Alkaloid diversification in the genus Palicourea (Rubiaceae: Palicoureeae) viewed from a (retro-)biogenetic perspective

Abstract

The species-rich genus Palicourea (Rubiaceae: Palicoureeae) is source of an intriguing diversity of alkaloids derived from tryptamine and its precursor tryptophan. So far simple tryptamine analogues, polypyrroloindoline, β-carboline, and, most importantly, monoterpene-indole, i.e., tryptamine-iridoid alkaloids of various structural types including javaniside, alstrostine and strictosidine derivatives have been identified. Here the diverse alkaloids that numerous studies have found in the genus are examined and organized according to their structures and biosynthetic groups. Using a parsimony-based approach that follows the concept of retro-biogenesis usually applied in synthetic chemistry, possible biosynthetic pathways are proposed and important steps and relationships between these alkaloids are highlighted. Understanding alkaloid diversification is of importance in studying the ecological significance and evolution of biosynthetic capabilities of the genus Palicourea, and should stimulate future investigations on the biochemical and genetic background.

A revised classification of the sister tribes Palicoureeae and Psychotrieae (Rubiaceae) indicates genus-specific alkaloid accumulation

“…Further described compound groups include cyclotides (Koehbach et al 2013), flavonoids and other polyphenols (Berger et al 2016) and iridoids (Berger 2012;Lopes et al 2004), highlighting the chemical diversity of the tribes. Alkaloids are particularly diverse in the genus Palicourea lending the group to a more in-depth analysis of alkaloid diversification and possible biosynthetic sequences (see Berger et al 2021).…”

“…Furthermore, the biosynthetic origin of the nitrogen atom from either of the amino acids tyrosine (in Carapichea) of tryptophan (other genera) are indicated by crossbars on the tree. In the present study a brief taxonomic introduction is given for each genus and the respective alkaloids are grouped and enumerated according to structural similarity and putative biosynthetic relationships (see also Berger et al 2021).…”

“…Other alkaloids types include polypyrroloindoline IA in ten species, b-carbolines in seven species, simple tryptamine analogues in five species and protoalkaloids in three species. For each group of alkaloids present in Palicourea, individual structures and their source plants are briefly discussed, and biosynthetic considerations for these are found in Berger et al (2021).…”

“…According to the mode of cyclisation and the number of monomers involved, simple IA may be divided in two subgroups, and they are discussed below. Respective biosynthetic considerations are presented by Berger et al (2021).…”

“…Among these, compounds possessing a secologanin moiety (''tryptamine-secologanin alkaloids'') and compounds with a loganin moiety (''tryptamineloganin alkaloids'') may be differentiated (see below). The cores of tryptamine-iridoid alkaloids may be subjected to various modifications, and corresponding biosynthetic considerations are found in Berger et al (2021).…”