induced expression of several SIS genes is independent of ethylene as well as SA.