Changes in phenylalanine ammonia‐lyase and peroxidase activities in wheat cultivars expressing resistance to the leaf‐rust fungus

Abstract

Phenylalanine ammonia‐lyase (PAL) and peroxidase activities increased slightly during pre‐haustorial stages of development of the leaf‐rust fungus (Puccinia recondita f. sp. tritici) in each combination of rust strain and wheat line used. Activity of both enzymes greatly increased during the formation of the first haustoria by avirulent and virulent strains in an Lr28‐bearing line but not in an Lr20‐bearing line. PAL increased markedly in activity at the time when an avirulent strain was known to cause hypersensitivity in the Lr28‐bearing line and when another avirulent strain was known to cause cellular changes that preceded the expression of resistance in the Lr20‐bearing line. PAL increased less markedly and not at all at corresponding times when virulent strains developed in the Lr28 and Lr20 lines, respectively. Increases in peroxidase activity occurred after the increases in PAL activity and were greater during resistance expression than in leaves infected with virulent strains. Cytochemical tests revealed enhancements of peroxidase activity in epidermal and guard cells in response to avirulent and virulent strains and in mesophyll cells that were becoming necrotic in response to avirulent strains only. The implications of the changes are that aromatic and phenylpropanoid synthesis and peroxidations may be enhanced in wheat by infection and particularly during resistance expression. The products of these processes may play roles in both Lr28‐ and Lr20‐based expression of resistance.