Orchids acquire fungal carbon for seed germination: pathways and players.

Orchids produce millions of dust-like and reserveless tiny seeds per capsule and are unable to germinate on their own in nature. Orchid seeds have developed strategies to exploit seed germination-promoting orchid mycorrhizal fungi (sgOMFs) for acquisition of exogenous nutrients to fuel germination. Once colonized by sgOMFs, orchid seeds can digest intracellular sgOMF hyphae to acquire fungal nutrients, especially those for carbon nutrition. However, the underlying carbon delivery pathway from sgOMFs to orchid cells remains elusive. Clarification of the identities of fungally derived carbohydrates and the main molecular players responsible for the sgOMF-to-orchid carbon flow is of great significance for better understanding of symbiotic germination. 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. [ABSTRACT FROM AUTHOR]