Ghirardi Botanic Garden, a “factory” of molecules

Botanic gardens are primarily experienced as environments where the plants often lose their identity to give rise to a whole that welcomes visitors. Adopting a different inquiring approach, it turns out that each plant is different from the other. The presence of several species native of different countries attests that botanic gardens are primarily places devoted to preserve plant biodiversity. Another point of view is to perceive botanic gardens as factories of molecules, that means evaluating their characteristics at a higher level than that visually described. Indeed, each plant communicates with the environment synthesizing and emitting secondary metabolites. The aim of this project is to investigate botanic gardens under this novel perspective, as factories of molecules. In this context, we planned a multidisciplinary research project on a pool of selected medicinal species preserved at the Ghirardi Botanic Garden (Toscolano Maderno, Bs) on the basis of their productivity in volatile compounds (VOCs), essential oils (EOs), epicuticular depositions and substances stored at tissue level. This heterogeneous set of molecules drives multiple functions for the plants themselves, and consequently for the ecosystem and the environment. The project involves multiple-scale analyses: (i) micromorphological description of the secretory structures; (ii) chemical characterization of the secondary metabolites; (iii) ecological study of the biotic interactions mediated by such chemicals; (iv) evaluation of their biological activity to assess their potential application in human nutrition as well as for therapeutic or cosmetic sectors. As preliminary steps, we addressed our attention on the morpho-anatomy of the secretory structure and on the phytochemistry and antibacterial activities of the EOs obtained from the aerial parts of six target-species: Cinnamomum camphora (L.) J.Presl., Laurus nobilis L., Myrtus communis L., Eugenia uniflora L., Santolina chamaecyparissus L. and Citrus japonica Thunb. The structures responsible for oil production and accumulation were found to be nondifferentiated cells (Lauraceae), glandular hairs (Asteraceae) and secretory ducts (schizogenous in Myrtaceae and schizolysigenous in Rutaceae). The analysis of EO composition allowed the overall identification of 165 different compounds, with the highest number detected in S. chamaecyparissus (72) and the lowest in M. communis (31). Oxygenated sesquiterpenes represented the most abundant chemical class in C. japonica and E. uniflora, whereas oxygenated monoterpenes dominated in the other examined species. Two common compounds were found in all the species: a-pinene and myrcene, most abundant in M. communis (39.0%) and in S. chamaecyparissus (3.7%), respectively. As a whole, a high level of chemical variability was found across the EOs, with S. chamaecyparissus exhibiting the most complex profile due to the occurrence of the highest number of exclusive compounds, accounting for 58.3% of the total. The EO antimicrobial activity was evaluated towards Staphylococcus aureus ATCC 6538, Escherichia coli ATCC 15325, Salmonella enterica thiphymurium ATCC 14028, Lysteria monocytogenes ATCC 7644, Enterococcus faecalis V583E and Pseudomonas aeruginosa ATCC 27853. The most active oils resulted S. chamaecyparissus EO against L. monocytogenes (MIC 1:256 v/v) and P. aeruginosa (MIC 1:256 v/v) and C. japonicum EO against S. aureus (MIC 1:256 v/v) and L. monocytogenes (MIC 1:512 v/v). These preliminary results were discussed in the light of the most recent literature contributions.