Morphological Characteristics, Nutrients, and Bioactive Compounds of Zizania latifolia, and Health Benefits of Its Seeds

Zizania latifolia (tribe Oryzeae Dum., subfamily Oryzoideae Care, family Gramineae) is native to East Asian countries. The seeds of Z. latifolia (Chinese wild rice) have been consumed as a cereal in China for >3000 years. Z. latifolia forms swollen culms when infected with Ustilago esculenta, which is the second most-cultivated aquatic vegetable in China. The current review summarizes the nutrients and bioactive compounds of Z. latifolia, and health benefits of its seeds. The seeds of Z. latifolia contain proteins, minerals, vitamins, and bioactive compounds, the activities of which—for example, antioxidant activity—have been characterized. Various health benefits are associated with their consumption, such as alleviation of insulin resistance and lipotoxicity, and protection against cardiovascular disease. Chinese wild rice may be used to prevent and treat metabolic disease, such as diabetes, obesity, and cardiovascular diseases. Various compounds were isolated from the swollen culm, and aerial parts of Z. latifolia. The former suppresses osteoclast formation, inhibits growth of rat glioma cells, and may act as antioxidants and immunomodulators in drugs or foods. The latter exerts anti-fatigue, anti-inflammatory, and anti-allergic effects. Thus, Z. latifolia may be used to produce nutraceuticals and functional foods.


Introduction
Zizania latifolia is the sole member of the tribe Oryzeae Dum., subfamily Oryzoideae Care, family Gramineae. It is an important gramineous crop, genetically related to the Oryza species [1]. Four Zizania species are found worldwide: Zizania aquatica, Zizania palustris, and Zizania texana in North America (U.S.A. and Canada), and Z. latifolia in East Asia (China, Japan, and Korea) [2,3]. Z. aquatica and Z. palustris are annual species, whereas Z. texana and Z. latifolia are perennial species [3,4]. The genetic diversity of the North American Zizania species is higher than that of Z. latifolia [5]. The reasons for the low genetic diversity of the latter include restricted gene flow among populations, self-fertilization as a mode of reproduction, and genetic drift within populations [6]. Rivers, mountains, and habitat fragmentation influence the landscape-scale genetic structure of Z. latifolia [7]. Molecular evidence indicates that Zizania species likely originated in North America, thereafter spreading to East Asia and other parts of the world via the Bering Strait [8].
In contrast to many review articles on the nutritional constituents and health benefits of wild rice (Zizania spp.) [3,4], few review articles on the morphological characteristics, nutrients,

History and Present Situation of the Z. latifolia Seeds (Chinese Wild Rice)
The Z. latifolia seeds have been consumed as a cereal in China for >3000 years, and are one of the earliest cultivated crops. The earliest record of the species comes from the annals of the Zhou Dynasty. 'Zhou Li', a classical book of ancient China, lists Chinese wild rice as one of the 'six grains' (the other five being rice, broomcorn millet, panicled millet, wheat, and beans) offered to the emperor for his enjoyment [24]. However, the habitat of Z. latifolia has changed drastically with the rapid increase of the Southern Chinese population and the drying of lakes for rice cultivation, both of which have reduced the area occupied by this species. It has gradually been outnumbered by regular rice and is consumed less frequently, mostly for relieving hunger. Currently, it is rarely cultivated as a crop and mainly survives untended in the wild. It should be noted that Z. latifolia is prevalent throughout China, except in Xinjiang and Tibet, and grows in lakes, ponds, rivers, marshes, and the wetlands [6,7]. It is particularly abundant in the middle and lower Yangtze River basin and Huai River basin [5,25,26]. A Z. latifolia survey of 10 lakes of the Eastern China plain over 5 consecutive years revealed growth over a 488-km 2 area, with the generation of ca. 1.8 × 10 7 kg seeds and 5.5 × 10 8 kg aerial parts [26]. Between 1981 and 2009, the area covered by Z. latifolia in the Wuchang Lake increased linearly, with 53% of the lake surface occupied by 2009; this overgrowth was mainly associated with changes in the water level [27].

