Composition and structure of Rosaceae leaf cuticles: insights into crystal formation and secondary alcohol...

The cuticle covers and protects aerial plant tissues from biotic and abiotic stressors, due in part to its unique composition of cuticular wax compounds and the presence of epicuticular wax crystals. The shape of these crystals is known to be dictated by specific compounds dominating the wax mixture. This study aimed to elucidate the chemical basis for such structures and understand the underlying wax biosynthetic mechanisms in two Rosaceae subfamilies: Amygdaloideae and Rosoideae.

Gas chromatography coupled with mass spectrometry and flame ionization detection was used to characterize and quantify the leaf wax constituents of 12 species across two Rosaceae subfamilies. Scanning electron microscopy was used to investigate the leaf surface micromorphology.

Total wax amounts varied considerably within and between subfamilies, as did the presence of epicuticular wax crystals. Wax tubules were found exclusively in Amygdaloideae and composed primarily of 10-nonacosanol, whereas elongated, irregular platelets occurred only in Rosoideae and were composed of C31 and C33 aliphatics. Isomer analysis showed that secondary alcohols across subfamilies had a conserved 1:2 asymmetry, with hydroxyl groups on even- and odd-numbered carbons, demonstrating shared biochemistry. Amygdaloideae species primarily accumulated alkane pathway products, namely secondary alcohols. Rosoideae species accumulated more primary alcohol pathway products, and more alkanes than secondary alcohols.

Although preferred biosynthetic mechanisms and wax structures are broadly grouped by subfamily, this organization breaks down at lower taxonomic ranks. In Rosaceae, cuticular biochemistry and structure may be quickly tuned within evolutionary time, reflecting yet unknown functional optimization of the cuticle.