Efficient, long-lasting resistance against the soft rot bacterium Pectobacterium carotovorum in calla lily provided by the plant activator methyl jasmonate

The potential of three externally applied chemical plant activators, Bion, BABA and methyl jasmonate, known to act only through the plant defence system and not on the pathogen directly, to induce resistance against wild-type Pectobacterium carotovorum was examined in white-flowered calla lily (Zantedeschia aethiopica). Following a 24-h induction period, plants were challenge-inoculated with P. carotovorum, originally isolated from calla lily or potato plants, previously transformed using a gfp broad-host-range promoter-probe vector. After another 24 h, Bion treatment (10 μg mL ^-1, as a drench) reduced disease symptoms more than sixfold and bacterial proliferation by four orders of magnitude. BABA treatment (5-10 μg mL^-1, also as a drench) reduced the rate of infection by 75-85%. However, the protection afforded by both inducers did not persist. Also, at higher concentrations both displayed a phytotoxic effect. By contrast, methyl jasmonate (10 mm, applied as a leaf spray) completely inhibited P. carotovorum development in calla lily leaves and afforded a long-lasting effect. It is suggested that the defence response of calla lily against P. carotovorum involves the SA-signalling pathway in the short term, but the jasmonate/ethylene-signalling pathway is required for durable protection.