Abstract
Waterproofing of the aerial organs of plants imposed a big evolutionary step during the colonization of the terrestrial environment. The main plant polymers responsible of water repelling are lipids and lignin, which play also important roles in the protection against biotic/abiotic stresses, regulation of flux of gases and solutes, and mechanical stability against negative pressure, among others. While the lipids, non-polymerized cuticular waxes together with the polymerized cutin, protect the outer surface, lignin is confined to the secondary cell wall within mechanical important tissues. In the present work a micro cross-section of the stem of Arabidopsis thaliana was used to track in situ the distribution of these non-carbohydrate polymers by Confocal Raman Microscopy. Raman hyperspectral imaging gives a molecular fingerprint of the native waterproofing tissues and cells with diffraction limited spatial resolution (~300 nm) at relatively high speed and without any tedious sample preparation. Lipids and lignified tissues as well as their effect on water content was directly visualized by integrating the 1299, 1600, and 3400 cm−1 band, respectively. For detailed insights into compositional changes of these polymers vertex component analysis was performed on selected sample positions. Changes have been elucidated in the composition of lignin within the lignified tissues and between interfascicular fibers and xylem vessels. Hydrophobizing changes were revealed from the epidermal layer to the cuticle as well as a change in the aromatic composition within the cuticle of trichomes. To verify Raman signatures of different waterproofing polymers additionally Raman spectra of the cuticle and cutin monomer from tomato (Solanum lycopersicum) as well as aromatic model polymers (milled wood lignin and dehydrogenation polymer of coniferyl alcohol) and phenolic acids were acquired.
Highlights
Arabidopsis thaliana is the model organism for plant genetics and biology due to, among other reasons, the small genome size, its short generation time, the knowledge of its whole genomic sequence, the large amount of genetic resources, and simple genetic transformation and cultivation protocols (Kaul et al, 2000)
To present an overview of the distribution of waterproofing polymers in the Arabidopsis stem Raman images were first calculated through integration over specific marker bands (Figures 1A,B)
High water content is represented by the white areas, whereas the dark areas point to low water content and impregnation of waterproofing polymers
Summary
Arabidopsis thaliana is the model organism for plant genetics and biology due to, among other reasons, the small genome size, its short generation time, the knowledge of its whole genomic sequence, the large amount of genetic resources (i.e., mutants), and simple genetic transformation and cultivation protocols (Kaul et al, 2000). Raman Imaging of Waterproofing Polymers genetic basis and study the different plant polymers e.g., cellulose, hemicellulose, pectin, lignin, cutin, wax, and suberin (Beisson et al, 2012; Atmodjo et al, 2013; Pauly et al, 2013; McFarlane et al, 2014; Barros et al, 2015). The non-polysaccharide polymers, lignin, cutin, and suberin played an evolutionary important role during the transition from water to land by conferring waterproofing properties and mechanical strength. Lignin stiffens the cell wall (Jones et al, 2001) and confers waterproofness and resistance to the negative pressure generated during water transport in the xylem as well as to biotic and abiotic stresses (Sarkanen and Ludwig, 1971). Arabidopsis has been used to study the secondary xylem development as model for wood formation (Chaffey et al, 2002) and lignification (Dima et al, 2015)
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