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Seed coating by a phenazine-producing bacterium,Pseudomonas chlororaphis O6, induced dose-dependentinhibition of germination in wheat and barley seds, but didnot inhibit germination of rice or cucumber seeds. In wheatseedlings grown from inoculated seds, phenazine productionlevels near the sed were higher than in the roots. Deletion ofthe gacS gene reduced transcription from the genes requiredphzI gene and thebiosynthetic phzA gene. The inhibition of seed germination andthe induction of systemic disease resistance against a bacterialsoft-rot pathogen, Erwinia carotovora subsp. carotovora, wereimpaired in the gacS and phzA mutants of P. chlororaphis O6.Culture filtrates of the gacS and phzA mutants of P. chlororaphisO6 did not inhibit seed germination of wheat, whereas that ofthe wild-type was inhibitory. Our results showed that theproduction of phenazines by P. chlororaphis O6 was correlatedwith reduced germination of barley and wheat seeds, and thelevel of systemic resistance in tobacco against E. carotovora.