Stress ossidativo, composti fenolici e piante officinali: il caso di Melissa officinalis esposta ad ozono

Oxidative stress is a complex chemical and physiological phenomenon that accompanies virtually all biotic and abiotic stresses in higher plants and develops as a result of overproduction and accumulation of reactive oxygen species (ROS). Anyway, plants have a lot of very efficient enzymatic and nonenzymatic antioxidant defense systems, which work in concert to protect plant cells from oxidative injury by scavenging ROS. Many companies in the pharmaceutical and food sectors are interested, especially, in a group of these non-enzymatic antioxidants: phenolics. Roles of officinal plants in human disease prevention and therapy have been attributed, at least in part, to the antioxidant activity of phenolics. Many phenolic products are synthesized as reaction to environmental stresses. In this review we summarize the results about responses of Melissa officinalis (lemon balm), a widely used officinal plant belonging to family Lamiaceae, under oxidative stress imposed by ozone (O3), the most important air pollutant because of its widespread occurrence and its known deleterious effects on vegetation, human health and manufacts. In the presence of alterations of photosynthetic function, membrane and visible injury induced by O3 treatment (200 ppb, 5 h), M. officinalis shows a regulatory adjustment of PSII efficiency, as indicated by the capacity to dissipate the excess of excitation energy and by the activation of a photoprotective mechanism, such as the xanthophyll cycle. Similar responses have been observed also when plants were exposed to lower O3 dose (80 ppb, 5h), making M. officinalis as a model O3-bioindicator for different regimes of this pollutant (such as changes in background concentrations). Moreover, ROS interact with hormonal signaling pathway, regulating cell death and the sensitivity of lemon balm to O3. Subsequently, attention was paid on secondary metabolism, especially phenolic compounds. The transcript levels of genes interested in the biosynthesis of rosmarinic acid [phenylalanine ammonia-lyase (PAL), 4-courmate:coenzyme A ligase (4CL), tyrosine aminotransferase (TAT) and rosmarinic acid synthase (RAS)] have been investigated by quantitative RT-PCR in plants exposed to O3 (80 ppb, 5h). A quick up-regulation of all genes occurs at 3 h of O3 exposure, but at 24 h from beginning of exposure (FBE) only RAS and PAL are up-regulated. The specific activity of RAS is closely related with a decrease of rosmarinic acid content. The specific activity of PAL increases at 12 h FBE to 163% in comparison to controls. This work proves insight into the effect of O3 stress on the formation of the main phenolic ingredient of pharmaceutically important plant M. officinalis, rosmarinic acid. Effects of O3 treatment (200 ppb, 3 h) have been also tested on the accumulation of main secondary metabolites in lemon balm shoot cultures, in order to define the biotechnological applications of this pollutant in the field of officinal plants as elicitor of bioactive compounds. Following the treatment, it has been observed: i) an activation of enzymes controlling the phenylpropanoid pathway; ii) a development of cellular barriers due to an increase of lignin; iii) an accumulation of phenolic compounds, in particular rosmarinic acid; iv) an increase of antioxidant ability. The biotechnological implications of these findings are worthy of further investigations.