2 PR-1 GENES FROM TOMATO ARE DIFFERENTIALLY REGULATED AND REVEAL A NOVEL MODE OF EXPRESSION FOR A PATHOGENESIS-RELATED GENE DURING THE HYPERSENSITIVE RESPONSE AND DEVELOPMENT

Pathogenesis-related (PR) proteins form a heterogeneous family of plan t proteins that are likely to be involved in defense and are inducible by pathogen attacks, One group of PRs, represented by the subfamily P R-1, are low-molecular-weight proteins of unknown biochemical function , Here we describe the cloning and characterization of two closely rel ated genes encoding a basic and an acidic PR-1 protein (PR1b1 and PR1a 2) from tomato (Lycopersicon esculentum). We present a comparative stu dy of the mode of transcriptional regulation of these two genes in tra nsgenic tobacco plants using a series of promoter-GUS fusions, Unexpec tedly, the chimeric PR1a2/GUS gene is not induced by pathogenic signal s but instead shows constitutive expression with a reproducible develo pmental expression pattern, It is expressed in shoot meristems, tricho mes, and cortical cells as well as in vascular and nearby tissues of t he mature stem, This constitutive expression pattern may represent pre emption of plant defenses against potential pathogens, Conversely, the chimeric PR1b1/GUS gene does not show any constitutive expression in the plant, but it is transcriptionally activated following pathogen at tack, Upon infection by tobacco mosaic virus, the PR1b1 gene is strong ly activated locally in tissues undergoing the hypersensitive response but not systemically in uninoculated tissues, Furthermore, its expres sion is induced by both salicylic acid and ethylene precursors, two si gnals that coexist and apparently mediate the activation of local defe nses during the hypersensitive response, We speculate that the differe nt mode of expression of the two genes presented here, together with t hat reported previously for the induction of other PR-1 genes in syste mic, uninoculated tissues, may all be complementary and necessary for die plant to acquire an efficient refractory state to resist pathogen attacks.