ABSTRACT
Chemotaxonomy has seldom been used in Basidiomycota systematics, and here we give a contribution, as concerns genus Russula. Terpenoids, in particular sesquiterpenes, are the most widely diffused secondary metabolites isolated from the basidiomes of Russula and Lactarius (Russulaceae). These compounds have been widely investigated, also because they are responsible of the acrid peppery taste of several species, which are harmful to human health, leading to troublesome poisonings. In the present contribution, we first review the known sesquiterpenes of Russula, discussing their chemotaxonomic implications. Afterwards, we focus on triterpenes, a much less widespread metabolite type in the family Russulaceae. In particular, the unusual (3,4)-seco-A ring cucurbitane skeleton has been found, so far, only in a few representative species of gen. Russula, from both Europe and Asia. These species either belong to sect. Roseinae (R. aurora and R. minutula) or to sect. Lepidinae (R. lepida and R. amarissima), two groups in the past placed in rather distant systematic units, based on morphological data. The presence of such triterpenes marks a significant difference with other Russula species whose chemical content has been analysed: In fact, most Russula (specifically those provided with the specialized cuticle cells known as dermatocystidia), contain sesquiterpenes of the protoilludane family. Other species lacking dermatocystidia do not synthesize sesquiterpenes, but the above triterpenoids have been found in none of them to date. Early outcomes of phylogenetic studies also show the proximity of R. lepida and R. aurora; hence, the presence of cucurbitane seco-triterpenoids may be considered a good chemotaxonomic marker of the sections. As bioactivities are concerned, Russula cucurbitanes are endowed with little or no cytotoxicity against some selected human tumour cell lines, but recently two cucurbitanes isolated from R. lepida were found to be very active in a PTP1B inhibition assay. Russula species containing cucurbitane triterpenes are little affected by insect larvae and other parasites; this empirical observation has often led to the discovery of antifeedant compounds, a property that has not been verified yet on Russula triterpenes. It is, therefore, evident that much still needs to be understood in this restricted class of compounds, first in the likely discovery of novel chemical structures (mainly if the genus is considered in a worldwide perspective), and then to gather more information about their evolutionary and ecologic role.
