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 (80 ppb for 5 h), 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 3 h of O-3 exposure, but at 24 h 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 12 h 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. officinalis. (C) 2013 Elsevier GmbH. All rights reserved.