Association between increased phenylalanine ammonialyase activity and cinnamic acid synthesis and the induction of temporary inaccessibility caused by Erysiphe graminis primary germ tube penetration of the barley leaf

Abstract

The interactions between barley leaves and compatible or incompatible races of Erysiphe graminis whose primary germ tubes (PGTs) had penetrated the host prior to appressorial penetration and E. pisi (which has no PGTs) were investigated for changes in phenolic metabolism during the early stages of the infection process. Cinnamic acid synthesis and the elevation of phenylalanine ammonia-lyase (PAL) activity were examined over the period during which PGT penetration of the leaves occurred. Increased synthesis of cinnamic acid began 3–4 h after inoculation with E. graminis, reached a maximum at 4–5 h, and then decreased. PAL activity began to increase 2 h after inoculation, reached a maximum at 4 h, and then declined in leaves inoculated with E. graminis. Neither the synthesis of cinnamic acid nor the activity of PAL was significantly affected by inoculation with E. pisi, or by treatment with heat-killed conidia of E. graminis. The fact that the state of temporary inaccessibility of host cells which is produced by PGT penetration coincides with the time of the increase of PAL activity and the increased synthesis of cinnamic acid suggests that both phenomena are closely associated.