Gene regulation in a pathogen-plant interaction: soft rot erwinias versus potato tubers
Abstract
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.
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