Tissue distribution and biosynthesis of 1,2-saturated pyrrolizidine alkaloids in Phalaenopsis hybrids (Orchidaceae)

Abstract

Phalaenopsis hybrids contain two 1,2-saturated pyrrolizidine monoesters, T-phalaenopsine (necine base trachelanthamidine) and its stereoisomer Is-phalaenopsine (necine base isoretronecanol). T-Phalaenopsine is the major alkaloid accounting for more than 90% of total alkaloid. About equal amounts of alkaloid were genuinely present as free base and its N-oxide. The structures were confirmed by GC–MS. The quantitative distribution of phalaenopsine in various organs and tissues of vegetative rosette plants and flowering plants revealed alkaloid in all tissues. The highest concentrations were found in young and developing tissues (e.g., root tips and young leaves), peripheral tissues (e.g., of flower stalks) and reproductive organs (flower buds and flowers). Within flowers, parts that usually attract insect visitors (e.g., labellum with colorful crests as well as column and pollinia) show the highest alkaloid levels. Tracer feeding experiments with 14C-labeled putrecine revealed that in rosette plants the aerial roots were the sites of phalaenopsine biosynthesis. However active biosynthesis was only observed in roots still attached to the plant but not in excised roots. There is a slow but substantial translocation of newly synthesized alkaloid from the roots to other plant organs. A long-term tracer experiment revealed that phalaenopsine shows neither turnover nor degradation. The results are discussed in the context of a polyphyletic molecular origin of the biosynthetic pathways of pyrrolizidine alkaloids in various scattered angiosperm taxa. The ecological role of the so called non-toxic 1,2-saturated pyrrolizidine alkaloids is discussed in comparison to the pro-toxic 1,2-unsaturated pyrrolizidine alkaloids. Evidence from the plant-insect interphase is presented indicating a substantial role of the 1,2-saturated alkaloids in plant and insect defense.