Abstract The effective component in Veratrum nigrum Linn， resveratrol， as an imporant phytoalexins， has broad applications. In recent years， broad and profound studies have been conducted on resveratrol in the fields including plant physiology， food， medicine， and health care products. In this paper， the research progress in its source， physiochemical properties， analysis and detection， extraction and antibacterial effect was reviewed， so as to provide valueful reference for further research and development of resveratrol.
　　Key words Resveratrol； Extraction； Analytical detection； Antibacterial effect
　　Veratrum nigrum Linn is a traditional Chinese herb. It is a perennial herb in Liliaceae family. According to incomplete statistics， there are about 40 species in the world， mainly distributed in Asia， Europe and North America. According to Flora of China， there are about 20 species in China， distributed in the Northeast China， North China， Shaanxi， Shandong， Sichuan and Guizhou[1]， mainly including V. maackii Regel， V. dahuricum （Turcz.） Loes. f.， V. patulum， V. nigrum L. var. ussuriense Nakai， V. grandiflorum （Maxim.） Loes. f.， V. nanchuanense S. Z. Chen et G. J. Xu， V. gongshanense S. Z. Chen et G. J. Xu and V. nigrum Linn. var. paniculatum Y. Ren[2]. The root and rhizome of V. Nigrum are used as a medicine and for treating diseases including stroke， epilepsy， pharyngitis and acariasis[3].
　　Resveratrol as one of the effective components in Veratrum plants has been studied by scholars at home and abroad continuously in recent years. In this article， the extraction， analysis and antimicrobial effect of resveratrol were reviewed， to as to provide valuable reference for further development of resveratrol.
　　Research about Chemical Components in V. nigrum
　　Scholars at home and abroad have deeply studied the chemical components in Veratrum plants， and the main components are as follows：
　　Steroidal alkaloids
　　There are various Veratrum alkaloids， which are mainly divided into isosteroidal alkaloids and cholest alkaloids. The isosteroidal alkaloids are divided into verazine group， cevine group and jervine group， and the cholest alkaloids are divided into solanidine group and new type of steroid alkaloids[4]. According to the research on Veratrum plants in 1984， the data showed that there are about 70 steroidal alkaloids in Veratrum plants， and in recent 20 years， more than 30 steroidal alkaloids were then separated from Veratrum plants[5]. The root part of Veratrum plants contains the most alkaloids， which account for about 1%-2% of the total content and are stable， so this part is suitable to be stored or dried when the plants are green[6]. 　　Stilbene compounds
　　Zhao et al.[7]isolated resveratrol and 2，3′，4，5′tetrahydroxystilbene from the rhizomes of Veratrum plants， and found verussustilbene from V. nigrum L. var. ussuriense Nakai. The first two substances were isolated from V. maackii Regel： resveratrol and 2，3′，4，5′tetrahydroxystilbene[8].
　　Flavonoid compounds
　　Foldesiova et al.[6]isolated from the leaves of V. lobelianum Bernh.， 3，5dihydroxy7methoxy flavone and 3，5，7trihydroxyflavone.
　　Dipeptide compounds
　　Zhao et al.[9-10]isolated from the rhizomes of V. nigrum L. var. ussuriense Nakai， two dipeptide compounds： ecinuline and aurantiamide.
　　Other compounds
　　Quan et al.[11]extracted V. dahuricum with ethanol and chloroform， and isolated βsitosterol and daucosterol. Yang et al.[12]extracted V. mengtzeanum Loes. with ethanol and chloroform， and isolated βsitosterol， daucosterol， cerotic acid and stearic acid. Liang et al. [13-15]extracted V. stenophyllum Diels in Notes Bot. with ethanol and chloroform， and isolated βsitosterol， stearic acid， and lignoceric acid.
　　Source and physicochemical properties of resveratrol in V. nigrum
　　Resveratrol has the chemical name （E）3，5，4trihydroxystilbene， which has the molecular formula C14H12O3， with the relative molecular weight of 228.25. The compound is white acicular crystal， and its melting points is 253-255 ℃. It would be sublimated at 261 ℃. Resveratrol is soluble in diethyl ether， chloroform， methanol， ethanol， acetone and ethyl acetate， and the solubleness ranks as acetone>ethanol>methanol>ethyl acetate>diethyl ether>chloroform. It could produce fluorescence under the UV irradiation at 365 nm， and could react with ferric trichloridepotassium ferricyanide to develop color. Resveratrol is more stable at low temperature in the dark， but not stable in alkaline environment. Resveratrol not only exists in cisform in the nature， but also in transform. However， resveratrol mainly exists in plants in transform， which has a biological activity stronger than cisform .
