Induced Systemic Protection in Cucumber: Time of Production and Movement of the Signal

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Abstract
An inducing inoculation of the first true leaf (leaf 1) of cucumber with Colletotrichum lagenarium systemically protected the plants against disease caused by subsequent challenge inoculation with the fungus. Several experiments involving detachment of the inducer leaf were designed to determine the dynamics of signal production and movement. Excising leaf 1 at intervals after inoculation revealed that protection of the second true leaf (leaf 2) of two cucumber cultivars was first evident by 72 hr. Protection increased gradually and reached a maximum by 144 hr. After excision fo the inducer leaf, the time of challenge (either immediately or several days later) did not affect the level of protection. Scions were previously shown to become protected when grafted onto rootstocks infected on leaf 1 with C. lagenarium. We found that if the inducer leaf was excised before translocation was established through the graft union, the scion never became protected. Protection was first detected when the inducer leaf was excised shortly after onset of translocation through the graft union; maximum protection required another 48-96 hr. The level of protection of leaf 2 did not differ whether the tip of leaf 1 or the whole leaf was excised at intervals after inoculation of the tip. These data suggest that the limiting step in development of systemic protection to signal production. Once produced, the signal moves swiftly through the plant and results in rapid sensitization of the plant''s defense mechanisms.