Research conducted: The majority of plant colors are produced by anthocyanin and carotenoid pigments, but coloration obtained by nanostructured materials (i.e., structural colors) is increasingly reported in plants. Here, we identify a multilayer photonic structure in the fruits of Lantana strigocamara and compare it to a previously described origin in Viburnum tinus.

Methods: We used a combination of transmission electron microscopy, serial EM tomography, scanning force microscopy, and optical simulations to characterize the photonic structure in L. strigocamara. We also examine the development of the structure during maturation.

Key results: We find that the structural color derives from a disordered, multilayered reflector consisting of lipid droplets of ~105 nm that form a plate-like structure in 3D. This structure begins to form early in development and reflects blue wavelengths of light with increasing intensity over time as the structure develops. The lipids used are likely polymers of lipid monomers.

Main conclusions: Lantana strigocamara is the second origin of a lipid-based photonic structure, convergently evolved with the structure in Viburnum tinus. Chemical differences between the lipids in L. strigocamara and those of V. tinus suggest a distinct evolutionary trajectory with implications for the signaling function of structural colors in fruits.

Samples for TEM imaging were embedded in Epon resin, ultramicrotomed to create 80nm thick sections on copper mesh grids, and then imaged on an FEI Tecnai T12. Here, we include the full-size images, which were rotated and (in some cases) cropped slightly for the purposes of visualization in the manuscript.

Images have been identified by the panel that they correspond with in the original figures in the paper.