　　Resveratrol is known as a kind of phytoalexin， which would be produced easily when the environment becomes worse or the plant is attacked by pathogens. The phytoalexin mainly exists in the xylem of plant. Resveratrol has been found in 72 species of 31 genera in 21 families， such as Polygonum in Polygonaceae family， Vitis and Ampelopsis of Vitaceae family， and Cassia and Arachis in Leguminous. The plants containing resveratrol are mostly medicinal plants， such as Polygonum cuspidatum， Cassia tora and V. nigrum. There are also foods， such as grape， peanut， mulberry and Gnetum montanum[16]. 　　Analytical Detection Methods of Resveratrol
　　Rapid accurate detection methods include the basic steps of extraction， isolation and purification， and are of great significance to the production and research of resveratrol.
　　Ultraviolet spectrophotometry
　　Zheng[17]determined the content of resveratrol in P. cuspidatum by ultraviolet spectrophotometry， and found that resveratrol had a good linear relation between concentration and absorbance within the range of 3-16 μg/ml. Though the method is simple with low cost， the detection error is high.
　　Fluorospectrophotometry
　　Zhang[18]determined the content of resveratrol extracted from peanut skin by fluorospectrophotometry， and discussed the optimal conditions for fluorospectrophotometry， and the excitation wavelength and emission wavelength were 324.1 and 400.0 nm， respectively. It was found that the fluorescence intensity of resveratrol had a good linear relation with its concentration within the range of 0-1.68×10-5 mol/L， and its detection limit was 8.14×10-10 mol/L. This method has high sensitivity and good selectivity， and is very beneficial to the determination of some microconstituents in traditional Chinese medicine.
　　High performance liquid chromatography （HPLC）
　　Stecher et al.[19]well isolated cis and transresveratrol in wine by reversedphase highperformance liquid chromatography， and then detected resveratrol in samples by ultraviolet spectrophotometer， fluorophotometer and mass spectrometer， respectively， and finally， the three detection methods were compared. It was found that the results of the three detection methods were accordant， but the detection results of mass spectrometer and fluorophotometer had higher sensitivity than the ultraviolet spectrophotometer， and among the various methods， electronic spray ionization mass spectrometer connected with HPLC had the highest sensitivity， which was 3-4 times of that of ultraviolet detection. Huang[20]performed HPLC scanning as well as ultraviolet scanning of specific chromatographic peak， and further confirmed the existence of resveratrol.
　　Gas chromatography/mass spectrography （GC/MS）
　　Luan[21]performed solidphase microextraction and methylsilylation on grape wine samples， and qualitatively and quantitatively analyzed transresveratrol in samples by GC/MS. Compared with conventional solidphase extraction， solidphase microextraction has the advantages of obvious separation of solidphase microextraction chromatographic peak， large peak area and less disturbance， and could used for gas chromatography as well as liquid chromatography. 　　Capillary electrophoresis
　　Capillary electrophoresis is a kind of highend separation and detection technique. Compared with HPLC， it has higher separation speed and high column efficiency， and consumes no solvent nearly， and less sample， only a few percent of HPLC. Currently， the electrophoresis mode for quantitative determination of resveratrol including capillary zone electrophoresis[22].
　　Extraction of resveratrol from plants
　　At present， researches at abroad mainly extract resveratrol from grape skin and grape seeds； and resveratrol is mainly extracted from P. cuspidatum as a raw material at home. The main extraction methods include solvent extraction method， enzymatic extraction， microwaveassisted extraction， supercritical CO2 extraction and ultrasonic extraction.
　　Solvent extraction method
　　Reflux extraction of resveratrol usually uses methanol， ethanol， acetone and ethyl acetate， among which the reflux extraction on P. cuspidatum using 60%-90% ethanol water solution is most common[23]. Other solvents are relatively less used with low efficiency and high toxicity. Zheng et al.[24]performed reflux extraction on P. cuspidatum using acetone， and the yield of resveratrol was 0.95%.
