This study investigates the coordination of gene expression in plants in response to agents that induce systemic acquired resistance (SAR). The research focuses on tobacco plants and shows that the onset of SAR is correlated with the coordinated induction of nine classes of mRNAs. Salicylic acid (SA), a candidate for the endogenous signal that activates the resistant state, induces the expression of these "SAR genes." A synthetic immunization compound, methyl-2,6-dichloroisonicotinic acid (INA), also induces resistance and SAR gene expression. These findings support the hypothesis that induced resistance results at least partially from the coordinated expression of these SAR genes. A model is presented that links pathogen-induced necrosis to the biosynthesis of salicylic acid and the induction of SAR. The study also examines the molecular mechanisms underlying SAR. It identifies nine gene families that are coordinately induced during the onset of SAR, including pathogenesis-related (PR) proteins and other proteins. These genes are expressed in response to various treatments, including SA and INA. The expression of these genes correlates with the onset of resistance, as evidenced by the reduction in TMV lesion size. The study further shows that the expression of these genes is regulated by factors such as light and circadian rhythms. The results indicate that the induction of SAR is associated with the coordinated expression of specific genes, including PR proteins and other enzymes. The study also highlights the role of salicylic acid in the induction of SAR, suggesting that it may act as a signal molecule. The findings provide insights into the molecular basis of induced resistance in plants and have implications for the development of strategies for crop protection through genetic engineering or the discovery of new chemical compounds that stimulate natural disease resistance mechanisms.This study investigates the coordination of gene expression in plants in response to agents that induce systemic acquired resistance (SAR). The research focuses on tobacco plants and shows that the onset of SAR is correlated with the coordinated induction of nine classes of mRNAs. Salicylic acid (SA), a candidate for the endogenous signal that activates the resistant state, induces the expression of these "SAR genes." A synthetic immunization compound, methyl-2,6-dichloroisonicotinic acid (INA), also induces resistance and SAR gene expression. These findings support the hypothesis that induced resistance results at least partially from the coordinated expression of these SAR genes. A model is presented that links pathogen-induced necrosis to the biosynthesis of salicylic acid and the induction of SAR. The study also examines the molecular mechanisms underlying SAR. It identifies nine gene families that are coordinately induced during the onset of SAR, including pathogenesis-related (PR) proteins and other proteins. These genes are expressed in response to various treatments, including SA and INA. The expression of these genes correlates with the onset of resistance, as evidenced by the reduction in TMV lesion size. The study further shows that the expression of these genes is regulated by factors such as light and circadian rhythms. The results indicate that the induction of SAR is associated with the coordinated expression of specific genes, including PR proteins and other enzymes. The study also highlights the role of salicylic acid in the induction of SAR, suggesting that it may act as a signal molecule. The findings provide insights into the molecular basis of induced resistance in plants and have implications for the development of strategies for crop protection through genetic engineering or the discovery of new chemical compounds that stimulate natural disease resistance mechanisms.