Orchids acquire fungal carbon for seed germination: pathways and players

Abstract

To germinate in nature, orchid seeds strictly rely on seed germination-promoting orchid mycorrhizal fungi (sgOMFs) for provision of carbon nutrients. The underlying delivery pathway, however, remains elusive. We develop here a plausible model for sugar transport from sgOMFs to orchid embryonic cells to fuel germination. Orchids exploit sgOMFs to induce the formation of pelotons, elaborate intracellular hyphal coils in orchid embryos. The colonized orchid cells then obtain carbon nutrients by uptake from living hyphae and peloton lysis, primarily as glucose derived from fungal trehalose hydrolyzed by orchid-specific trehalases. The uptake of massive fungally derived glucose is likely to be mediated by two classes of membrane proteins, namely, sugars will eventually be exported transporters (SWEETs) and H⁺-hexose symporters. The proposed model serves as a launch pad for further research to better understand and improve orchid seed germination and conservation.

Keywords: SWEETs; invertase; mycorrhizal fungi; orchid; seed germination; sugar transporters; symbiosis; trehalose.