　　Enzymatic extraction
　　Hou[25]performed beating on P. cuspidatum. The material was stirred at 10-50 ℃ for 24-120 h and extracted with ethyl acetate， and the yield of resveratrol was improved by nearly two times compared with the direct extraction method. In another method， P. cuspidatum was soaked in water， followed by fermentation at 15-50 ℃ for 24-96 h and extraction with ethyl acetate， and the content of resveratrol was improved by two times compared with the direct extraction method[26]. In this method， cellulase and distillers yeast are generally used as ectoenzyme. Li[27]added P. cuspidatum powder into cellulase and water to perform enzymolysis， and the filter residue was subjected to reflux extraction， achieving the yield of resveratrol up to 1.5%. Lin[28]reported that after the enzymolysis of P. cuspidatum powder with a proper amount of distillers yeast at 40 ℃ for 3 h， the yield was improved by nearly two times compared with the direct extraction. Currently， the reports about the acquisition of resveratrol from enzymolysis of polydatin have increased gradually. Wang[29]added a proper amount of Synceplzalastrum racemosum 3.264 in P. cuspidatum， followed by fermentation and ethanol extraction， and the yield of resveratrol was higher than 2%. Tian[30]demonstrated that Rhizopus T34 could convert polydatin into resveratrol with a conversion rate up to 98%. Lyu et al.[31]inoculated Aspergillus niger I to ferment P. cuspidatum， and the yield of resveratrol reached 1.197%， which was significantly improved compared with those achieved by conventional selfenzymolysis and alcohol extraction process. 　　Microwave extraction
　　Li[32]investigated the effects of main operation parameters in microwave extraction on the yield of resveratrol from traditional Chinese herb P. cuspidatum， and obtained following optimal extraction conditions： methanol or acetone as solvent， the extraction time of 15 min， the microwave power at 800-900 W and the size of P. cuspidatum particles of 0.30-0.45 mm， and the water content in the raw material was 20%.
　　Supercritical CO2 extraction
　　Cao[33]reported the supercritical CO2 extraction of resveratrol from P. cuspidatum. The extraction was carried out under the following conditions： the extractor pressure of 5.7 MPa and temperature at 46 ℃； the separator pressure of 25 MPa and temperature at 50 ℃； and the mixed solvent of 2propanol and anhydrous ethanol as modifier. The resveratrol content in extract was 18%， and the extraction yield was 75%.
　　Ultrasonic extraction
　　The action mechanism of ultrasonic extraction： Ultrasonic wave could be converted to heat energy rapidly and improve the internal temperature in materials rapidly within ultrashort time， thereby accelerating the dissolution of effective components. When ultrasonic wave acts on extraction liquid， the liquid would be tore to many small cavities which would be closed instantly， resulting in an instant high pressure， which accelerates the effective components in the material entering the extraction liquid， and improves the extraction rate of effective components[34]. Jiang et al.[35]compared the reflux extraction， ultrasonic extraction and supercritical CO2 extraction method for extracting resveratrol from traditional Chinese herb P. cuspidatum， and the results showed that the ultrasonic extraction method had the best effect. Zhou[36]studied the effects of different solvents on the extraction of resveratrol from P. cuspidatum by ultrasonic method， and found that the yield of resveratrol reached 0.373%-1.83%. Li et al.[37]extracted resveratrol from grade residues ultrasonically， and the yield of resveratrol reached 0.35%.
　　Agricultural Biotechnology2018
　　Antibacterial Effect of Resveratrol
　　Resveratrol has the activity of resisting methicillinresistant Staphylococcus aureus， its MIC90 is 0.512 mg/ml， and the inhibitory effect of resveratrol to methicillinresistant S. aureus is the same that to methicillinsensitive S. aureus. The MIC90 of resveratrol to coagulase negative Staphylococcus was determined to be 0.256 mg/ml， but it has no remarkable effect on Gramnegative E. coli and Pseudomonas aeruginosa. Resveratrol has a destructive effect on Staphylococcus[38]. The growth of S. aureus standard strain was inhibited after the action of resveratrol. At the resveratrol concentrations of 1/4MIC and 1/2MIC， the normal growth state of S. aureus standard strain was delayed or destroyed， and at the resveratrol concentrations of 1MIC and 2MIC， the growth of S. aureus standard strain was completely inhibited， showing a gentle growth curve， indicating that resveratrol has a remarkable inhibiting effect on S. aureus standard strain[39]. The transresveratrol had an inhibiting rate of 80% on S. aureus at the mass concentration of 0.125 mg/ml[40]. Su et al. [41]demonstrated that resveratrol has an inhibiting effect on both Enterococcus faecalis and E. faecium. Docherty et al.[42]carried out an invitro antibacterial experiment of resveratrol on the clinical isolates of Neisseria gonorrhoeae， during which the MIC50 and MIC100 to N. gonorrhoeae were determined， and the results showed that resveratrol has certain inhibiting effect on it. Ferreira et al.