Onion thrips is an important insect pest of onion causing damage to foliage and reducing bulb and seed yields (Diaz-Montano et al., 2010; Elmore, 1949). Thrips also vector serious diseases such as iris yellow spot virus (Gent et al., 2004; Shock et al., 2008). Onion thrips populations can increase rapidly during warmer conditions requiring frequent pesticide applications, and consequently the insect has developed resistance to pyrethroid and organophosphate insecticides (Allen et al., 2005; Herron et al., 2008; Shelton et al., 2006). Because it is desirable to reduce pesticide use and increase integrated pest management strategies in vegetable crops (Eigenbrode and Trumble, 1994), genetic resistance to onion thrips would be beneficial for growers and, combined with fewer pesticide applications, offers a potentially affordable and sustainable management strategy for this important pest.
Reduced amounts of leaf epicuticular waxes (often referred to as glossy or bloomless phenotypes) have been associated with insect resistance in several crops. Bloomless sorghum [Sorghum bicolor (L.) Moench.] showed resistance to greenbug [Schizaphis graminum Rondani (Starks and Weibel, 1981)], glossy wheat (Triticum aestivum L. emend. Thell.) to grain aphid [Sitobion avenae F. (Lowe et al., 1985)], and glossy Brassica oleracea L. to cabbage worm (Artogeia rapae L.) and cabbage aphid (Brevicoryne brassicae L.) (Stoner, 1990). An explanation for these observations is that insects respond to cues from the chemistry of the waxes, and the broad diversity of compounds found in epicuticular waxes (long-chain fatty acids, esters, ketones, alkanes, and alcohols) may help insects to identify their host plants (Blaney and Chapman, 1970; Eigenbrode and Espelie, 1995; Städler, 1986; Thibout et al., 1982).
The biosynthetic pathway for epicuticular waxes has been studied in several plants (Kunst and Samuels, 2003). De novo fatty acid synthesis occurs in the plastid (Ohlrogge and Browse, 1995) and a substantial portion of fatty acids in epidermal cells is allocated toward wax production (Liu and Post-Beittenmiller, 1995). C18 fatty acids destined for wax synthesis are catalyzed into long chain molecules by fatty acid elongases associated with the endoplasmic reticulum (Fehling and Mukherjee, 1991). In leek (Allium ampeloprasum L.), peak activities of these enzymes were detected in basal regions of leaves immediately before the accumulation of epicuticular waxes (Rhee et al., 1998). Two main pathways have been suggested for production of epicuticular waxes (Millar et al., 1999). Long-chain fatty acids are converted into primary alcohols and esters through the acyl reduction pathway or into secondary alcohols and ketones through the decarbonylation pathway (Millar et al., 1999). Aldehydes are an intermediate step in both pathways and alkanes are either a product or intermediate step in the decarbonylation pathway. Enzymes that catalyze many of the later steps of these pathways are not well understood (Kunst and Samuels, 2003).
Natural variation exists in onion for amounts and types of epicuticular waxes (Damon et al., 2014; Molenaar, 1984). The most prevalent epicuticular waxes on the foliage of wild-type (waxy) onion are ketones (hentriacontanone-16), alkanes (2-methyl octacosane, 1-ethenyloxy octadecane, and heptacosane), and fatty alcohols (heptadecanol-1, hexacosanol-1, octacosanol-1, and triacontanol-1) (Damon et al., 2014). Amounts of these waxes are significantly less on leaves of the glossy (lowest amounts of waxes) and semiglossy (intermediate amounts of waxes) phenotypes relative to waxy plants (Damon et al., 2014). Although glossy onions show significant resistance to onion thrips (Alimousavi et al., 2007; Damon et al., 2014; Jones et al., 1934; Molenaar, 1984), this phenotype is not commercially useful because of susceptibility to leaf pathogens, excessive transpiration, and spray injury (Baker, 1982; Mohan and Molenaar 2005; Wirth et al., 1991). Semiglossy onions possess epicuticular wax amounts intermediate between the waxy and glossy phenotypes (Damon et al., 2014) as well as show significant resistance to onion thrips (Damon et al., 2014; Diaz-Montano et al., 2010, 2012). Therefore, this phenotype may be useful for integrated management of this important pest.
Genetic analyses of amounts and types of epicuticular waxes may reveal genes controlling the different branches of the wax–biosynthetic pathway and lead to a better understanding of epicuticular wax accumulation in plants. Quantitative trait loci (QTL) controlling amounts of epicuticular waxes have been mapped in several crops, including canola [Brassica napus L. (Pu et al., 2013)], rice [Oryza sativa L. (Srinivasan et al., 2008)], maize [Zea mays L. (Liu et al., 2012)], and sorghum (Burow et al., 2009). Mapping of QTL controlling amounts and types of waxes in onion would provide markers associated with onion thrips resistance and contribute to our understanding of the inheritance of different foliar wax phenotypes. In this study, we developed a waxy by semiglossy mapping population to identify QTL controlling amounts and types of epicuticular waxes in onion.
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