| Salicylic acid (SA) plays an important role in plant defense against pathogens. During an incompatible plant-pathogen interaction which results in localized cell death, termed a hypersensitive response (HR), SA accumulates both in the infected tissue and in as-yet uninfected tissue. The accumulation of SA leads to the induction of pathogenesis-related proteins, and increased resistance to subsequent infection by the same or unrelated pathogens, called systemic acquired resistance (SAR).;The application of molecular, genetic and biochemical methods has led to the identification of SA as a signal for SAR in tobacco, cucumber and Arabidopsis. Little has been known, however, about the biosynthesis and role of SA in the monocotyledonous plants, such as rice. This dissertation addresses the role of SA and its biosynthesis in rice and investigates SA metabolism in tobacco plants.;In healthy rice (Oryza sativa L.) SA is present in a form of free acid with the levels as high as those found in pathogen-inoculated tobacco plants. After inoculation with either a compatible pathogen, Magnaporthe grisea, or the HR-inducing pathogen, Pseudomonas syringae pv syringae, SA levels did not change, suggesting that SA does not play a signaling role in disease resistance in rice.;Rice shoots converted labeled cinnamic acid to lignin precursors, while labeled benzoic acid was converted to SA, suggesting that in rice, as in tobacco, SA is synthesized from cinnamic acid via benzoic acid.;A role for active oxygen species (AOS) in the biosynthesis of SA was investigated. In contrast to previously published data no increase in hydrogen peroxide accumulation was detected in virus-inoculated tobacco plants, however, inhibition of AOS formation reduced the accumulation of SA and HR in virus-inoculated tobacco.;Gaseous methyl salicylate (MeSA), a major volatile produced in tobacco mosaic virus-inoculated tobacco plants, serves as an airborne defense signal. Using an analytical method developed for this purpose, MeSA accumulation was detected in pathogen-inoculated tobacco tissues, paralleling the increases in SA. Transgenic NahG tobacco, unable to accumulate SA, did not show increased resistance to virus inoculation following MeSA treatment, suggesting that conversion to SA is required for biological activity of MeSA.;An activity present in crude extracts of tobacco leaves that catalyzes the methylation of SA to MeSA was characterized. SA:methyltransferase activity was induced by virus inoculation and SA application, suggesting its involvement in defense responses. |
