GENE ACTIVATION BY UV-LIGHT, FUNGAL ELICITOR OR FUNGAL INFECTION IN PETROSELINUM-CRISPUM IS CORRELATED WITH REPRESSION OF CELL CYCLE-RELATED GENES

The effects of UV light or fungal elicitors on plant cells have so far been studied mostly with respect to defense-related gene activation. Here, an inverse correlation of these stimulatory effects with the act ivities of several cell cycle-related genes is demonstrated. Concomita nt with the induction of flavonoid biosynthetic enzymes in UV-irradiat ed cell suspension cultures of parsley (Petroselinum crispum), total h istone synthesis declined to about half the initial rate. A subclass o f the histone H3 gene family was selected to demonstrate the close cor relation of its expression with cell division, both in intact plants a nd cultured cells. Using RNA-blot and run-on transcription assays, it was shown that one arbitrarily selected subclass of each of the histon e H2A, H2B, H3 and H4 gene families and of the genes encoding a p34(cd c2) protein kinase and a mitotic cyclin were transcriptionally repress ed in UV-irradiated as well as fungal elicitor-treated parsley cells. The timing and extent of repression differed between the two stimuli; the response to light was more transient and smaller in magnitude. The se differential responses to light and elicitor were inversely correla ted with the induction of phenylalanine ammonia-lyase, a key enzyme of phenylpropanoid metabolism. Essentially the same result was obtained with a defined oligopeptide elicitor, indicating that the same signali ng pathway is responsible for defense-related gene activation and cell cycle-related gene repression. A temporary (UV light) or long-lasting (fungal elicitor) cessation of cell culture growth is most likely due to an arrest of cell division which may be a prerequisite for full co mmitment of the cells to transcriptional activation of pathways involv ed in UV protection or pathogen defense. This conclusion is corroborat ed by the observation that the histone H3 mRNA level greatly declined around fungal infection sites in young parsley leaves.