The induction of phenylpropanoid biosynthetic enzymes by ultraviolet light or fungal elicitor in cultured parsley cells is overriden by a heat-shock treatment.

The normal (25° C) response of parsley (Petroselinum crispum Mill.) cell suspension cultures to ultraviolet (UV) light was suppressed by a simultaneous 37° C heat-shock treatment, as indicated by the loss of the inducibility of two enzymes of flavonoid biosynthesis, phenylalanine ammonia-lyase (EC 4.3.1.5) and chalcone synthase. The effects on enzyme activity and on enzyme synthesis in vitro and in vivo were similar, indicating that regulatory control is at an early step of gene expression, presumably transcription. When heat shock was given during the course of an ongoing UV induction, both enzyme synthesis and enzyme activities ceased rapidly. Likewise, the induction of phenylalanine ammonia-lyase by an elicitor from Phytophthora megasperma f. sp. glycinea was terminated upon transfer from 25° C to 37° C. Based on these and previously published data, it is concluded that stress responses in this system are preferentially expressed in the order of heat shock, fungal elicitor and UV light.