Studies on molecular mechanisms in the signal transduction cascade from signal perception to the expression of defense responsive genes in a plant-pathogen interaction

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In this study, we attempted to clarify the molecular mechanisms in the signal transduction cascade between perception of fungal signal molecules and the expression of defense responsive genes.Our findings are summarized as follows :1. ATPase and polyphosphoinositide metabolism in pea plasma membranes are regulated by the clicitor and suppressor from Mycosphaerella pinodes. The suppressor, however, blocked nonspecifically them even in plasma membranes isolated from non-host plants of the fungus.2. The M.pinodes-elcitor enhanced nonspecifically the activities of ATPase (NTPase), peroxidase and ascorbic acid oxidase in NaCl-soluble fraction from cell walls isolated from pea and cowpea, whereas the suppressor blocked these activities in a species-specific manner.3. Superoxide anion and as yet unidentified infection-inhibitor, that were generated in isolated pea cell wall by the elicitor, were inhibited by the suppressor. On the other hand, those in cowpea cell wall were inversely elicited … More by the suppressor alone.4. Signal transduction leading to phytoalexin production was mediated by connection of certain vitronectin-like molecules in cell wall and vitronectin receptor-like molecules on plasma membrane.5. In the promoter sequence of PSPALs and PSCHSs, we found conserved sequence motifs, such as Box-1 (homologous to Box L), Box-2 (homologous to Box P) and Box-4 (AT-box) that were essential for response to the elicitor.6. In PSCHSL the directly repeated AT-rich sequences, TAAAATACT were capable of forming a low mobility complex (LMC) by a gel mobility shift assay and in vitro DNase i-footprinting analysis. The finding, that treatment of the nuclear factors with alkaline phosphatase abolished LMC formation, indicates phosphorylation of nuclear protein (s) is necessary for the formaiton of complex machinery.7. The cis-elements such as Box1, Box2 and G-box were essential for the promoter of elicitor-responsive CHS genes.From these results, we propose the stream of signal transduction leading to defense responses as follows : 1) signal molecules are percepted by respective receptors on cell wall ; 2) the second messengers are carried out into the cells via vitronectin receptor-like proteins on the plasma membranes ; 3) polyphosphoinositide metabolism in plasma membranes is also activated to produce the 3rd messenger ; 4) the messenger stimulates phosphorylation of nuclear factor (s) ; 5) The transcription of defense responsive genes are activated ; 6) the rejection reaction is expressed in the clicitor-treated plant tissues. Less