In this study, cuticular wax load, its chemical composition, and biosynthesis, was studied during development of wild type (WT) bilberry fruit and its natural glossy type (GT) mutant. GT fruit cuticular wax load was comparable with WT fruits. In both, the proportion of triterpenoids decreased during fruit development concomitant with increasing proportions of total aliphatic compounds. In GT fruit, a higher proportion of triterpenoids in cuticular wax was accompanied by a lower proportion of fatty acids and ketones compared to WT fruit as well as lower density of crystalloid structures on berry surfaces. Our results suggest that the glossy phenotype could be caused by the absence of rod-like structures in GT fruit associated with reduction in proportions of ketones and fatty acids in the cuticular wax. Especially CER26-like, FAR2, CER3-like, LTP, MIXTA, and BAS genes showed fruit skin preferential expression patterns indicating their role in cuticular wax biosynthesis and secretion.

The cuticle and cuticular waxes play vital roles as the interface between a plant and its environment. Cuticular waxes are composed of very long-chain fatty acids (VLCFAs) and their derivatives as well as triterpenoids, which are specialized metabolites. Although triterpenoids constitute a major fraction of cuticular waxes, their functions remain poorly understood. In this review, we provide an overview of the chemical diversity of triterpenoids in the cuticular waxes of leaves and fruits of angiosperms and discuss changes in triterpenoid content during their development. The methods and challenges for the analysis of cuticular wax triterpenoids are summarized. The biosynthesis of triterpenoids is also reviewed along with recent knowledge of the potential mechanisms of triterpenoid transportation and secretion from the site of biosynthesis to the plant surface. Considering ongoing climate change, the current understanding of the effects of environmental conditions and abiotic stress factors on cuticular wax triterpenoids is reviewed. We also present an overview of the current knowledge on the roles of triterpenoids in cuticular waxes in transpiration and defense against biotic stressors and their possible role in cuticle stabilization. Moreover, owing to the generally high levels of triterpenoids in fruit cuticular waxes, the provided information on the variation and role of cuticular wax triterpenoids during postharvest fruit storage might help in the development of strategies for better fruit quality maintenance and preservation.

Cuticular wax plays an important role in fruits in protection against environmental stresses and desiccation. In this study, biosynthesis and chemical composition of cuticular wax in wild type (WT) bilberry fruit was studied during development and compared with its natural glossy type (GT) mutant. The cuticular wax load in GT fruit was comparable to WT fruit. In both fruits, triterpenoids were the dominant wax compounds with decreasing proportion during the fruit development accompanied with increasing proportion of aliphatic compounds. Gene expression studies supported the pattern of compound accumulation during fruit development. Genes CER26-like, FAR2, CER3-like, LTP, MIXTA , and BAS exhibited prevalent expression in fruit skin indicating role in cuticular wax biosynthesis and secretion. In GT fruit, higher proportion of triterpenoids in cuticular wax was accompanied by lower proportion of fatty acids and ketones compared to WT fruit as well as lower density of crystalloid structures on berry surface. Our results suggest that a marked reduction in ketones in cuticular wax may play a significant role in the formation of glossy phenotype leading to the loss of rod-like structures in epicuticular wax layer of GT fruit. Highlight: Chemical composition and morphology of cuticular wax along with gene expression for wax biosynthetic genes varied between glossy type mutant (GT) and wild type (WT) fruit.