The taxonomic relevance of Naphthoquinones in tropical pitcher plants (Nepenthes L., Nepenthaceae)

Abstract

The distribution of the naphthoquinones ramentaceone and plumbagin was studied among 50 taxa of the genus Nepenthes. Naphthoquinone patterns support classifications based on homology of plastid and/or nuclear genes to some extent, with plumbagin predominant in sections Nepenthes, Urceolatae, Tentaculatae, and Regiae, ramentaceone predominant in sections Insignes and Villosae, and both isomers present without clear predominance in sections Pyrophytae and Montanae. Only 9 of 96 studied species contained both isomers in the same plant. Naphthoquinone data from artificial hybrids of known parentage allowed conclusions on the biosynthesis of these compounds and the heredity of the respective enzymatic steps.

Distribution of Acetogenic Naphthoquinones in Droseraceae and Their Chemotaxonomic Utility

“…In comparison to other groups of comparable distribution and size, plants of the order Nepenthales belonging to other families than Droseraceae have been subject to rather thorough investigation with respect to their secondary metabolites [1,47,49,[55][56][57]. However, whereas the naphthylisoquinoline alkaloids of Ancistrocladaceae are fairly well known [59], only three of 21 species of Ancistrocladus have been investigated for naphthoquinones so far [1].…”

“…While naphthoquinones may aid in preserving animal prey and protecting plant traps during digestion [44], and precursors acquired by carnivory are channeled into their biosynthetic pathway [45], the speculation that this may be an "important adaptation" across all carnivorous Nepenthales [46] is flawed by the fact that there are a few species of Nepenthes [47] and a comparatively large number of species of Drosera (Table 1, [21,22,27]) in which neither plumbagin nor ramentaceone have been detected. Although it was mentioned that the acetogenic naphthoquinones of carnivorous Nepenthales are lacking in all other lineages of carnivorous plants [46] where, interestingly, other characteristic acetogenic polyketides occur in some families such as the alkaloid coniine in Sarracenia [48], it was not appropriately considered that plumbagin is present in the closely related but noncarnivorous Plumbaginaceae (consecutive sister to the carnivorous clade of Nepenthales [9]) and Ancistrocladaceae (carnivory lost secondarily [9]).…”

“…The tendency that both monotypic genera (Aldrovanda, Dionaea) of Droseraceae and the earliest branching lineage in Drosera (D. regia) contain plumbagin (Table 1, Figure 4) confirms the general trend that this isomer is also more widespread outside Droseraceae. Within Nepenthales, the only other family in which a reasonable number of species have been identified to contain the other isomer (ramentaceone) is Nepenthaceae [47], the immediate sister of Droseraceae [9]. [43].…”