How do background ozone concentrations affect the biosynthesis of rosmarinic acid in Melissa officinalis?

Lemon balm (Melissa officinalis; Lamiaceae) plants were exposed to background ozone (O 3 ) dosages (80ppb for 5h), because high background levels of O 3 are considered to be as harmful as episodic O 3 peaks. Immediately at the end of fumigation the plants appeared visually symptomless, but necrotic lesions were observed later. The biosynthesis of rosmarinic acid (RA) comprises eight enzymes, among them phenylalanine ammonia-lyase (PAL), 4-coumarate:coenzyme A ligase (4CL), tyrosine aminotransferase (TAT) and rosmarinic acid synthase (RAS). The transcript levels of these genes have been investigated by quantitative RT-PCR. There was a quick up-regulation of all genes at 3h of O 3 exposure, but at 24h from beginning of exposure (FBE) only RAS and PAL were up-regulated. The specific activity of RAS was closely correlated with a decrease of RA concentration in lemon balm leaves. The specific activity of PAL increased at 12h FBE to 163% in comparison to control levels. This work provides insight into the effect of O 3 stress on the formation of the main phenolic ingredient of the pharmaceutically important plant M. officinali