Structural colour from a helicoidal cellulose architecture in the secondary cell wall: Optical properties, cell wall composition, structure and morphology of components, and their assembly and interactions.
Creators: name: Steiner, Lisa Maria
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: Structural colour
Margaritaria nobilis
Microsorum thailandicum
Pollia condensata
Plant cell wall
Secondary cell wall
Cellulose
Cellulose microfibrils
Xylan
Iridescence
Cell wall composition
Helicoid
Helicoidal architecture
Nanostructure
Living light
Cellulose helicoidal architecture
Circular polarisation
Cellulose biosynthesis
Cell wall biosynthesis
Xylan biosynthesis
Adaxial epidermis
Abaxial epidermis
Cellulose isolation
Cellulose crystallinity
Glucuronoxylan

description: Structural colours are produced by constructive interference of light scattered from periodically arranged interfaces within nanostructured materials. A common strategy of plants to achieve structural coloration consists of assembling cellulose microfibrils into helicoidal structures. Examples of these architectures can be found in phylogenetically distant species and in different tissues of plants, such as in the endocarp of the fruit of $\textit{Margaritaria nobilis}$, an eudicot, and in the fronds of $\textit{Microsorum thailandicum}$, a fern. In this thesis, I studied the optical response and anatomical features of the adaxial and abaxial epidermal layers of cells of $\textit{M. thailandicum}$. The optical variation between fronds, between cells, and within cells, both for the adaxial and abaxial surface, were quantified by polarised optical microscopy and microspectroscopy. The adaxial reflection is mainly in the blue range (400-550 nm), with a narrow distribution of reflection peak maxima and peak widths, while the abaxial reflection spans almost the entire visible range (400-650 nm), and spectra are broader and show more varied features. The anatomical structure responsible for this optical behaviour was confirmed by electron microscopy. Numerical modelling based on the microscopy data indicates that there is much more complexity in the helicoidal architecture than just simple local defects. The significant difference between the adaxial and abaxial epidermal cell walls hints at fundamental differences in their biosynthesis. For the endocarp of the fruit of $\textit{M. nobilis}$, I correlated the optical response to the ultrastructure of the secondary cell wall, and the morphology and chemical structure of its main components, cellulose and xylan. The composition of the endocarp was characterised as 9 \% extractives, 36 \% cellulose, 23 \% xylan, and 30 \% lignin. The cellulose microfibrils were isolated in a long series of chemical purifications, and found to be extremely short - around 500 nm - and highly crystalline, as determined via nuclear magnetic resonance spectroscopy and X-ray diffraction. Xylan was found to be the most abundant hemicellulose and its primary structure was characterised via enzyme digestions, gel electrophoresis and mass spectrometry. It has acetyl groups on every other xylose unit, no arabinose and around 4 \% of glucuronic acid decoration. By combining the information on the xylan with the morphological characteristics of the cellulose microfibrils, I speculate about their assembly into the helicoidal architecture during the cell wall biosynthesis, based on findings from coarsed-grain modelling, and on their interactions in the final tissue, based on small angle X-ray scattering studies.