Nutrients of Chinese Wild Rice
The Z. latifolia seed (Chinese wild rice) is a type of whole grain with high nutritional value; it contains proteins, minerals, and vitamins, as well as bioactive compounds [3,4,28]. According to laboratory and population-based studies, Chinese wild rice is a safe and unique tasting food, the consumption of which shows a protective effect [24]. The nutrients of Chinese wild rice are shown in

History and Present Situation of the Z. latifolia Seeds (Chinese Wild Rice)
The Z. latifolia seeds have been consumed as a cereal in China for >3000 years, and are one of the earliest cultivated crops. The earliest record of the species comes from the annals of the Zhou Dynasty. 'Zhou Li', a classical book of ancient China, lists Chinese wild rice as one of the 'six grains' (the other five being rice, broomcorn millet, panicled millet, wheat, and beans) offered to the emperor for his enjoyment [24]. However, the habitat of Z. latifolia has changed drastically with the rapid increase of the Southern Chinese population and the drying of lakes for rice cultivation, both of which have reduced the area occupied by this species. It has gradually been outnumbered by regular rice and is consumed less frequently, mostly for relieving hunger. Currently, it is rarely cultivated as a crop and mainly survives untended in the wild. It should be noted that Z. latifolia is prevalent throughout China, except in Xinjiang and Tibet, and grows in lakes, ponds, rivers, marshes, and the wetlands [6,7]. It is particularly abundant in the middle and lower Yangtze River basin and Huai River basin [5,25,26]. A Z. latifolia survey of 10 lakes of the Eastern China plain over 5 consecutive years revealed growth over a 488-km 2 area, with the generation of ca. 1.8 × 10 7 kg seeds and 5.5 × 10 8 kg aerial parts [26]. Between 1981 and 2009, the area covered by Z. latifolia in the Wuchang Lake increased linearly, with 53% of the lake surface occupied by 2009; this overgrowth was mainly associated with changes in the water level [27].

Nutrients of Chinese Wild Rice
The Z. latifolia seed (Chinese wild rice) is a type of whole grain with high nutritional value; it contains proteins, minerals, and vitamins, as well as bioactive compounds [3,4,28]. According to laboratory and population-based studies, Chinese wild rice is a safe and unique tasting food, the consumption of which shows a protective effect [24]. The nutrients of Chinese wild rice are shown in Table 1. The vitamin (vitamin B1, vitamin B2, and vitamin E) content in Chinese wild rice from five lakes of Jiangsu Province is appreciably higher than in polished Indica rice from Jiangsu Province [29]. In addition, it contains higher amounts of essential amino acids than white rice, barley, and maize [30,31]. The amino acid score of Chinese wild rice is 66.6, which is considerably higher than that of polished Indica rice (51.1) [28]. The first and second limiting amino acids of Chinese wild rice are originally considered to be threonine and lysine, respectively [31]. However, a subsequent study by Jiang et al. [28] shows that lysine and isoleucine are the first and second limiting amino acids of Chinese wild rice, respectively. The protein efficiency ratio of Chinese wild rice is 2.75, which is higher than that of enriched white flour (0.6), rice (2.18), and soybean (2.32) [29]. Therefore, proteins derived from Chinese wild rice are considered to be high quality. The lipid content of North American wild rice ranges from 0.7 to 1.1%, compared to 2.7% in regular brown rice [35]. The main lipids in North American wild rice are linoleic (35-37%), linolenic (20-31%), palmitic (14.1-18.4%), and oleic (12.8-16.2%) acids [35]. The amount of moisture, protein, fat, ash, and dietary fiber in Chinese wild rice is similar to that in North American wild rice; however, iron, vitamin B1, and vitamin E content of Chinese wild rice is higher, whereas methionine, zinc, and vitamin B2 content is lower [30,31]. The amount of vitamin B1 in Chinese wild rice is 0.52-0.63 mg/100 g, whereas that in North American wild rice is 0.36-0.50 mg/100 g; the total vitamin E content of North American wild rice is reportedly 0.2 mg/100 g lipids, whereas it is 0.48 mg/100 g lipids in Chinese wild rice [31]. Although the total carbohydrate content of Chinese wild rice is lower than that in polished Indica rice, the levels of other components and elements are higher, including protein (2.16-fold), fat (1.54-fold), total dietary fiber (7.62-fold), and total mineral levels (1.91-fold), as well as calcium (3.32-fold), chromium (4.00-fold), copper (1.83-fold), iron (2.39-fold), magnesium (3.14-fold), manganese (1.31-fold), phosphorus (2.93-fold), potassium (2.83-fold), sodium (1.57-fold), and zinc (1.39-fold) [28]. Therefore, Chinese wild rice is a rich source of minerals (Table 1).

