RESPONSE OF GRACILARIA CONFERTA (RHODOPHYTA) TO OLIGOAGARS RESULTS IN DEFENSE AGAINST AGAR‐DEGRADING EPIPHYTES

Elicitation of Gracilaria conferta (Schousboe ex Montagne) J. et G. Feldmann with oligoagars resulted in a defense response that was strong enough to kill epiphytic bacteria associated with the alga. Up to 60% of the resident bacterial flora of healthy plants was eliminated within 60 min after addition of neoagarohexaose to the algal medium. Single isolates of agar‐degrading bacteria that had been isolated previously from healthy or decaying algal tissues proved to be more sensitive. Some of them were generally unable to survive on healthy G. conferta. Others survived on unelicited plants. Approximately 90% of these more resistant agar degraders were eliminated within 15 min after elicitation. The bacterial degradation of dead tissue of G. conferta resulted in a release of elicitors. The elicitors accumulated in the medium and reached high enough concentrations within 24 h to induce a hypersensitive response in healthy algae. The eliciting agent could be destroyed with β‐agarase and was thus probably oligoagar. Application of antibiotics prevented the accumulation of the elicitor, which indicated that bacteria were responsible for its release from the algal biomass. The hypersensitive response of G. conferta after contact with oligoagars is thus a true defense response, because it enables the plant to protect itself efficiently from enzymatic attacks on its cell wall.