Understanding the functional basis of orchid mycorrhiza - associations

Many species of orchid are amongst the rarest of all plants yet conservation efforts are made more difficult by a fundamental lack of knowledge into their basic biology. Orchids are amongst the newest and most rapidly evolving plants, comprising the largest plant family with over 20,000 species worldwide. Rapid diversification of orchids is facilitated by their vast seed production, a single plant typically releasing over 25,000 seeds per year. Such prolific reproduction is only possible by minimising seed size, and orchid seeds are so reduced that they contain insufficient reserves to permit germination and establishment on their own. Instead, these plants have forged a symbiotic alliance (living together) with soil fungi to provide the resources required to establish from seed. The first stage of seedling development is the formation of an initially leafless plantlet called a protocorm in which all of the carbon and mineral nutrients required to grow are supplied by its fungus partner. Most other plants also form symbiotic associations with soil fungi (termed mycorrhizas, literally meaning 'fungus root'), but in the majority of these cases the plant supplies carbon, which it has fixed through photosynthesis, to the fungus in return for mineral nutrients taken up from soil. This is a mutualistic symbiosis where both partners derive a benefit the relationship. What is unusual about the orchid-fungal symbiosis is that the fungus supplies both nutrients and carbon to the young plants without any obvious benefit to itself. Why does the fungus allow itself to be apparently parasitised in this way? My recent studies have shed light on the interactions between orchid and fungus. Using carbon, nitrogen and phosphorus sources enriched with specific isotopes of these elements, I have been able to unravel some of the mysteries surrounding this unusual symbiosis, answering two key questions for the first time; showing that the fungus can, in one species, benefit from this symbiosis and that the adult orchid continues to benefit from this symbiosis. Despite this major breakthrough with one species of orchid, we do not yet know whether any other green orchids behave in this way, how the carbon and nutrients are transferred between plant and fungus and whether being able to acquire carbon from a fungus has enabled some adult orchids to live in low light conditions where they are unable to fix enough carbon into sugars to meet their needs and effectively live as a parasite on the fungus. This project aims to address these key areas where there is a fundamental lack of understanding of the basic biology of orchid-fungus symbioses.