Bioactive Compounds in Chinese Wild Rice
In addition to nutrients, Chinese wild rice also contains bioactive compounds ( Table 1). The flavonoid, saponin, and phytosterol content in Chinese wild rice are 52.25-, 12.12-, and 4.29-times higher, respectively, than in white rice [28]. Importantly, anthocyanins and chlorophyll, which were not detected in white rice, are present in high quantities in Chinese wild rice. Although the bioactive compounds of Chinese wild rice have not been extensively studied, relatively more data are available regarding the flavonoid, phenolic acid, cell wall hydroxycinnamate, γ-oryzanol, and phytosterol of North American wild rice [35][36][37][38][39]. Among these, the total phenolic content of North American wild rice varies from 2472 to 4072 mg of ferulic acid equivalents (FAE)/kg, which is higher than that in white rice (279 mg of FAE/kg) [38]. Flavonoid glycosides (diglucosyl apigenin, glucosyl-arabinosyl apigenin, and diarabinosyl apigenin) and flavan-3-ols (catechin, epicatechin, and oligomeric procyanidin) are the main antioxidants identified in North American wild rice [38]. Ferulic acid (FA) is the most abundant phenolic acid (up to 355 mg/kg), followed by sinapic acid, in North American wild rice; the other monomeric phenolic acid compounds present in North American wild rice are p-coumaric acid, vanillic acid, syringic acid, p-hydroxybenzoic acid, p-OH-benzaldehyde, and vanillin [39]. Phenolic acid dehydrodimers are cell wall-bound, and are only detectable in the methanol-insoluble fractions; they are represented by diferulic and disinapic acids [39]. FA is the most abundant phenolic acid (394.2 mg/kg) among the insoluble dietary fibers in North American wild rice, although significant amounts of sinapic acid (51.8 mg/kg) and p-coumaric acid (14.2 mg/kg) were also detected [37]. Five feruloylated oligosaccharides were isolated and identified as arabinoxylan ferulate fragments, and the new feruloylated tetrasaccharide [37]. North American wild rice contains more γ-oryzanol, a natural mixture of FA esters of triterpene alcohols and sterols, and an important bioactive component in rice bran oil, than regular brown rice (1352 vs. 688 mg/kg); cycloartenol ferulate is the most abundant sterol ferulate in North American wild rice, whereas 24-methylenecycloartenol ferulate is the most abundant in regular brown rice [36]. There are more saturated ferulates in regular brown rice compared to North American wild rice; four additional γ-oryzanol compounds are identified as caffeates and cinnamates of cycloartenol and campesterol in North American wild rice [36]. To our knowledge, North American wild rice is a source of bioactive phytosterols; the main sterols of North American wild rice found in an unsaponified fraction were: campesterol (14-52%), β-sitosterol (19-33%), ∆ 5 -avenasterol (5-12%), and cycloartenol (5-12%) [35]. Moreover, the level of omega-3 fatty acids in North American wild rice is up to 18 times higher than in brown rice [35]. Anthocyanins are thought to be the major functional components in pigmented rice [40], which may be the major functional components in North American wild rice [38]. The bioactivities and health benefits of wild rice may be attributed to the synergistic effects of its functional components. Notably, the inhibitory effects of dietary polyphenols against α-glucosidases, α-amylases, and aldose reductases have attracted interest among researchers [41][42][43].

