Anthocyanins have become sought-after natural products due to potential for medicinal and industrial uses. These metabolites have a number of health-promoting properties; increasing demand for nutraceuticals, fruits, and vegetables containing anthocyanins (Deroles, 2009; He and Giusti, 2010; Wrolstad and Culver, 2012). Production of textiles, cosmetics, and solar panels are examples of industrial applications where anthocyanins are also being increasingly used to replace synthetic dyes (Hao et al., 2006; Mansour et al., 2013; Wongcharee et al., 2007).
The use of anthocyanin extracts for the above applications is limited by the small number plant sources of anthocyanins as well as the cultural limitations of these species: annual life cycle, slow growth, limited harvest, high input, etc. (Deroles, 2009). The use of plant tissue culture has been proposed as a means of large-scale anthocyanin production; however, to date, these techniques have not been able to produce anthocyanins at levels sufficient to meet current industrial needs (Delgado-Vargas, 2000; Vogelien et al., 1990; Yamamoto and Mizuguchi, 1982). One way to meet the increasing demand for anthocyanins is to employ nonconventional plant species, such as poaceous grasses. The anatomy and perennial nature of turfgrasses make them attractive anthocyanin production systems.
Turfgrasses accumulate anthocyanins in leaves that originate from meristematic tissues that sit at or below the soil surface (Christians, 2011). Leaf tissue can therefore be harvested while keeping meristematic tissues intact, allowing for year-round production from the same stand of plants. Cool-season turfgrasses (C3 photosynthetic) devote greater than 60% of photosynthate toward leaf and sheath growth, and the potential yield of anthocyanin-containing tissue could be upwards of 3 Mg·ha−1 after a single harvest (Krans and Beard, 1980; Landschoot and Waddington, 1987; Younger, 1969). Relative to fruit crops, currently the most used anthocyanin source, turfgrasses provide numerous advantages. For example, turfgrasses could be harvested for anthocyanins within weeks of seeding, and leaf tissue could be harvested at least once per month. Further, since turfgrasses do not undergo secondary growth, a greater proportion of photosynthate could be devoted toward anthocyanin synthesis.
Rough bluegrass (P. trivialis L.) is known to constitutively produce the anthocyanins cyanidin-3-glucoside and cyanidin-3-malonylglucoside in the leaf sheath (Fossen et al., 2002; Hurley, 2010). This turfgrass is a fast-growing perennial under field conditions, and high tissue yield could therefore be expected (Atkin et al., 1996). Still, to employ this species as an industrial crop, it would be necessary to increase anthocyanin production in rough bluegrass to levels greater than those currently observed in the field. Environmental stress, light in particular, is one factor that is known to increase anthocyanin synthesis (Boldt et al., 2014).
Transient anthocyanin accumulation occurs with changes in light quantity and/or quality, and has been documented in a variety of plants including: tomato, Arabidopsis, maize, sorghum, rye, red cabbage, and mustard (Chalker-Scott, 1999; Mancinelli, 1985; Mol et al., 1996). Light is a requirement for anthocyanin synthesis, and anthocyanin production is photoregulated (Downs and Siegelman, 1963; Lange et al., 1971; Mancinelli, 1985; Rabino et al., 1977; Vyas et al., 2014). Photomanipulation could therefore be used to increase anthocyanin content in rough bluegrass.
Phytochrome has been shown to regulate anthocyanin synthesis through the absorption of red or far-red (FR) light (Kerckhoffs and Kendrick, 1997; Mancinelli, 1985; Neff and Chory, 1998; Oh et al., 2014; Wade et al., 2001). However, blue light also regulates anthocyanin synthesis, although this is accomplished through the activity of multiple photoreceptors, including both cryptochromes and phototropins (Galvão and Frankhauser, 2015). Cryptochromes are well known to regulate anthocyanin synthesis, whereas phototropins have only been recently implicated in anthocyanin regulation (Folta and Carvalho, 2015; Fox et al., 2012; Hong et al., 2009; Kadomura-Ishikawa et al., 2013; Poppe et al., 1998; Vandenbussche et al., 2007).
Treatment with red and blue light may also increase anthocyanin synthesis through photoreceptor coaction. In other words, red light may be noninductive on its own, but when applied with blue light, anthocyanin synthesis may be increased compared with blue light alone (Drumm and Mohr, 1978; Mancinelli, 1985; Mohr and Drumm-Herrel, 1983; Wade et al., 2001). In addition, anthocyanin synthesis has also been shown to be regulated through photosynthesis and increased under high-intensity light (Kumar Das et al., 2011; Mancinelli et al., 1976; Mancinelli and Rabino, 1978; Mancinelli, 1985; Schneider and Stimson, 1971; Weiss and Halevy, 1991). Therefore, anthocyanin content may increase through a combination of photosynthetic and photoreceptor-mediated regulation when blue, red, and/or combinations of blue and red light are applied.
Given the previously established regulation of anthocyanin synthesis in several monocot crops (i.e., sorghum and maize), we hypothesized that anthocyanin production in rough bluegrass could be manipulated through exposure to specific light regimes. The objectives of this research were to determine conditions that favor anthocyanin synthesis in rough bluegrass by first evaluating whether treatment with high-intensity light could increase anthocyanin content. Second, the wavelength(s) of light capable of upregulating anthocyanin synthesis was determined to optimize light conditions. Finally, the role of photosynthesis on anthocyanin production in rough bluegrass was evaluated.
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