Iridescence in plants: a shining example of research

Winner of a L'Oreal/UNESCO 'Women in Science' Award, Dr Heather Whitney reflects on the research inspired by a Hibiscus on the Systematic Beds.
'My research focuses on the occurrence of iridescence. Iridescence is a form of structural colour where the observer experiences a change in hue according to the angle from the iridescent structure is viewed. Very little is known about iridescence in plants, despite the fact that it is widespread throughout photosynthetic organisms, with at least five methods of iridescence generation in land plants. This includes the production of iridescence through diffraction gratings in flowers.

I was part of the team that recently discovered this novel mechanism by which flowers can produce colour when I was working with Dr Beverley Glover at the Department of Plant Sciences and collaborating with Professor Ulli Steiner at the Department of Physics in Cambridge. While carrying out this study we relied heavily on the wonderful diversity of plants grown at the Botanic Garden. It was actually at the Botanic Garden that the phenomenon was first observed – a sunny day lit up the vary-coloured base of Hibiscus trionum (which is grown in the Systematic Beds if you want to take a look for yourself!), and prompted the enquiry into the possible advantages a flower might have if it could produce iridescence.
Since moving to the University of Bristol, I have focused on plants that produce iridescence All leaf iridescence described to date is produced by a range of multilayered structures, in which light interacts at the boundaries of media with different refractive indices such that some wavelengths of light are reflected and the others are transmitted. While it is currently recorded that leaf iridescence is found in at least 64 different species in 26 families, the structures underlying the iridescence have only been characterised in seven of these species. These include multilayers of cellulose laminae in the epidermal wall in the clubmoss Selaginella uncinata, a helicoidal multilayered cell wall in the fern Microsorum thailandicum, and highly modified plastids containing multilayers (iridoplasts) in the flowering plant Begonia pavonina. However, the function of this multilayer iridescence is not known. I have three hypotheses: iridescence could act as disruptive camouflage against herbivores; it could enhance light capture in low light conditions; or it could be a photoprotective mechanism to protect shade-adapted plants against high light levels. These three hypotheses are not mutually exclusive: plant surface features are frequently multifunctional and each function may vary in importance in different environments.

I have just been awarded a European Research Council (ERC) starting grant to look at the mechanisms and development of iridescence production and its genetic basis in Selaginella uncinata. This species can produce a vivid metallic blue iridescence over its surface, dependent on light conditions. The ERC starting grant is a lab-based plant development and molecular biology project focusing on developing Selaginella uncinata as a model system, and provides funding for a transcriptome-based approach to identify the genes controlling the production of iridescence.

I have also recently been awarded a UNESCO/L’Oreal Women in Science fellowship to take this lab-based project out into the field. At least 23 species of tropical Selaginella produce iridescence, and many of them grow in neo-tropical environments. Prof Iván Valdespino (Universidad de Panamá), an expert in neotropical Selaginella species has kindly allowed me to visit Panama with the aim of observing these plants in their native habitat. I am also hoping to test how the herbivores and pollinators that live in neo-tropical habitats respond to iridescence.'
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