Histochemical Detection and Role of Phenolic Compounds in the Defense Response of Lactuca spp. to Lettuce Downy Mildew (Bremia lactucae)

The occurrence of phenolic compounds (PC) in the defence reaction of Lactuca spp. was detected by histochemical methods. Four staining methods, inclu- ding three for light and one for ¯uorescence microscopy, were used for imaging the location of PC in cells of genotypes with di€erent resistance mechanisms after infection by Bremia lactucae, race NL16. The results showed that the major role of phenolic compounds in studied Lactuca spp. is connected with their overexpres- sion and localized accumulation during a hypersensitive response (HR). In incompatible interactions a slight accumulation of phenols near the cell wall of infected cells was detected. The negative reaction to staining with aniline sulphate veri®ed the absence of lignin creation. In both compatible and incompatible interactions structural modi®cations in the host cells occurred as a callose deposition. Frequently, these deposits were widespread in susceptible genotypes. Intensive and rapid accumulation of auto¯uorescent phenolics was linked with the onset of HR, the main cytological feature of resistance to lettuce downy mildew in Lactuca spp.