Cuticular Waxes of Arabidopsis

Abstract

As in most terrestrial plants, the cuticle on Arabidopsis thaliana (L.) Heynh. forms a continuous lipid membrane over the apical epidermal cell walls of essentially all aerial plant organs. Epicuticular waxes form the outermost layer over this membrane and are visible on Arabidopsis inflo-rescence stem and silique surfaces as a bluish-white colored coating called glaucousness or waxy bloom. Intracuticular waxes are intermeshed within the cuticle membrane and not visible to the naked eye. Close examination of epicuticular waxes on Arabidopsis stems and siliques using scanning electron microscopy (SEM) at around 3000x magnification best reveals their diverse crystalline structures (Figure 1A). The stem and silique epi-cuticular wax morphology is composed primarily of colum-nar-shaped crystals (of ~1.0 μm diameter), although rods, tubes, vertical plates, dendritic-, and umbrella-like structures are also typically present. The non-glaucous rosette and cauline leaf surfaces of Arabidopsis lack wax crystals detectable at the level of SEM (Figure 1B). Interestingly, other organs possess epicuticular wax crystals, including those of the pistil (Bowman, 1993). Descriptions of wax crystalline morphology on other flower parts, seeds, seedlings, roots, and other organs of Arabidopsis have not been reported. Whereas the term epicuticular wax is used to describe wax crystals above of the cuticle, cuticular wax is being used here to describe those long chain lipids extracted by submersion of tissues in solvents like hexane and chloro-form since this extraction procedure likely removes both epicuticular and intracuticular waxes. The chemical composition of cuticular waxes on Arabidopsis leaves and stems are typical of those on many dicotyledonous plants, being composed primarily of saturated free fatty acids, aldehydes, alkanes, primary alcohols, secondary alcohols, ketones, and wax esters (Figure 2). Within these component classes, homologues occur as aliphatic chains of between 16 and 33 carbons, except the wax esters, which are composed of even more carbons. The dominant wax class on Arabidopsis leaves and stems is the alkanes, although primary alcohols comprise a significant wax fraction on these surfaces. Stems and siliques possess relatively high amounts of ketones and secondary alcohols, whereas rosette and cauline leaves possess these constituents in trace or undetectable amounts. Arabidopsis lacks the wax hydroxy-ß-diketones, ß-diketones, and alkan-2-ol esters found on certain monocots (Bianchi and Bianchi, 1990), the estolides only reported in gym-nosperms, and other minor constituents that occur idio-syncratically in plants that have been examined (Walton 1990). Inflorescence stems of wild-type Arabidopsis can produce over ten fold more total wax per area than leaves, and stem wax chain length distribution is shorter than leaves, with the C 29 alkane homologue dominating stem waxes but the C 31 alkane dominating leaf waxes (Jenks et al., 1995). Rosette leaves possess lower relative amounts of primary alcohols than cauline leaves, whereas rosette and cauline leaves, and siliques, have lower relative amounts of the C 30 aldehydes than the stems (Todd et al., 1999). Interestingly, pollen grains are also coated with waxes, these being dominated as for stems by the C 29 alkanes, secondary alcohols, and ketones, and the C 30 aldehydes (Preuss et al., 1993; Fiebig et al., 2000). Wax composition on other Arabidopsis organs have not been reported. Little is known about metabolic and regulatory processes associated with synthesis of cuticular waxes by Arabidopsis. Similarities between wax constituents on Arabidopsis and many other plant species however suggest that the basic mechanisms for wax production are highly conserved within the plant kingdom. The Arabidopsis close-relative Brassica oleraceae L. has been used to describe many of these reactions (see