Cuticular waxes in Rosaceae : insights into crystal formation, biosynthesis, and the cuticular water barrier

The plant cuticle is a hydrophobic barrier composed of a complex mixture of wax compounds that coats all aerial surfaces, including leaves and fruit. This layer protects the plant against biotic and abiotic stressors, due in part to (i) its unique composition of very-long-chain aliphatics and pentacyclic triterpenoids and (ii) the presence of epicuticular wax crystals. The most important role of cuticular waxes, however, is in minimizing non-stomatal water loss. Despite their importance, little is known about the composition, biosynthesis, structure, and functions of cuticular waxes in non-model species like Rosaceae. In my first chapter, I surveyed species of two Rosaceae subfamilies and used gas chromatography to characterize their leaf cuticular wax mixtures. Additionally, I used scanning electron microscopy to investigate leaf surface micromorphology. My first aim was to elucidate the chemical basis for subfamily-specific wax crystal morphologies: tubules and irregular platelets. In pairing chemical and micromorphological data, I hypothesized that tubules are made of 10-nonacosanol and co-crystallizing diols, and that irregular platelets are made of C₃₁ and C₃₃ co-crystallizing alkanes and secondary alcohols. My second aim was to characterize ubiquitous and specialized wax biosynthesis, specifically of secondary alcohols. I found that secondary alcohols between subfamilies differed in preferred chain lengths but are likely synthesized from a similar mechanism from their conserved 1:2 asymmetry and hydroxyl positioning on even- and odd-numbered carbons. In my second chapter, I paired chemical and functional data to address whether correlations exist between cuticular wax constituents and cuticular water loss in Rosaceae leaf and fruit cuticles. From these comparisons, I found that leaf and fruit wax mixtures of the same species largely differ in quantity but not relative wax composition. Neither total wax nor the amounts of VLC aliphatics and triterpenoids were significantly correlated with permeability. The absence of significant relationships from these comparisons hints at molecular structure rather than composition of the cuticle determining the strength of the cuticular water barrier in Rosaceae. While these results provide valuable insight, they highlight the complexity of the water barrier in plants and the need for multi-disciplinary approaches to better understand the function of this vital structure.