Erwinia soft rot is a widespread disease destructive to numerous important crop plants. Damage to plants is primarily due to celldegrading enzymes (CDEs) secreted by the bacteria. I am interested in potato (Solanum tuberosum) soft rot because it is of agricultural importance and it represents an ideal model system for understanding molecular events in plant-pathogen interactions. Much has been learned in vitro about the molecular genetics of CDEs in the past decade; however, little is known about their expression in plantae To study expression of genes for these enzymes during pathogenesis and plant responses to erwinias or their enzymes, I developed a membrane-separated system for simultaneous studies of potato and bacterial gene expression. This system facilitates the isolation of plant tissue-free bacterial cells and bacteria-free plant tissue for subsequent analysis of gene expression by RNA blot hybridization. Using this system, I demonstrated that in compatible interactions, rnRNAs for three Erwinia carotovora subsp. carotovora (Ecc) CDE genes were induced to high levels and were induced sequentially: exo-pectate lyase (PL), endo-PL, and then endopolygalacturonase (PG) with maximal mRNA accumulations at 6, 9, and 12 hr, respectively. Induction of these mRNAs was well correlated with tissue maceration. In the incompatible interaction, however, induction of all three Ecc genes was reduced several-fold compared to the compatible interaction. The kinetics of mRNA accumulation during pathogenesis were distinct from those of in vitro accumulation induced by polygalacturonic acid. My results confirm that in planta expression of these genes was induced by exo-PL reaction products as suggested by other researchers. In studies of plant genes correlated with plant responses to pathogens and environmental stresses [plant defenseresponse (PDR) genes], I also showed Ecc triggered active responses distinct from wound responses. I used gene probes for phenylalanine ammonia- lyase (PAL) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), key genes in the biosynthesis of phenylpropanoid- and terpenoidderived compounds believed to be important in plant defenses. Ecc inoculation caused much more rapid and greater increases in PAL mRNA and enzyme activity levels in potato tuber than wounding alone. Escherichia coli, a non-plant pathogen, carrying a plasmid which encodes Ecc endo-PL, also induced PAL mRNA accumulation. Ecc induced a specific HMGR isogene (HMGR1) not activated by wounding. My results support the existence of an HMGR mul-ci-gene family. Wounding resulted in a rapid and transient accumulation of HMGR2 mRNA followed by a slower accumulation of HMGR3 mRNA. These isogenes are distinct from the Ecc-induced HMGRI gene. / Ph. D.