Iridoplasts, out of the shadows at last

Plastid is the name applied to a family of organelles peculiar to plants and plant-like organisms that have a bounding double membrane – an ‘envelope’ – and a range of internal membranes and other structures within. As plant scientists we should be more than familiar with chloroplasts – green-coloured members of the family with their heavily-stacked internal pigment-carrying membranes (thylakoids) – which are the sites of photosynthesis. We should also be knowledgeable about colourless amyloplasts (especially in their role as statoliths in gravity perception). As botanists with an appreciation of the wider ecological relevance of plants we should also know at least a little about chromoplasts, and the importance of the colours they give to certain flowers and fruits. And as avid followers of the Annals of Botany, and/or AoB Blog, we should at least be on nodding terms with the tannosome*, a specialised plastid involved in formation of condensed tannins. But how many of us can honestly say we know much, if anything, about iridoplasts? I, for one, had forgotten I’d even heard of this member of the plastid super-family before a certain ‘colourful news item’ was one of the big plant stories of recent times, catapulting this organelle into the ‘spotlight’.

Named by Kevin Gould and David Lee, iridoplasts** are plastids that contain poorly stacked internal membrane aggregations running the length of the organelle. Identified in the adaxial epidermis of leaves of Begonia pavonina (Begoniaceae – the aptly-named Peacock Begonia), and Phyllagathis rotundifolia (Melastomataceae) they were inferred to contribute the blue iridescence of those species that grow as understorey plants in Malaysian forests.

Iridoplasts came to more recent prominence in a study by Matthew Jacobs et al. who investigated these structures in the interspecific hybrid Begonia grandis × B. pavonina. That study concluded that these plastids have a photonic crystal (‘periodic optical structures that can control the flow of light’) structure formed from the periodic arrangement of the light-absorbing thylakoid membranes. These structural features enhance photosynthesis, both by increasing light capture at the predominantly green wavelengths, which are common in the shaded conditions of the plant’s understorey habitat, and by directly enhancing quantum yield, by 5–10% under such low-light conditions. They further conclude that the iridoplast is a highly modified chloroplast whose structure is adapted to optimise use of the extremely low-light conditions in the tropical forest understorey, where it is found.

Still to be investigated is whether enhanced photosynthesis in the epidermis-sited iridoplasts translates to enhanced translocatable photosynthate, and an overall boost to the carbon economy of the entire plant, or whether products of these plastids’ photosynthetic prowess are used only locally, within the adaxial epidermal cells. Regardless, this work adds to the growing body of studies reflecting upon the significance of structural colours and plant iridescence in this low light environment and the potential biotechnological roles of such photonic structures, which may extend to engineering more efficient use of water by plants the better to adapt to a changing climate. This is definitely one plant stor(e)y that is, ‘to be continued’…

* But, why is it not called a tannoplast? At least that would be nomenclaturally consistent with other members of the plastid super-family.

** Not to be confused with iridosome, a cellulosic structure beyond the plasma membrane of the epidermal cells within the cell wall of fruits such as those of Elaeocarpus angustifolius (Elaeocarpaceae) and Delarbrea michieana (Araliaceae), and which causes their iridescent blue coloration. Nor should iridoplast be confused with iridoplasty, a medical procedure to correct an eye problem.