In a variety of plant species, the development of necrotic lesions in response to pathogen infection leads to induction of generalized disease resistance in uninfected tissues. A well-studied example of this "immunity" reaction is systemic acquired resistance (SAR) in tobacco. SAR is characterized by the development of a disease-resistant state in plants that have reacted hypersensitively to previous infection by tobacco mosaic virus. Here, we show that the onset of SAR correlates with the coordinate induction of nine classes of mRNAs. Salicylic acid, a candidate for the endogenous signal that activates the resistant state, induces expression of the same "SAR genes." A novel synthetic immunization compound, methyl-2,6-dichloroisonicotinic acid, also induces both resistance and SAR gene expression. These observations are consistent with the hypothesis that induced resistance results at least partially from coordinate expression of these SAR genes. A model is presented that ties pathogen-induced necrosis to the biosynthesis of salicylic acid and the induction of SAR.
Acquired resistance is an important component of the complex disease resistance mechanism in plants, which can result from either pathogen infection or treatment with\synthetic, resistance-inducing compounds. In this study, Arabidopsis, a tractable genetic system, is shown to develop resistance to a bacterial and a fungal pathogen following 2,6-dichloroisonicotinic acid (INA) treatment. Three proteins that accumulated to high levels in the apoplast in response to INA treatment were purified and characterized. Expression of the genes corresponding to these proteins was induced by INA, pathogen infection, and salicylic acid, the latter being a putative endogenous signal for acquired resistance.Arabidopsis should serve as a genetic model for studies of this type of immune response in plants.
In a variety of plant species, the development of necrotic lesions in response to pathogen infection leads to induction of generalized disease resistance in uninfected tissues. A well-studied example of this "immunity" reaction is systemic acquired resistance (SAR) in tobacco. SAR is characterized by the development of a disease-resistant state in plants that have reacted hypersensitively to previous infection by tobacco mosaic virus. Here, we show that the onset of SAR correlates with the coordinate induction of nine classes of mRNAs. Salicylic acid, a candidate for the endogenous signal that activates the resistant state, induces expression of the same "SAR genes." A novel synthetic immunization compound, methyl-2,6-dichloroisonicotinic acid, also induces both resistance and SAR gene expression. These observations are consistent with the hypothesis that induced resistance results at least partially from coordinate expression of these SAR genes. A model is presented that ties pathogen-induced necrosis to the biosynthesis of salicylic acid and the induction of SAR.
An extracellular, acidic chitinase was purified to homogeneity from tobacco necrosis virus-infected leaves of Cucumis sativis. The amino acid sequences of the intact protein and of peptides isolated following endoproteinase Lys-C digestion, cyanogen bromide cleavage, and trypsin digestion were determined. Oligonucleotide probes derived from this sequence were used to isolate a cDNA clone encoding this protein. No significant homology was found between this chitinase and either the basic chitinase isolated from bean or tobacco or the chitinase isolated from Serratia marcescens; however, strong homology was found between the cucumber chitinase and a lysozyme/chitinase from Parthenocissus quinquifolia. The induction of the protein by tobacco necrosis virus infection or salicylate was found to be at the level of RNA accumulation. Genomic Southern analysis indicates that a single gene in the cucumber genome encodes this protein.Many plants infected with necrotizing pathogens develop local or systemic resistance against subsequent infections (1, 2). This induced resistance is accompanied by a number of biochemical changes in the host plant, including the production of pathogenesis-related (PR) proteins (for review see refs. 3 and 4). The accumulation of these acid-extractable, low molecular weight, protease-resistant proteins correlates with induced resistance, but their function has largely remained elusive. Recently, it has been shown that of the 10 well-characterized PR proteins in tobacco, two (PR-P and -Q) have chitinase activity (5-7), three (PR-O, -N, and -2) have /3-1,3-glucanase activity (8), and two (PR-R and -S) are structurally similar to a maize protease/a-amylase inhibitor (9, 10).From cucumber, we have recently purified one PR protein and identified it as a chitinase (11). After infection with tobacco necrosis virus (TNV), this Mr 28,000 protein accumulates in the intercellular space of the infected, as well as the uninfected, parts of the plant (11, 12). Here, we report the purification of this protein to homogeneity and the determination of 55% of the amino acid sequence (148 of 267 residues) of the mature protein. Oligonucleotide probes were synthesized based on the protein sequence analysis and used to isolate cDNA clones encoding this protein from a library constructed with RNA isolated from TNV-infected cucumber leaves. We have sequenced the cDNA clones and find no significant homology to known chitinase genes. However, striking homology was found between the deduced amino acid sequence and the partial amino acid sequence of a bifunctional lysozyme/chitinase purified from Parthenociccus quinquifolia (13).In preliminary studies on the regulation of chitinase gene expression, we show that there is one gene encoding chitinase in the cucumber genome and the accumulation of this protein after TNV infection or salicylic acid induction correlates with the accumulation of mRNA. MATERIALS AND METHODSProtein Purification and Sequencing. Chitinase protein was isolated from TNV-infected cucumber (Cucu...
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