Endogenous and exogenous elicitors of a hypersensitive response in Gracilaria conferta (Rhodophyta)

Certain forms of oligocellulose and certain bacterially excreted peptides were identified as endogenous and exogenous elicitors, respectively, of a tip bleaching response in Gracilaria conferta (Schousboe ex Montagne) J. et G. Feldmann. The half-maximal tip bleaching response was observed when 31.1 mu M cellobiose or 11.6 mu M cellotetraose were present in the growth medium. In contrast, no response was detected after exposure to glucose, cellotriose, cellopentaose or maltooligosaccharides. The response was thus strongly dependent on the molecular size of the oligocellulose and only saccharides that consisted of an even number of glucose residues were elicitor-active. Three bacterial species that had earlier been identified as potential inducers of the tip bleaching symptom excreted elicitor-active compounds into the growth medium. These compounds were protease-sensitive and thus peptides or proteins. The tip bleaching-inducing compound that was excreted by one Cytophaga-like organism was partially purified. It could be extracted from culture supernatants with chloroform and its molecular size was between 700 and 1500 Da, corresponding with a structure of 4-20 amino acids. Various endogenous and exogenous elicitors are thus recognized by G. conferta and allow this alga to respond hypersensitively to the maceration of its cell wall skeleton or just to the presence of certain epiphytic organisms.