Liang Xu
Email Address
eriliaxu@nus.edu.sg
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Publication Growth and glucosinolate profiles of a common Asian green leafy vegetable, Brassica rapa subsp. chinensis var. parachinensis (choy sum), under LED lighting(Elsevier B.V., 2020-02-10) TAN WEE KEE; GOENADIE VIANDRA; LEE HUI WEN; LIANG XU; Loh Chiang Shiong; ONG CHOON NAM; TAN TIANG WAH,HUGH; NUS ENVIRONMENTAL RESEARCH INSTITUTE; SAW SWEE HOCK SCHOOL OF PUBLIC HEALTH; BIOLOGICAL SCIENCESTechnological advancements in light-emitting diode (LED) technology have led to the production, under controlled indoor conditions, of value-added crops that are high in nutritional or nutraceutical contents. In this study, the growth and glucosinolate (GSL) profiles of a common Asian vegetable, Brassica rapa subsp. chinensis var. parachinensis (choy sum or cai xin) were determined under LED lighting. The growth of choy sum was influenced by both the LED light intensity and spectrum, with plants grown under 160 µmol m-2 s-1 red-blue light (160RB) producing the highest shoot biomass (~100–300% increment in adult plants). GSL content was found to vary across growth stages regardless of LED treatment. Total GSLs per gram fresh weight were four times higher in one-leafed seedlings than in adult plants. A shift from a high proportion of aliphatic GSLs in one-leafed seedlings to indolic and aromatic GSLs in three-leafed seedlings and adult plants was attributed to an increase in the proportion of glucobrassicin and/or 4-hydroxy-glucobrassicin, and gluconasturtiin which are known to have anti-cancer properties. Our results will be useful for the production of GSL-enhanced vegetables through urban farming.Publication Nutritional metabolites in Brassica rapa subsp. chinensis var. parachinensis (choy sum) at three different growth stages: microgreen, seedling and adult plant.(Elsevier B.V., 2021-03-11) ZOU LI; TAN WEE KEE; DU YUANYUAN; LEE HUI WEN; LIANG XU; LEI JIAJIA; STRIEGEL LISA; WEBER NADINE; RYCHLIK MICHAEL; ONG CHOON NAM; NUS ENVIRONMENTAL RESEARCH INSTITUTE; SAW SWEE HOCK SCHOOL OF PUBLIC HEALTHChoy sum is a commonly consumed Asian green leafy brassica vegetable. A comprehensive spectrum of nutritional important metabolites, including amino acids, plant sugars, essential minerals, vitamins (A, B9, E, and K1) and glucosinolates were systematically quantified using LC-QQQ-MS, GC-QQQ-MS and ICP-MS. Significant metabolic profile shifts were observed during the three major developmental stages (microgreen, seedling and adult) studied. Primary metabolites, especially essential amino acids decreased while most plant sugars increased from microgreens to seedlings. Carotenoids, such as violaxanthin, neoxanthin, together with vitamin K1 were higher in the seedlings whereas CHO-folate vitamers and -cryptoxanthin were much lower in adult plants. Most essential minerals were concentrated in the microgreens, while sodium increased in adult plants. Aliphatic glucosinolates in microgreens were converted to indolic glucosinolates in the seedlings and further to aromatic glucosinolates in the adults. Overall findings reveal that most of the nutritional metabolites were concentrated either in the microgreens or seedlings.Publication Characterization of plant volatiles reveals distinct metabolic profiles and pathways among 12 brassicaceae vegetables(2018) Liu, Y; Zhang, H; Umashankar, S; Liang, X; Lee, H.W; Swarup, S; Ong, C.N; BIOLOGY; NUS ENVIRONMENTAL RESEARCH INSTITUTE; SAW SWEE HOCK SCHOOL OF PUBLIC HEALTH; BIOLOGICAL SCIENCESPlants emit characteristic organic volatile compounds (VOCs)with diverse biological/ecological functions. However, the links between plant species/varieties and their phytochemical emission profiles remain elusive. Here,we developed a direct headspace solid-phasemicroextraction (HS-SPME) technique and combined with non-targeted gas chromatography-high-resolution mass spectrometry (GC-HRMS) platform to investigate the VOCs profiles of 12 common Brassicaceae vegetables (watercress, rocket, Brussels sprouts, broccoli, kai lan, choy sum, pak choi, cabbage, Chinese cabbage, cauliflower, radish and cherry radish). The direct HS-SPME sampling approach enabled reproducible capture of the rapid-emitting VOCs upon plant tissue disruption. The results revealed extensive variation in VOCs profiles among the 12 Brassicaceae vegetables. Furthermore, principal component analysis (PCA) showed that the VOC profiles could clearly distinguish the 12 Brassicaceae vegetables, and that these profiles well reflected the classical morphological classification. After multivariate statistical analysis, 44 VOCs with significant differences among the Brassicaceae