Franziska Zahn, postdoc at University of Bayreuth (Germany), presents her work “Stable isotope analysis indicates partial mycoheterotrophy in arbuscular mycorrhizal woody seedlings in tropical forests”. She discusses the collaborative network established between plants and fungi, how little we know about C-transfer in plants, and advises everyone to focus on their research despite the fascinating questions ecology offers to us.
Thinking of plants, we often picture lush greenery soaking up sunlight, converting it into energy through photosynthesis – a process central to life on Earth. This incredible ability for photosynthesis allows plants to transform sunlight, carbon dioxide, and water into glucose and oxygen, powering ecosystems worldwide.
But what happens when plants inhabit environments where light is scarce, or when the conditions for photosynthesis are otherwise not ideal?
Remarkably, some plants have evolved alternative ways to acquire carbon and sustain themselves. Take fully mycoheterotrophic plants, for example. These ghostly, pale plants lack chlorophyll – the green pigment that allows most plants to photosynthesise – and instead form a symbiotic belowground relationship with fungi in their roots to acquire the carbon they need.
Some plants take a more balanced approach. They can photosynthesise but still tap into fungal belowground networks for additional carbon gain. This lifestyle, referred to as partial mycoheterotrophy has been detected in quite a number of herbaceous forest plants growing in recent years, and it might be much more widespread than previously thought.
About the paper
In the dark understory of tropical forests conditions are highly unfavourable to photosynthesis – yet tree seedlings can survive there for decades. It is well-known that most tree species in tropical lowland forests form associations with arbuscular mycorrhizal (AM) fungi. A specific morphotype of AM fungi in plant roots is considered a prerequisite for mycoheterotrophic carbon gain: the Paris-morphotype – named after its occurrence in the herbaceous species Paris quadrifolia. In a literature search, we noticed that many abundant tropical tree species are in plant genera, where this morphotype had been found. In our paper, we therefore asked the question whether tree seedlings in the understory of tropical lowland forests may gain carbon from mycorrhizal fungi in addition to photosynthesis, that is, whether they are partially mycoheterotrophic. As indicator for fungus-to-plant carbon transfer, we compared the natural abundance signatures of stable isotopes of seedlings with the Paris-morphotype of AM and of co-occuring seedlings without it.
We collected root and leaf samples of seedlings of 41 species within 20 genera and 15 families in the understory of tropical lowland forests in Central Panama. We microscopically examined the roots to determine the mycorrhizal morphotype. We found many more plant species to have the Paris-type than we expected based on literature, which surprised me. In our research paper we showed for the first time that tropical woody species (6 out of the 21 species with Paris-type AM) had a stable isotope signature that is indicative of partial mycoheterotrophy – supporting the idea that carbon can be transferred from fungi to tropical tree seedlings across arbuscular mycorrhizal networks.
About the research
Our study on partial mycoheterotrophy in tropical tree seedlings builds upon the recent interest and discourses on Common Mycorrhizal Networks in the scientific and popular literature. The ongoing discussion highlights that we require a much better scientific understanding for popular claims (such as increased tree seedling performance through resource transfer via Common Mycorrhizal Networks) to be substantiated and more data are urgently needed, particularly from natural forest ecosystems to capture their complexity. We know very little about whether, how, and to what extent carbon is transferred from fungi to plants and between plants via belowground fungal network, with the scarce evidence varying depending on the ecosystem.
Our findings suggest for the first time that some woody seedlings may exploit fungi in arbuscular mycorrhizal networks for supplementary carbon from adult trees, which in turn may have pervasive implications for understanding the mechanisms of forest regeneration.
Yet, many questions remain to be answered, e.g. about environmental conditions, evolutionary drivers and ecological relevance of such carbon transfers. I believe we require a diverse set of methods (including both field- and laboratory studies with their challenges) to further support our initial evidence. We also need to find out what exactly determines the fungal morphology in arbuscular mycorrhiza and which role the morphology, e.g. coiled fungal hyphae (Paris-type), plays for carbon and nutrients transfer to mycoheterotrophic plants.
About the author
I currently work as a postdoctoral researcher at the University of Bayreuth (Germany), where I’ve just recently finished my Doctorate at the Laboratory for Isotope-Biogeochemistry. Being always fascinated by the natural world, studying Geography and Geoecology allowed me to explore various ecosystems from multiple perspectives. The inter-kingdom mycorrhizal interaction between plants and fungi is certainly a field of research that is most exciting for me. During my PhD I therefore focused on examining fungus-to-plant carbon transfer in mainly orchid but also arbuscular mycorrhizal associations (this paper) through stable isotope approaches.
From field to lab: The author Franziska organising the root samples for the microscopic observations (Credit: Alicia Knauft).
Like for most publications, there is a team of authors behind this paper, and I am very grateful to be part of this joint effort. The opportunity to collaborate with amazing people from around the world, all working together to tackle some of the many fascinating questions in science is something I enjoy most about being a scientist and that inspires me a lot. Now, my advice for other young scientists, both in and beyond the field of Ecology: While there are countless intriguing questions out there, it’s important to keep your focus on your main goal. Concentrate on what you find most promising, but also remain open to new perspectives and alternatives, stay curious, and enjoy doing research (even if it’s not always easy). Giving advice is always easier than following it, and I often must remind myself to take my own advice as well.
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