Antioxidant Activity
Whole grains are rich in bioactive substances, such as antioxidants, which protect the body against oxidative stress [44]. As a whole-grain food, Chinese wild rice contains high levels of antioxidants [28], which can effectively remove free radicals from the body, and enhance the function of antioxidant systems. When incorporated into different meat products, wild rice retards lipid oxidation, with phytic acid being one of the potent antioxidants of wild rice [45]. The antioxidant activity of raw Z. palustris seeds is 30 times higher than that of control white rice, and the total phenolic content is highly correlated with the total antioxidant activity of Z. palustris seeds [38]. The diphenylpicrylhydrazyl (DPPH) free radical-scavenging activities of the soluble and insoluble phenolic acids suggest that the antioxidant activity of Z. palustris seeds is partially associated with its phenolic acid profile [39]. The main contributors to the antioxidant activity of free flavonoids in Z. aquatica seeds are epigallocatechin, epicatechin, and rutin, and the main contributors to the antioxidant activity of free phenolic acids in Z. aquatica seeds are ferulic, vanillic, ellagic, sinapic, and syringic acids [46]. Chinese wild rice also contains the trace elements selenium and copper, as well as vitamin E, which contribute to its enzymatic and non-enzymatic antioxidant activities [29]. Consumption of Chinese wild rice prevents the accumulation of oxidative stress in rats fed a high saturated fat and cholesterol diet; this is because of increased antioxidant and superoxide dismutase activities, and reduced malondialdehyde concentration in the serum and liver [47,48] (Table 2). Owing to its superior antioxidant capacity, Chinese wild rice can inhibit the deleterious effects of lipid peroxidation to ameliorate chronic metabolic diseases.

Alleviation of Insulin Resistance and Lipotoxicity
Chinese wild rice is abundant in dietary fiber, resistant starch, and phytosterols, as well as polyphenols, such as flavonoids, saponins, and anthocyanins [28] (Table 1). Replacing white rice and processed wheat starch with Chinese wild rice as the main source of dietary carbohydrate in rats fed high saturated fat and cholesterol diet ameliorated abnormal glucose metabolism by suppressing diet-induced insulin resistance [49,51]. Animal studies revealed that Chinese wild rice prevented increase in body weight and fat accumulation in insulin-resistant mice, and lowered the serum levels of glucose, insulin, and free fatty acids [49]. Chinese wild rice inhibits the expression of protein tyrosine phosphatase 1B in the liver to promote the phosphorylation of insulin receptor substrate-2, thereby enhancing insulin signal transduction and reducing insulin resistance ( Table 2). Dietary carbohydrate replaced with Chinese wild rice reduced triglyceride and free fatty acid levels in the liver, leading to increase in serum adiponectin levels, reduction in serum lipocalin-2 and visfatin concentrations, up-regulation of the expressions of adiponectin receptor 2, peroxisome proliferator-activated receptor alpha, and gamma, and down-regulation of leptin and lipocalin-2 expression [51] ( Table 2). Because of its low glycaemic index, Chinese wild rice alleviated insulin resistance in rats induced by high saturated fat and cholesterol diet when it was used to replace 50% of the enriched rice and flour content in the diet [50]. The total dietary fiber content of Chinese wild rice is 7.62 times that of polished Indica rice; its resistant starch content (11.73 g/100 g) is significantly higher than that of rice flour (7.71 g/100 g) and wheat flour (7.79 g/100 g) [28]. The ratios of omega-6 to omega-3 fatty acids in Z. palustris seeds range from 1.1 to 1.8, whereas those in regular brown rice range between 20.2 and 22.4 [35]. Reports indicate the beneficial effects of the lower ratios of omega-6 to omega-3 fatty acids in North American wild rice on human blood lipid level. In rats fed high saturated fat and cholesterol diet, Chinese wild rice effectively reduced fat deposition, and lowered serum total cholesterol, triglyceride, tumor necrosis factor (TNF)-α, C-reactive protein, and free fatty acid levels; in contrast, it increased high-density lipoprotein cholesterol levels by enhancing lipid metabolism. Lipotoxicity is a metabolic syndrome that results from the accumulation of lipid intermediates in non-adipose tissue, leading to cellular dysfunction and death, and is involved in heart failure, obesity, and diabetes [56]. Notably, Chinese wild rice reduced hepatic lipotoxicity, lowered blood lipid levels in rats with dysregulated lipid metabolism, and reduced low-level inflammation caused by hyperlipidaemia [52,53]. It also prevented the up-regulation of sterol-regulatory element binding protein-1c, fatty acid synthase, and acetyl-CoA carboxylase, and down-regulated lipoprotein lipase and adipose triglyceride lipase induced by high saturated fat and cholesterol diet [54] (Table 2). Therefore, Chinese wild rice belongs to foods of low glycaemic index, which can alleviate insulin resistance and lipotoxicity.