vegetables were identified. Pathway analysis showed that three secondary metabolism pathways, including the fatty acid pathway, methylerythritol phosphate (MEP) pathway and glucosinolate (GLS) pathway, behave distinctively in these vegetables. These three pathways are responsible for the generation and emission of green leaf volatiles (GLVs), terpenes and isothiocyanates (ITCs), respectively. Correlation analysis further showed that volatile metabolites formed via the common pathway had significantly positive correlations, whereas metabolites from different pathways had either non-significant or significantly negative correlations. Genetic influences on these metabolites across various vegetable types were also evaluated. These findings extend our phytochemical knowledge of the 12 edible Brassicaceae vegetables and provide useful information on their secondary metabolism. © 2018 by the authors.Publication Simultaneous determination of carotenoids, tocopherols and phylloquinone in 12 Brassicaceae vegetables(Elsevier B.V., 2020-05-25) LEE HUI WEN; ZHANG HUI; LIANG XU; ONG CHOON NAM; NUS ENVIRONMENTAL RESEARCH INSTITUTE; SAW SWEE HOCK SCHOOL OF PUBLIC HEALTHAt present, analysis of carotenoids, tocopherols and phylloquinone in vegetables are performed independently, suggesting a need for an effective analytical method to determine these lipophilic micronutrients simultaneously and accurately. A LC-APCI-MS/MS analytical method was developed and validated for the simultaneous determination of seven carotenoids, two tocopherols and phylloquinone in 12 Brassicaceae vegetables. The analytical run time was 35 min, with the full separation of lutein and zeaxanthin, using LCMS-friendly mobile phases comprising methanol and isopropanol mixtures. Limits of detection and quantification were 0.10–21.5 μg/g dry weight (DW) and 1.10–47.6 μg/g DW respectively. Intra-day and inter-day repeatability were less than 20% relative standard deviation (RSD), and recoveries were 89–128%. Data indicated that lutein and α-tocopherol were the predominant carotenoid and tocopherol in green leafy vegetables. Highest amounts of carotenoids, tocopherols and phylloquinone were found in rocket salad (1061 ± 11 μg/g DW), broccoli (286 ± 85 μg/g DW) and watercress (55 ± 4 μg/g DW), respectively. Our findings suggest that these vegetables are excellent sources of lipophilic micronutrients. Overall, this analytical method is ideal for the accurate and rapid determination of carotenoids, tocopherols and phylloquinone in vegetables.Publication An integrated metabolomics study of glucosinolate metabolism in different brassicaceae genera(MDPI AG, 2020) Liu, Y.; Rossi, M.; Liang, X.; Zhang, H.; Zou, L.; Ong, C.N.; NUS ENVIRONMENTAL RESEARCH INSTITUTE; DEAN'S OFFICE (SSH SCH OF PUBLIC HEALTH); SAW SWEE HOCK SCHOOL OF PUBLIC HEALTHGlucosinolates are a group of plant secondary metabolites that can be hydrolyzed into a variety of breakdown products such as isothiocyanates, thiocyanates, and nitriles. These breakdown products can facilitate plant defense and function as attractants to natural enemies of insect pests. As part of the diet, some of these compounds have shown cancer-preventing activities, and the levels of these metabolites in the edible parts of the plants are of interest. In this study, we systematically examined variations in glucosinolates, their precursors, and their breakdown products in 12 commonly consumed vegetables of the Brassicaceae family with gas chromatography—quadrupole time-of-flight mass spectrometer (GC-Q-TOF/MS), liquid chromatography–quadrupole time-of-flight mass spectrometer (LC-Q-TOF/MS), and liquid chromatography—triple quadrupole mass spectrometer (LC-QQQ/MS), using both untargeted and targeted approaches. The findings were integrated with data from literature to provide a comprehensive map of pathways for biosynthesis of glucosinolates and isothiocyanates. The levels of precursor glucosinolates are found to correlate well with their downstream breakdown products. Further, the types and abundances of glucosinolates among different genera are significantly different, and these data allow the classification of plants based on morphological taxonomy. Further validation on three genera, which are grown underground, in damp soil, and above ground, suggests that each genus has its specific biosynthetic pathways and that there are variations in some common glucosinolate biosynthesis pathways. Our methods and results provide a good starting point for further investigations into specific aspects of glucosinolate metabolism in the Brassica vegetables. