Expression Analysis of Flavonoid-related Genes in Green and White Asparagus Spears

Asparagus (Asparagus officinalis L.) is one of the most widely produced vegetables in the world. Green and white spears originate from the same cultivar but are produced using different cultivation methods in Japan. White spears are shielded from sunlight, i.e., spears that grow in the absence of light, and green and white spears contain different phytochemicals due to these differences (presence vs. absence of light). Green spears contain rutin, while white spears do not have rutin but are rich in saponin (protodioscin) (Maeda et al., 2005, 2008, 2012). Rutin is one of the most significant flavonoids that has been reported to have biological activities, such as hypertension prevention, anti-inflammatory, anti-tumor, anti-bacterial/ viral, and potent radical-scavenging properties. Rutin also has protective effects against capillary fragility and arteriosclerotic vascular changes (Griffith Jr. et al., 1944; Hellerstein et al., 1951; Middleton et al., 2000; Calabro et al., 2005; Guo et al., 2007). Consumer interest in functional foods, such as flavonoids, is increasing; and this interest has been stimulated by the potential health benefits that have arisen from the antioxidant activities of these compounds (Maeda et al., 2006). The main genes involved in rutin biosynthetic pathway are chalcone synthase (CHS), chalcone isomerase (CHI), favanone-3-hydroxylase (F3H), favonoid-3¢-hydroxylase (F3¢H), and flavonol synthase (FLS), glucosyltransferase (GT) and rhamnosyltransferase (RT) (Fig. 1). Studies have revealed that light is one of the most important environmental signals regulating flavonoid biosynthesis (Fuglevand et al., 1996; Jenkins, 1997, 2001; Wade et al., 2001; Maeda et al., 2010; Kopsell and Sams, 2013; Carvalho and Folta, 2014). Supplemental lighting is introduced when asparagus is grown using the “Fusekomi” forcing culture technique, which is a unique cropping technique developed in Japan. With this technique, it was found that the amount of rutin increased as the number of lamps and the duration of light exposure increased, while spear color improved (Wambrauw et al., 2016). Nonetheless, the mechanisms behind this rutin enhancement have not been fully clarified. In addition, only a few studies have addressed the molecular basis of flavonoid metabolism regulation in asparagus (Yi et al., 2019), the effect of light on the accumulation of flavonoid-related genes and the role of light-regulated genes in enhancing rutin production. Therefore, we investigated the effects of light on rutin-related expressions of biosynthetic genes (CHS, CHI, F3H, F3¢H, FLS) and the amount of rutin in green (light-exposed) and white (light-shielded) spears.