Accumulation of diferulic acid during the hypersensitive response of oat leaves to Puccinia coronata f.sp. avenae and its role in the resistance of oat tissues to cell wall degrading enzymes

The relationship between diferulic acid formation and locally induced resistance was investigated in the interactions of two physiological races of Puccinia coronata f.sp. avenae with Shokan 1 oat. Diferulic acid was detected in the hemicellulosic fraction extracted with oxalic acid from the cell wall. The amount of ferulic acid bound in the cell wall matrix polysaccharides did not vary in uninoculated and inoculated primary leaves of oats during the time course of the experiment. In contrast, the formation of diferulic acid bound to wall polysaccharides became significantly higher in the incompatible than the compatible interaction at approx. 36 h after inoculation. The timing of increased formation of diferulic acid was correlated with the accumulation of avenalumins and inhibition of protoplast liberation from infected primary leaves. Activities of soluble, and of ionically- and covalently-bound cell wall peroxidase from uninoculated and inoculated leaves were measured. Ionically-bound cell wall peroxidase activity increased in the incompatible interactions at about 36 h after inoculation. This was the same time that increased levels of diferulic acids were observed. This may indicate that the cross-linking of matrix polysaccharides by ferulic acids bound in cell walls is caused by a coupling reaction initiated by ionically-bound cell wall peroxidases. It is postulated that the diferulic acid formation occurs in oat cell walls at the infection sites in incompatible interactions and may contribure to locally induced resistance in oats by providing a barrier to pathogen ingress.