Cardiovascular Disease Prevention
Soluble dietary fiber can reduce blood glucose levels and bind to bile acid for excretion in feces, thereby lowering cholesterol levels and reducing the risk of heart disease, atherosclerosis, and other conditions associated with high cholesterol levels [52]. Owing to its high resistant starch content, Chinese wild rice lowers cholesterol and blood lipid levels, which is associated with a reduced incidence of cardiovascular diseases, including coronary heart disease and atherosclerosis [57]. Consumption of Z. palustris seeds appreciably reduced the size and severity of atherosclerotic lesions in the aortic roots of male and female low-density lipoprotein receptor-knockout (LDLr-KO) mice, by 71% and 61%, respectively, compared to the gender-matched controls [58]. The phytosterol content of Chinese wild rice (71.28 mg/100 g) was 4.29 times higher than that of polished Indica rice [28]. Consumption of 60% (w/w) Z. palustris seeds in combination with 2% (w/w) phytosterols appreciably reduced the size and severity of atherosclerotic lesions in the aortic roots of LDLr-KO mice compared to the control group [59]. This effect was associated with an appreciable reduction of total plasma, low-density lipoprotein, and very low-density lipoprotein cholesterol levels, as well as increase in fecal cholesterol excretion. Chinese wild rice lowered blood cholesterol concentration in rats with high lipid levels, and inhibited the formation of atherosclerotic plaques and the occurrence of fatty liver, while simultaneously increasing antioxidant activity in serum and suppressing inflammation associated with atherosclerosis [55] (Table 2). When compared to white rice, Z. palustris seeds inhibit monocyte adhesion to the aorta, atherosclerosis, and the concentration of inflammatory and fibrinolytic regulators in the cardiovascular tissue of LDLr-KO mice [60]. The anti-atherosclerotic effect of wild rice in LDLr-KO mice may be associated with its inhibition of monocyte adhesion and inflammatory modulators [60].

Nutrients and Bioactive Compounds of the Aerial Parts of Z. latifolia
Although the aerial part of Z. latifolia is not eaten directly as food, several studies have reported the main nutrients contained therein. Qian et al. [32] reported the following nutrients in the leaf sheath: moisture, 870.00 g/kg FW; protein, 11.71 g/kg DW; fat, 18.10 g/kg FW; starch, 25.86 g/kg DW; total soluble sugars, 220.53 g/kg DW; reducing sugars, 182.73 g/kg DW; ascorbic acid, 0.01 g/kg DW; polyphenols, 6.30 g/kg FW; ash, 10.10 g/kg DW; total dietary fiber, 106.20 g/kg FW; soluble dietary fiber, 28.90 g/kg FW; insoluble dietary fiber, 77.40 g/kg FW; lignin, 8.19 g/kg FW. Another study reported the biomass chemical composition of the aerial parts of Z. latifolia to be as follows: cellulose, 28.5%; hemicellulose, 12.9%; lignin, 31.0%; ash, 3.0%; extractives, 18.9% [2]. Considering their high protein, cellulose, hemicellulose, and lignin content, the aerial parts of Z. latifolia could be used as animal feed, culture material for edible mushrooms, feed stock for bioethanol production, and activated carbon [2,32,69].

