Fungal development and irreversible membrane damage in cells of Lactuca sativa undergoing the hypersensitive reaction to the downy mildew fungus Bremia lactucae

Abstract

A plasmolytic method was developed to examine irreversible membrane damage (IMD) occurring during the early stages of the hypersensitive reaction (HR) of lettuce ( Lactuca sativa L.) to Bremia lactucae Regel. Symptoms associated with incompatibility determined by Dm genes 1–8 in lettuce were compared. The timing of IMD varied according to the Dm gene expressed and occurred within 1·5 h of penetration during the expansion of the primary vesicle in the most rapid reactions which were determined by Dm 2 and Dm 5/8. By contrast, with Dm 1 or Dm 3, IMD occurred only after initiation of the secondary vesicle, from 3 h after penetration, and rarely occurred during the first 12 h after inoculation of genotypes possessing Dm 4, Dm 6 or Dm 7. The genetic background of the host had little effect on the expression of Dm genes 1, 3 or 5/8, but some fungal isolates, thought to have the same gene for avirulence, differed in their behaviour on resistant cultivars. The time of occurrence of IMD in the first 12 h after inoculation had a direct bearing on the extent of fungal development during the subsequent 36 h. In lettuce cv. Valmaine ( Dm 5/8 gene for resistance) growth of the avirulent isolate VO/11 of B. lactucae was checked at the time of IMD. In all the combinations of isolates and cultivars studied, plasmolysis demonstrated lack of adhesion between primary and secondary vesicles and the host plasmalemma during compatible or incompatible interactions. Variation existed between isolates in their speed of germination and initiation of penetration on different varieties, but variation was not related to the expression of Dm genes.