Answering a four decade-old question on epicuticular wax biosynthesis

In this issue of Journal of Experimental Botany (pages 2715–2730) Schneider et al. report the identity of three genes from barley described in the 1970s as important for the synthesis of β-diketone cuticular waxes, thereby revealing a novel polyketide synthase pathway responsible for their production. It is a perfect example of how modern sequencing technologies can resolve age-old questions on important food crops. All land plants possess a lipophilic layer coating their aerial surfaces: a cuticle. It consists of waxes embedded within and overlaying an esterified polymer of oxygenated fatty acids and glycerol (cutin). As such, plant cuticles form one of the largest biological interfaces on the planet, providing the first points of contact with the surrounding, often hostile, environment. The key function of cuticle is to prevent non-stomatal water loss, but other functions include defense against bacterial and fungal pathogens, mediating interactions with insects, and protection from excess levels of ultraviolet radiation. Cuticular waxes are chemically complex mixtures of hydrophobic molecules, most typically even and odd chain fatty acid derivatives of carbon lengths C24 and higher. Epicuticular waxes form the outermost layer of cuticle and often form crystals, imparting a whitish bloom to the organ surface. The amount and composition of cuticular wax varies widely between plant species and even between organs of the same plant; and cuticle chemistry can also change during development of the organ and is altered by environmental conditions. β-diketones are very-long-chain (typically C29–C31) oxygenated hydrocarbons that have long been known to be cuticular wax components of some plants (Horn and Lamberton, 1962; Tulloch and Weenink, 1966; Jackson, 1971; von Wettstein-Knowles, 1972; Evans et al., 1975; Jenks et al., 2002). They are well described in the cuticular waxes of diverse plant species, including Eucalyptus, Rhododendron and Hosta. However, they are best described in graminaceous species (e.g. barley, wheat and oats) (Box 1). Knowledge of β-diketone biosynthesis has come from extensive genetic and radiotracer feeding studies in barley by Penny von Wettstein-Knowles and colleagues (von Wettstein-Knowles, 1995; 2012). The largest collection of cuticular wax-deficient mutants, termed eceriferum (cer), is in barley, consisting of more than 1500 mutants in about 75 complementation groups. The Cer-cqu cluster, which affects β-diketone, hydroxy β-diketone and esterified alkan-2-ol production, is represented by over 500 distinct mutations and is made up of three complementation groups (Cer-c, -q and -u) (Box 1). The identities of the genes in this locus have remained elusive – until now. Box 1. Epicuticular waxes of barley flag leaf sheaths (A) Wild type (WT) barley ‘Bonus’ and cer-c, cer-q, and cer-u mutants demonstrating the glossy phenotypes of the cer-c and cer-q mutants plus reduced wax on the cer-u mutant resulting from reductions in β-diketones and their derivatives. ...

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