3D (x-y-t) Raman imaging of tomato fruit cuticle: microchemistry during development

The cuticle of tomato fruits was studied in-situ using Confocal Raman Microscopy. Microsections from cuticles isolated at different developmental stages were scanned to reveal the distribution of cuticle components with a spatial resolution of 342 nm by univariate and multivariate data analysis. From the three main components, cutin, polysaccharides and aromatics, the latter one exhibit the strongest Raman scattering intensity. Therefore, Raman imaging opened the view on phenolic acids and flavonoids within the cuticle and resulted in three schematic cuticle models depicting development. At the earliest stage of development, which corresponded to the procuticle layer, phenolic acids were found across the entire cuticle. Based on a mixture analysis with reference component spectra, the phenolic acids were identified as mainly esterified p-coumaric acid together with free p-hydroxybenzoic acid. Later in development, during the cell expansion period of growth, phenolic acids accumulated in an outermost layer of the cuticle and in the middle region of the pegs. In these stages of development cellulose and pectin were appeared towards the epidermal layer, where later during ripening the flavonoid impregnation started. In the first ripening stage chalconaringenin was observed, while methoxylated chalcones were chosen by the algorithm to fit the mature cuticle spectra. The co-location with carbohydrates and esterified p-coumaric acid and methoxylated chalconaringenin suggest that they link polysaccharide and cutin domains. Within the cutin matrix, aromatics confer mechanical and thermal functions, while the outermost phenolic acid layer displays UV-B protection of the plant tissue. One-sentence summaryNew insights into the distribution of cutin, carbohydrates and phenolics along cross sections of green and mature tomato fruit cuticles by Raman mapping and multivariate data analysis.