Scientists have discovered some novel adaptations in Australian orchids that may explain why they are so hardy, in comparison to other orchids around the world.
Fig 1a: Helper fungus seen as threads.Using a state-of-the-art approach, a collaboration between Kings Park, The University of Western Australia and the Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth in Germany have found that Australian orchids have developed novel ways of gaining nutrients from our bushland soils.
The orchid family comprises species that, at maturity, obtain carbon and nitrogen partially from fungal sources - so-called mycorrhiza (see Figure 1a and 1b; click to enlarge). Known as partial mycoheterotrophs, these orchids are often associated with fungi that simultaneously form ectomycorrhizas with trees.
Fig 1b: Greenhood orchid grown in lab
The study, undertaken by research students from BayCEER, looked at the mycorrhizal fungi of 35 green and one non-green (achlorophyllous) orchid species, whose identification were analysed using molecular methods. Nutritional mode was identified for 27 species by C (carbon) and N (nitrogen) isotope abundance analysis in comparison to non-orchids (shrubs and trees) from the same habitat. Analysis of the non-orchids could show that the orchids 'cheat' by using shared fungi to steal nitrogen meant for the non-orchid.
The stable isotope approach proved challenging for assigning most orchids to a specialised nutritional mode; therefore, these orchids were classified as autotrophic at maturity, that is, fully capable of sourcing their nitrogen and carbon from their own associations with non-shared mycorrhiza.
Fig 2: Spider orchid - Caladenia sp.Using radioactive 13CO2 pulse labeling confirmed full autotrophy for six orchid species showing they have self-reliant systems for accessing carbon with none arising from their fungal association (Figure 2; click to enlarge).
Nonetheless, at least three orchid species (Gastrodia lacista, Prasophyllum elatum and Corybas recurvus) were identified as nutritionally distinctive from autotrophic orchids and reference plants (Figure 3a and b; click to enlarge). These species most likely have a dependency on their mycorrhiza for carbon nutrition.
Fig 3a: Gastrodia seedlingsDespite the orchid-rich flora in southwestern Australia being among the most diverse terrestrial orchid flora globally, partial mycoheterotrophy among these orchids is less common than in other parts of the world, most likely because most associate with saprotrophic fungi rather than ectomycorrhizal fungi. What this means is that our orchids have evolved to live without depending on other non-orchids for providing their nitrogen and carbon.
Such a situation probably evolved in response to the dynamics of vegetation change over climate shift periods where non-orchid shrubs and trees may have come and gone from environments leading to orchids having to rely on their own in-built physiology and fungal partners for nutrition.
Fib 3b: Excavated Gastrodia tubersThis exciting study opens up new possibilities for explaining why orchids in southwest Australia may have broader ecological tolerances than shown in other orchid floras and provides hope for reintroducing rare species into restored landscapes.
Author: Professor Kingsley Dixon.
This article has been reproduced from Breakthrough: a bi-monthly enewsletter compiled by the Botanic Gardens and Parks Authority's Science Team.
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