CHARACTERIZATION OF A DIFFUSIBLE SIGNAL CAPABLE OF INDUCING DEFENSE GENE-EXPRESSION IN TOBACCO

Treatment of tobacco (Nicotiana tabacum) cell-suspension cultures with cryptogein, an elicitin protein from Phytophthora cryptogea, resulted in the release of a factor(s) that diffused through a 1000-D cutoff d ialysis membrane and was capable of inducing sesquiterpene cyclase enz yme activity (a key phytoalexin biosynthetic enzyme in solanaceous pla nts) when added to fresh cell-suspension cultures. The diffusible fact or(s) was released from cells over a 20-h period and induced a more ra pid induction of cyclase enzyme activity than did direct treatment of the cultures with pure elicitin protein. The diffusible factor also in duced a more rapid accumulation of transcripts encoding for sesquiterp ene cyclase, acidic and basic chitinase, and hsr203 (a putative hypers ensitive response gene) than did elicitin treatment. The diffusible fa ctor(s) was resistant to protease, pectinase, DNase, and RNase treatme nts, was not extractable into organic solvents, and was not immunoprec ipitable when challenged with polyclonal antibodies prepared against e licitin protein. The diffusible factor(s) could not induce the release of more factor, suggesting that it was a terminal signal. These resul ts are consistent with the notion that cells directly challenged or st imulated by pathogen-derived elicitors release diffusible secondary si gnal molecules that orchestrate the induction of complementary defense responses in neighboring cells.