by Nathan Smith
If you were presented with a plant and a fungus and asked to pick the parasite, chances are you’d pick the fungus. Whilst this is often the case, there are significant and widespread cases of the relationship being the other way around. Enter Orchids.
Orchids are family of plants distinct in physical appearance and are renowned for their sweet scent and aesthetic beauty, despite the fact they more closely related to rice than they are to roses. Their seeds contain rather small reserves of nutrients and they are unable to photosynthesise immediately after germination, instead going through an achlorophyllic stage; in fact some orchids are not capable of photosynthesis during their lifespan. Usually small reserves and an initial inability to photosynthesise would be considered a bad strategy for a plant, but Orchids are still thriving and there is a good reason why.
Throughout their non-photosynthesising stage, and indeed throughout their entire existence, they are the dominant partners in what can best be described as an uneven symbiotic relationship with a fungal partner. In fact, a fungal partner is required by the orchid for them to germinate ‘in the wild’. Orchids can be germinated in sterile conditions; however this requires exposure to the ‘fungal sugar’ trehalose. So what is the trade between the orchid and the fungus? The fungus supplies the plant with organic carbon, a source of nitrogen, phosphorous, and other minerals and nutrients, and in return, gets… well, not much really. This uneven relationship continues once the plant gains the ability to photosynthesise and there is little evidence that the fungus gains a significant amount of reduced carbon from its photosynthetic symbiont. The fact that the fungus enters into a symbiosis with the plant in the first place, and continues this relationship throughout the plant’s life, suggests the fungus gains something from the relationship or that the plant emits a strong attractant, however there is little to no evidence for this and so these hypothesises remain little more than speculation.
What about the orchids that never photosynthesise? These plants, for instance the Bird’s-nest Orchid, have a habit of forming symbioses with fungi that also associate with tree roots. This allows them to use the fungus like a straw and indirectly parasitise what they need from the unsuspecting trees. Clever stuff.
Orchids are beautiful and interesting plants and deserve to be admired, but it doesn’t mean it’s the good guy. Next time spare a thought for the poor little fungus.
Photography by Leanne Massie