Effects of phytoplasma presence on the chemical composition of essential oils from Grindelia robusta Nutt., Echinacea purpurea Moench., and Monarda fistulosa L.
Plants are well recognized to change the secondary metabolites pattern of biosynthetic pathways in response to virus, bacteria, phytoplasma or fungi infections, and/or attacks by phytophagous insects. Most of these secondary metabolites are represented by volatile semiochemicals used as chemical defence in the sense of repellency. Such changes in the phytochemical profile greatly affect the quality and the commercial value of plants used for medicinal and cosmetic purposes. As a continuation of our studies aimed to verify the effect of phytoplasma infections on the volatile chemicals production, we report the differences detected between healthy and infected plants of Grindelia robusta Nutt. (Asteraceae), Echinacea purpurea Moench. (Asteraceae), and Monarda fistulosa L. (Lamiaceae), all species of great importance in the medicinal practice and grown at the Herb Garden of Casola Valsenio (Ravenna, Emilia Romagna region, northern Italy). Before flowering, only a few yellow symptoms were present on the leaves; however in some cases, the plants showed reduction of leaf size and stunting; whereas, at blooming, severe virescence and phyllody symptoms were observed. Samples from symptomatic plants were collected and tested for phytoplasma presence. Nested PCR/RFLP analyses on 16Sr DNA gene confirmed that in all the symptomatic samples examined phytoplasmas identified belonged to ribosomal subgroup 16SrI-B (Aster yellows: AY, \u2018Candidatus Phytoplasma asteris\u2019) in the case of G. robusta and E. purpurea, and to ribosomal soubgroup 16SrXII-A stolbur, for M. fistulosa. These two phytoplasmas are associated with a number of economically important diseases worldwide and recent reports indicated the presence of stolbur phytoplasmas in hyssop (Hyssopus officinalis) and in Parietaria spp., and of AY in woolly foxglove (Digitalis lanata).
Essential oils were obtained from aerial parts of healthy and phytoplasma-infected plants by hydrodistillation followed by GC/MS analysis using a reported procedure. In all cases, significant differences in the chemical composition of essential oils were observed. In G. robusta infected plants, a higher percentage of limonene, borneol (both almost > 50%), and borneol acetate (15% in healthy plants and 21.3% in infected ones) were detected. Essential oil of E. purpurea revealed differences for limonene (2.2% in healthy plants and 4.4% in infected ones), cis-verbenol (1.8% and 5.6% respectively), verbenone (2.7% and 11.4%), and to a lesser extent carvone (0.8% to 2.5%). Infection by phytoplasmas belonging to ribosomal soubgroup 16SrXII-A stolbur in M. fistulosa led to an increase of monoterpene hydrocarbons such as \uf061-thujene, \uf061-pinene, \uf0443-carene, and myrcene and to a decrease in aromatic monoterpene like p-cymene and thymol. Phytoplasmas have an appreciable influence in the content of secondary metabolites and these three species increase the concentration of the above cited monotherpens as chemical response to the presence of these pathogens