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.Publication Serum amino acids in association with prevalent and incident type 2 diabetes in a Chinese population(2019) Lu, Y; Wang, Y; Liang, X; Zou, L; Ong, C.N; Yuan, J.-M; Koh, W.-P; Pan, A; NUS ENVIRONMENTAL RESEARCH INSTITUTE; SAW SWEE HOCK SCHOOL OF PUBLIC HEALTH; DUKE-NUS MEDICAL SCHOOLWe aimed to simultaneously examine the associations of both essential and non-essential amino acids with both prevalent and incident type 2 diabetes in a Chinese population. A case-control study was nested within the Singapore Chinese Health Study. Participants included 144 cases with prevalent and 160 cases with incident type 2 diabetes and 304 controls. Cases and controls were individually matched on age, sex, and date of blood collection. Baseline serum levels of 9 essential and 10 non-essential amino acids were measured using liquid chromatography tandem mass spectrometry. We identified that five essential (isoleucine, leucine, lysine, phenylalanine, and valine) and five non-essential (alanine, glutamic acid, glutamine, glycine, and tyrosine) amino acids were associated with the prevalence of type 2 diabetes; four essential (isoleucine, leucine, tryptophan, and valine) and two non-essential (glutamine and tyrosine) amino acids were associated with the incidence of type 2 diabetes. Of these, valine and tyrosine independently led to a significant improvement in risk prediction of incident type 2 diabetes. This study demonstrates that both essential and non-essential amino acids were associated with the risk for prevalent and incident type 2 diabetes, and the findings could aid in diabetes risk assessment in this Chinese population. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.Publication Profiling of phenolic compounds and antioxidant activity of 12 cruciferous vegetables(MDPI AG, 2018) Li Z.,; Lee H.W.; Liang X.; Liang D.; Wang Q.,; Huang D.; Ong C.N.; FOOD SCIENCE & TECHNOLOGY; NUS ENVIRONMENTAL RESEARCH INSTITUTE; SAW SWEE HOCK SCHOOL OF PUBLIC HEALTHThe phenolic profiles of 12 cruciferous vegetables (pakchoi, choysum, Chinese cabbage, kailan, Brussels sprout, cabbage, cauliflower, broccoli, rocket salad, red cherry radish, daikon radish, and watercress) were studied with UHPLC-MS/MS. Antioxidant activity and total phenolic content (TPC) were also evaluated. A total of 74 phenolic compounds were identified, including 16 hydroxycinnamic acids and derivatives, and 58 flavonoids and derivatives. The main flavonoids identified were glycosylated quercetin, kaempferol and isorhamnetin, and the main hydroxycinnamic acids were ferulic, sinapic, caffeic and p-coumaric acids. Principal component analysis (PCA) revealed that the distribution of phenolic compounds in different genera of cruciferous vegetables was in accordance with their conventional taxonomy. The DPPH, ORAC and TPC values ranged from 1.11 to 9.54 µmoles Trolox equivalent/g FW, 5.34 to 32.92 µmoles Trolox equivalent/g FW, and 0.16 to 1.93 mg gallic acid equivalent/g FW respectively. Spearman’s correlation showed significant (p < 0.05) positive correlations between TPC, flavonoids and antioxidant activity. © 2018 by the authors.Publication Simultaneous Quantification of 22 Glucosinolates in 12 Brassicaceae Vegetables by Hydrophilic Interaction Chromatography-Tandem Mass Spectrometry(American Chemical Society, 2018) Liang, X.; Lee, H.W.; Li, Z.; Lu, Y.; Zou, L.; Ong, C.N.; NUS ENVIRONMENTAL RESEARCH INSTITUTE; SAW SWEE HOCK SCHOOL OF PUBLIC HEALTHGlucosinolates, which are unique to Brassicaceae vegetables, have diverse biological activities, including antimicrobial, antioxidant, and anticancer actions. In this study, we applied hydrophilic interaction chromatography-tandem mass spectrometry (HILIC-MS/MS) to the simultaneous quantification of 22 glucosinolates in 12 Brassicaceae vegetables, including pak choi, choy sum, Chinese cabbage, cauliflower, cabbage, broccoli, Kai Lan, Brussels sprouts, rocket salad, daikon radish, red cherry radish, and watercress. Significant differences in concentration and composition of glucosinolates were observed among these vegetables. Cabbage had the highest level of total glucosinolates (?g/g dry weight: 19 551.2 � 1317.7), whereas Kai Lan had the lowest level (7611.3 � 868.4). Aliphatic and indole glucosinolates were the major components in the 12 vegetables ranging from 76 to 100%, except watercress (37%). On the basis of the content of glucosinolates, the 12 vegetables were well distinguishable and classified according to their morphological taxonomy. This study presents a HILIC-MS/MS approach for quantification of glucosinolates, and demonstrates the potential of glucosinolate profiles for Brassicaceae species identification. Copyright � 2018 American Chemical Society.