The role of salicylic acid in plant defense

Plants resistant to pathogen attack often display a hypersensitive response (HR) characterized by localized tissue death at the site of pathogen ingress. Following the HR, some plant species show systemic resistance to pathogen attack against the inducing pathogen as well as unrelated pathogens. This induced resistance is termed systemically acquired resistance (SAR). Salicylic acid (SA) has been postulated to be a signal for SAR of tobacco (Nicotiana tabacum L.): application of SA induces disease resistance and endogenous levels of SA increase following the inoculation of TMV-resistant tobacco with TMV. The results presented in this dissertation support SA playing a role in expression of SAR in tobacco and further delineate the role of SA in disease resistance.; For SA to be a signal in tobacco SAR it must move systemically in the plant. The amount of SA in phloem exudates from leaves of TMV-resistant (N-gene) tobacco increased 72 h after TMV-inoculation. Increases in exuded SA were proportional to TMV dose and severity of symptoms. Export of SA, as well as increases in endogenous SA, were confined to tissues undergoing the HR.; Increases in tissue SA following the HR are not limited to N-gene tobacco. Inoculation of tobacco with or without the N-gene with tobacco necrosis virus or Pseudomonas syringae pv. tomato, resulted in large increases in SA. Stem inoculation of KY-14 tobacco with sporangia of Peronospora tabacina, resulted in a stem lesion, SAR to subsequent challenge with P. tabacina, and large increases in SA in the upper leaves.; Rice (Oryza sativa) has the highest SA levels of any plant species surveyed, 50-fold higher than tobacco. Inoculation of M-201 rice with either an HR-inducing P. syringae, or with the rice pathogens Magnaporthe grisea or Rhizoctonia solani, did not result in increased tissue SA. However, leaf SA levels in 28 rice varieties showed a correlation with generalized resistance to M. grisea, indicating that SA may play a role as a pre-formed constitutive defense compound, and may be a biochemical marker for blast resistance.; In rice, SA is present largely as the free acid. In tobacco when SA levels increase following the HR to levels equivalent to those observed in rice, SA is converted to 2-{dollar}beta{dollar}-{dollar}o{dollar}-D-GlucosylSA (GSA). Feeding SA to rice shoots also induced the accumulation of GSA and an increase in SA-glucosyltransferase (Gtase) activity. The lack of a sizable GSA pool in rice suggests that there is spatial separation of endogenous SA and GTase.; Ethylene is produced during the HR. Incubation of TMV-inoculated Xanthi-nc tobacco leaf disks in the presence of ethylene resulted in a slight reduction of SA accumulation. However, incubation of tobacco with SA methyl ester increased endogenous SA levels, induced PR proteins, and increased resistance to TMV.