Nutrients and Bioactive Compounds of the Aerial Parts of Z. latifolia
Although the aerial part of Z. latifolia is not eaten directly as food, several studies have reported the main nutrients contained therein. Qian et al. [32] reported the following nutrients in the leaf sheath: moisture, 870.00 g/kg FW; protein, 11.71 g/kg DW; fat, 18.10 g/kg FW; starch, 25.86 g/kg DW; total soluble sugars, 220.53 g/kg DW; reducing sugars, 182.73 g/kg DW; ascorbic acid, 0.01 g/kg DW; polyphenols, 6.30 g/kg FW; ash, 10.10 g/kg DW; total dietary fiber, 106.20 g/kg FW; soluble dietary fiber, 28.90 g/kg FW; insoluble dietary fiber, 77.40 g/kg FW; lignin, 8.19 g/kg FW. Another study reported the biomass chemical composition of the aerial parts of Z. latifolia to be as follows: cellulose, 28.5%; hemicellulose, 12.9%; lignin, 31.0%; ash, 3.0%; extractives, 18.9% [2]. Considering their high protein, cellulose, hemicellulose, and lignin content, the aerial parts of Z. latifolia could be used as animal feed, culture material for edible mushrooms, feed stock for bioethanol production, and activated carbon [2,32,69].
The aerial parts of Z. latifolia have been shown to exert anti-fatigue, anti-inflammatory, and anti-allergic effects [70][71][72][73][74]. A methanol extract of the aerial parts of Z. latifolia inhibits compound 48/80-induced degranulation, antigen-induced β-hexosaminidase release, and PMA plus A23187-induced TNF-α production in RBL-2H3 mast cells [71]. In contrast, the chloroform fraction of the plant tissue suppresses allergic inflammatory response by blocking the release of β-hexosaminidase and TNF-α from RBL-2H3 cells stimulated with dinitrophenyl and bovine serum albumin, and inhibiting the expression of cyclooxygenase-2 and activation of the mitogen-activated protein kinase [74].

Conclusions
Chinese wild rice contains proteins, minerals, vitamins, and bioactive substances. It exhibits various bioactivities, including antioxidant activity. A number of health benefits are associated with its consumption, e.g., alleviation of insulin resistance and lipotoxicity, and protection against cardiovascular disease. Therefore, Chinese wild rice may be used to prevent and treat metabolic disease, such as obesity, diabetes, and cardiovascular diseases. However, no human dietary  (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) isolated from the aerial parts of Z. latifolia.

Conclusions
Chinese wild rice contains proteins, minerals, vitamins, and bioactive substances. It exhibits various bioactivities, including antioxidant activity. A number of health benefits are associated with its consumption, e.g., alleviation of insulin resistance and lipotoxicity, and protection against cardiovascular disease. Therefore, Chinese wild rice may be used to prevent and treat metabolic disease, such as obesity, diabetes, and cardiovascular diseases. However, no human dietary intervention studies have been performed, although studies where human subjects will be administered realistic doses of Chinese wild rice as a part of their daily diet are required. Although considerable progress has been made toward understanding the bioactivities and health benefits of Chinese wild rice, relatively fewer studies focus on the bioactivities and structure-activity relationship of the isolated compounds. Consequently, future studies will focus on the isolation of compounds with high bioactivities and stable chemical structures, and elucidate their structure-activity relationship. Compounds from the swollen culm of Z. latifolia have been shown to suppress osteoclast formation and inhibit the growth of glioma cells. Their role as potential antioxidants and immunomodulators for use in drugs or food products have also been investigated. Furthermore, compounds from the aerial parts of Z. latifolia can be used to treat fatigue, inflammation, and allergic reaction.

Conflicts of Interest:
The authors declare no conflicts of interest.