Comparative study of interactions between Bremia lactucae and lettuce cells in suspension culture and in cotyledons

Abstract

A critical comparison of the relationship between fungal development and irreversible membrane damage (IMD) in lettuce cells penetrated by Bremia lactucae in suspension cultures or in the epidermis of the cotyledon petiole has revealed the following important differences between interactions occurring in vitro and in the plant. (1) Large fungal colonies comprising intercellular hyphae and the formation of numerous haustoria did not develop in suspension cultures. (2) IMD occurred at earlier stages of fungal development in cultured cells than in cotyledons of susceptible cultivars. (3) In resistant cells undergoing the hypersensitive reaction (HR) fungal development was more advanced in vitro than in the plant at the time of IMD. (4) Suppression of the plant's metabolic activity either by heat-shock or blasticidin S (an inhibitor of protein synthesis) had less effect on the HR in cultured cells than in the plant. Despite these differences, B. lactucae did produce characteristic infection structures in cultured cells and, in certain cultivar/isolate interactions e.g. cvs. Diana ( Dm 1 + Dm 3 + Dm 7 + Dm 5/8) and Cobham Green (no known resistance genes) with isolates V0/11 (with at least one dominant avirulence allele at loci A 1, A 3, A 7 and A 5/8) and TV (with no dominant avirulence alleles at loci A 1, A 3, A 7 or A 5/8), the expected compatibility or incompatibility was clearly expressed. By contrast, the expected rapid HR of cells of cv. Valmaine ( Dm 5/8) challenged by isolate V0/11 was not observed in cultured cells. The influence of nutrient status on the responses of cultured lettuce cells is discussed.