The phytoalexin response of lettuce to challenge by Botrytis cinerea, Bremia lactucae and Pseudomonas syringae pv. phaseolicola

Lettucenin A was confirmed to be the principal phytoalexin in lettuce. The accumulation of lettucenin A was directly related to the number of cells undergoing the hypersensitive reaction (HR) during incompatible interactions with Bremia lactucae governed by the Dm5/8 and Dm7 resistance genes. Very little phytoalexin accumulation occurred during the successful colonization of cotvledons by a virulent isolate of B. luctucae. The KR to Pseudomonas syringae pathovar phaseolicola was also associated with the localized accumulation of lettucenin A. inoculation of leaves with conidia of Botrytis cinerea caused the formation of limited lesions within which fungal growth was restricted. Lettucenin A reached high concentrations in tissue bearing limited lesions and within inoculum droplets on the leaf surface. Mycelial inocula of B. cinerea produced spreading lesions. During successful colonization by B. cinerea an initial increase in lettucenin h concentration at infection sites was followed by a decrease as tissue became completely colonized and rotted by the grey mould fungus. The success or failure of B. cinerea to invade lettuce appeared to be based on a balance between phytoalexin accumulation and degradation. Lettucenin A at 10(-6) M concentrations was found to possess considerable activity against B. lactucae, B. cinerea and P. syringae pv. phaseolicola and probably has an important role in the resistance of lettuce tissues to microbial colonization.