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 inflorescence 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 epicuticular wax morphology is composed primarily of columnar-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 chloroform 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 gymnosperms, and other minor constituents that occur idiosyncratically 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 ...

Analyses of Tomato Fruit Brightness Mutants Uncover Both Cutin-Deficient and Cutin-Abundant Mutants and a New Hypomorphic Allele of GDSL Lipase

“…In Arabidopsis, stem glossiness is generally indicative of altered epicuticular wax crystallization and is either due to general wax load reduction or to alteration of specific wax compounds (Jenks et al, 2002). Similar observations have been made in maize (Zea mays) and in rice (Oryza sativa) (Jenks et al, 2002;Islam et al, 2009).…”

The Cytochrome P450 Enzyme CYP96A15 Is the Midchain Alkane Hydroxylase Responsible for Formation of Secondary Alcohols and Ketones in Stem Cuticular Wax of Arabidopsis

“…As a result, alkane is assumed to be the in vivo substrate for the hydroxylation reactions in the wax biosynthetic pathway yielding secondary alcohols and ketones ( Fig. 1; Kolattukudy, 1996;Rashotte et al, 2001;Jenks et al, 2002). However, because these biosynthetic steps had not been confirmed to date, the alternative scenario could not be ruled out that acyl or aldehyde precursors may also be hydroxylated and the resulting hydroxyacids or hydroxyaldehydes transformed into secondary alcohols .…”

“…Finally, MAH1 gene activity was highest in the actively growing parts of the inflorescence stems in the same regions where rapid cuticle formation occurs. Chemical analyses had shown that this is also the part of the plant surface where secondary alcohols and ketones accumulate most rapidly (Suh et al, 2005), whereas they are almost absent from the leaf wax (Jenks et al, 2002). Based on the perfect correlation between expression patterns and metabolite occurrence, it can be concluded that MAH1 functions mainly, or even exclusively, as a hydroxylase in the decarbonylation pathway of stem wax biosynthesis.…”

“…Most notably, cer20 stems had been found to show lower levels of secondary alcohols and ketones than the wild type (Rashotte et al, 2001;Jenks et al, 2002), thus resembling mah1 stem wax. Based on this phenotype, it had been suggested that CER20 might be an enzyme responsible for the midchain oxidation of alkanes.…”

Disruption of Glycosylphosphatidylinositol-Anchored Lipid Transfer Protein Gene Altered Cuticular Lipid Composition, Increased Plastoglobules, and Enhanced Susceptibility to Infection by the Fungal Pathogen Alternaria brassicicola

“…The first barrier between plants and environmental stresses is the cuticle, which is composed of a lipophilic cutin polymer matrix and waxes (Holloway, 1982;Jeffree, 1996;Kunst et al, 2005;Nawrath, 2006). The cuticular waxes, which consist of very long chain fatty acids (VLCFAs; C20 to C34) and their derivatives, are embedded within and encase the cutin polymer matrix, a polyester framework composed of hydroxy fatty acids (C16 and C18) and glycerol monomers (Kolattukudy, 2001;Jenks et al, 2002;Nawrath, 2002;Heredia, 2003;Kunst and Samuels, 2003;Stark and Tian, 2006). The cuticle plays a role in limiting nonstomatal water loss and gas exchange, repelling lipophilic pathogenic spores and dust, providing mechanical strength and viscoelastic properties (Baker et al, 1982;Hoffmann-Benning and Kende, 1994;Riederer and Schreiber, 2001), preventing cell fusions (Lolle et al, 1998;Sieber et al, 2000), and protecting plants from environmental stresses (Schweizer et al, 1996).…”