Role of flavan-3-OLS in resistance of apple trees to venturia inaequalis

Phenolic compounds are frequently attributed with a role in resistance of apple to scab (Venturia inaequalis). However, no clear evidence has yet been offered. The application of new analytical methods (HPLC with post column derivatization) and histochemical techniques have revealed high levels of preformed flavanols in resistant genotypes as well as an accumulation of flavan-3-ols in apple tissues after infection by Venturia. Treatment with AOPP, an inhibitor of phenylalanine ammonia-lyase, broke the resistance of 'Sir Prize', a scab-resistant cultivar. The fungus sporulated and the defence reaction via the synthesis of flavanols did not start. The mechanism of resistance with respect to the role of flavanols in the cell wall is discussed. The paper further deals with the influence of environmental conditions on the biosynthesis of flavanols and hypothesizes that resistance genes against apple scab are regulatory genes of phenol synthesis.