Exogenous hormones influence Brassica napus leaf cuticular wax deposition and cuticle function

Background. Cuticular waxes cover plant surface and play important roles in protecting plants from abiotic and biotic stresses. The variations of wax deposition and chemical compositions under changing environments have been shown to be related to plant adaptations. However, it is still not clear whether the wax depositions could be adjusted to increase plant adaptations to stressed conditions. Methods. In this study, exogenous methyl jasmonate (MeJA), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and salicylic acid (SA) were applied to test their effects on cuticular wax deposition in two Brassica napus cultivars, Zhongshuang 9 (ZS9, low wax coverage ) and Yuyou 19 (YY19, high wax coverage). Next, we measured the water loss rate and the transcriptional expression of genes involved in wax biosynthesis as well as genes related to disease defense. Results. Seven wax compound classes, including fatty acids, aldehydes, alkanes, secondary alcohols, ketones, and unbranched as well as branched primary alcohols, were identified in B. napus leaf wax mixtures. MeJA, SA and ACC treatments had no significant effect on total wax amounts in YY19, whereas ACC reduced total wax amounts in ZS9. Overall, hormone treatments led to an increase in the amounts of aldehydes and ketones, and a decrease of secondary alcohol in ZS9, whereas they led to a decrease of alkane amounts and an increase of secondary alcohol amounts in YY19. Concomitantly, both cultivars also exhibited different changes in cuticle permeability, with leaf water loss rate per 15 min increased from 1.57% (averaged across treatments) at 1.57% (averaged across treatments) at 15 min to 3.12% at 30 min for ZS9 (except for ACC treated plant) and decreased for YY19. MeJA-treated plants of both cultivars relatively had higher water loss rate per 15 min when compared to other treatments. Conclusion. Our findings that B. napus leaf wax composition and cuticle permeability are altered by exogenous SA, MeJA and ACC suggest that the hormone treatments affect wax composition, and that the changes in wax profiles would cause changes in cuticle permeability.