Short communicationEffect of blue LED light intensity on carotenoid accumulation in citrus juice sacs
Introduction
Carotenoids are wide-spread organic pigments, and fulfill a variety of critical functions in plants (Cunningham and Gantt, 1998, Havaux, 1998, Cazzonelli and Pogson, 2010, Nisar et al., 2015). To date, more than 700 carotenoids have been identified in nature of which mainly 6 carotenoids are detected at a high level in human plasma, including β-carotene, α-carotene, lutein, lycopene, β-cryptoxanthin and zeaxanthin (Al-Delaimy et al., 2005). The plasma carotenoid level is associated with less likely risks of cardiovascular diseases, cancers and age related diseases (Khachik et al., 2006, Pouchieu et al., 2014). Recently, some epidemiological studies have shown that the β-cryptoxanthin is one of the most important carotenoids in human plasma; dietary intake of β-cryptoxanthin from citrus fruits can reduce the risks of certain diseases, especially cancers, diabetes and rheumatism because of its antioxidative activity (Cerhan et al., 2003, Yamaguchi et al., 2006, Sugiura et al., 2011, Takayanagi et al., 2011, Yamaguchi, 2012, Iskandar et al., 2013). In the past 10 years, to meet the growing demand of consumers for the carotenoid-rich diets, carotenoid biosynthesis in plants has been extensively investigated, and several efforts have been devoted to enhancing carotenoid concentration by genetic modification (Paine et al., 2005, Naqvi et al., 2009, Welsch et al., 2010, Huang et al., 2013).
Citrus fruits, one of the most economically important fruits grown in temperate regions are a rich and complex source of carotenoids. In citrus, carotenoids are the pigments responsible for the external and internal coloration of fruits, and their contents and compositions are important indexes for the quality of fruits. In the recent years, the accumulation of carotenoid in fruits of various citrus varieties has been studied, showing that the composition of carotenoid in juice sacs vary greatly among different varieties during the ripening process (Kato et al., 2004, Rodrigo et al., 2004, Rodrigo and Zacarías, 2007, Ríos et al., 2010, Ma et al., 2013, Wei et al., 2014). In Satsuma mandarin (Citrus unshiu Marc.), β-cryptoxanthin is predominantly accumulated in the juice sacs (Fig. 1). In Valencia orange (Citrus sinensis Osbeck), in contrast, the content of β-cryptoxanthin is quite low, and violaxanthin is the major carotenoid in the juice sacs (Fig. 1). It has been proven that the differences in the carotenoid composition between Satsuma mandarin and Valencia orange were attributed to their different gene expression profiles (Kato et al., 2004, Kato et al., 2006). In Satsuma mandarin, the expression levels of the upstream synthesis genes (CitPSY, CitPDS, CitZDS, and CitLCYb1) were higher than those in Valencia orange. In contrast, the expression levels of downstream synthesis genes (CitHYb and CitZEP) were lower in Satsuma mandarin than those in Valencia orange. In addition, β-carotene is converted to zeaxanthin via β-cryptoxanthin by a two-step hydroxylation, which is catalyzed by HYb. In a previous study, we reported that CitHYb predominantly catalyzed the conversion of β-carotene to β-cryptoxanthin (Fig. 1). In Satsuma mandarin, the gene expression of CitHYb increased with a peak in December, which led to the accumulation of β-cryptoxanthin (from 1 μg g−1 in August to 15 μg g−1 in December) during the ripening periods (Kato et al., 2004).
In citrus fruits, carotenoid metabolism is a complicated process, which is regulated by developmental stages and environmental conditions (Kato et al., 2004, Zhang et al., 2012, Rodrigo et al., 2013). As the growing conditions on trees are not uniform, it is difficult to evaluate the effects of environmental factors on carotenoid accumulation in fruits ripening on tree. In our previous study, a culture system was set up using the juice sacs of Satsuma mandarin and Valencia orange, in which the juice sacs enlarged gradually with carotenoid accumulation and no callus formed during cultured in vitro for eight weeks (Zhang et al., 2012). In this system, blue LED light irradiation was effective for inducing carotenoid accumulation, while red LED light irradiation did not significantly affect the carotenoid content in the juice sacs of Satsuma mandarin and Valencia orange (Zhang et al., 2012). Except for light quality, light intensity was also a key factor regulating carotenoid metabolism in plants (Demmig-Adams and Adams, 1992, Bohne and Linden, 2002, Bramley, 2002, Keyhaninejad et al., 2012, Lado et al., 2015). In the present study, to further elucidate the roles of blue light in regulating carotenoid accumulation, the effects of the intensity of blue LED light on carotenoid biosynthesis and expression of the carotenoid biosynthetic genes were investigated in the juice sacs of Satsuma mandarin and Valencia orange in vitro. In general, blue light can not penetrate the peel in an intact fruit, thus, it is difficult to evaluate the effects of blue light on carotenoid accumulation of the juice sacs using the intact fruit. In this study, we treated the juice sacs directly with different intensities of blue LED light, the results presented herein will contribute to a better understanding of the differential blue-light regulated accumulation of various carotenoids between Satsuma mandarin and Valencia orange.
Section snippets
Plant materials
Satsuma mandarin (C. unshiu Marc.) and Valencia orange (C. sinensis Osbeck) in the mature green stage were used as materials. The samples were harvested at the National Institute of Fruit Tree Science, Department of Citrus Research, Okitsu (Shizuoka, Japan).
In vitro culture system and treatments
The fruits were surface-sterilized by a 10-min soak in 70% ethanol, a 30-min soak in 1% (w/v) NaOCl, and rinsed in sterile water. Juice sacs excised from the equatorial region of the fruit were placed on 10 mL of agar medium in culture tubes
Effect of the intensity of blue LED light on carotenoid content and composition
In the present study, the effects of blue LED light intensity on the accumulation of the main carotenoids in the juice sacs were investigated. In Satsuma mandarin, total carotenoid content under 50B or 100B was 2–3 fold that of the control during the experimental periods (Fig. 2A). In the second week, the contents of β-carotene, β-cryptoxanthin, 9-cis-violaxanthin, α-carotene, lutein and total carotenoid were lower in the 100B treatment than those in the 50B treatment in Satsuma mandarin. In
Discussion
Blue light is an effective activator of carotenoid accumulation in plants. In the sprouts of Tartary buckwheat (Fagopyrum tataricum Gaertn.), the expression levels of genes related to the carotenoid biosynthesis increased under the blue light irradiation from the 2nd day after sowing, and peaked at the 6th day after sowing (Tuan et al., 2013). Johkan et al. (2010) found that carotenoid content in the leaves of lettuce seedlings was increased by blue LED light treatment, and the elevated content
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