[43]reported that resveratrol had an antiListeria monocytogenes effect， and its MIC value to L. monocytogenes was 200 μg/ml. Meanwhile， resveratrol also showed very strong antibiofilm activity. Wang et al.[44]found that resveratrol could inhibit the migration and growth of Proteusmirabilis and the expression of pathogenic factor. Mahady et al.[45]determined the invitro MICs of 15 H. pylori clinical isolates and one standard strain according to CLSI standards， and the results showed that resveratrol has stronger inhibiting effect on them. Erin et al. [46]concluded through the detection of minimum bactericidal concentration that resveratrol not only has antibacterial effect， but also could kill Hemop hilus ducreyi. Resveratrol has hardly any killing effect on Lactobacillus inherent in sexual organs. In addition， the MIC of resveratrol to Bacillus megaterium is 0.250 g/L[47]. Resveratrol also has good inhibiting effect on Fs dysentery bacillus and Providencia rettgeri[48]. Hwang et al.[49]showed the MIC of resveratrol to E. coli is 456 μg/ml. The antibacterial rate of transresveratrol with the mass concentration of 1.00 mg/ml to E. coli is 40%[50]. Kim[51]illustrated that resveratrol could protect host cells from the poisoning and infection caused by Vibrio vulnificus. Nimmy et al.[52-53]also found that resveratrol inhibits Vibro cholera intestinal infectious diseases by hindering the formation of biofilm. 　　Inhibitory Activity of Resveratrol against Pathogenic Fungi
　　Inhibiting effect of resveratrol on growth of Capnodium salicinum Mont.[50]
　　Resveratrol remarkably inhibits the growth of C. salicinum. The inhibiting rate reached 31.86% at the concentration only of 5 mg/ml， and was the highest of 50.10% at the concentration of 160 mg/ml. The inhibiting rate increased with the concentration increasing. Variance analysis showed that there were no significance differences between different concentrations of resveratrol.
　　Inhibiting effect of resveratrol on growth of Alternaria tenuis Nees[50]
　　Under the concentration lower than 20 mg/ml， the inhibiting rate of resveratrol increased with the increase of concentration， but over the concentration， the inhibiting rate had no remarkable change with the concentration increasing. At the concentration of 160 mg/ml， the inhibiting rate was the highest of 33.27%. It could be seen from this that resveratrol has a better inhibiting effect on ulcer bacteria. Variance analysis showed that there were very significant differences between different concentrations of resveratrol.
　　Inhibiting effect of resveratrol on growth of Coniothyrium populicola Miura oo.[50]
　　The highest inhibiting rate of resveratrol to C. populicola was 23.17% when the concentration of resveratrol was 160 mg/ml. However， at the concentration of 5 mg/ml， the antifungal rate was only of 6.79%. It indicates that resveratrol only has certain inhibiting effect on C. populicola at higher concentration. The variance analysis showed that there were significant differences in the antifungal rate between different concentrations of resveratrol.
　　Inhibiting effect of resveratrol on growth of Glomerella cingnlata（Stonem.）Spauld.et Schrenk[50]
　　The highest inhibiting rate of resveratrol on G. cingnlata was only of 17.17% when the concentration of resveratrol was 80 mg/ml. However， at the concentration of 5 mg/ml， the antibacterial rate was only of 4.99%. It could be seen that the inhibiting effect of resveratrol on G. cingnlata was not remarkable. Variance analysis showed that there were significant differences in the antifungal rate between different concentrations of resveratrol.
　　Inhibiting effect of resveratrol on other fungi[54]
　　In order to make clear the prospect of plantsourced active components stilbenes in the control of pests and diseases in agriculture， Wu et al.[54]determined the inhibiting effect of resveratrol on the mycelial growth of plant pathogens and the their effects on mycelial morphology and spore germination， and studied the control effect of resveratrol by leaf method and greenhouse pot culture method. The results showed that the five tested compounds， resveratrol， oxyresveratrol， 3，5dihydroxy4′methoxy stilbene， 3hydroxy4′5dimethoxy stilbene and 3，4′，5trimethoxy stilbene， had different degrees of inhibiting effect on mycelial growth of Alternaria solani， Botryosphaeria dothidea， Botrytis cinerea， Colletotrichum gloeosporioides， Rhizotonia cerealis， and Trichothcium rosum on nine plants. The inhibiting activity was the highest on A. solani， and among the five tested compounds， 3，5dihydroxy4′methoxy stilbene had the highest inhibiting effect. Resveratrol caused mycelial malformation and inhibited the spore germination with no changes in the morphology of A. solani. The leaf method and greenhouse pot culture method gave the same conclusion， i.e.， resveratrol could inhibit the infection of pathogens and protects plants to a certain degree. 　　In view of the antifungal effect， the effects of resveratrol on solution protein and DNA content in mycelia on membrane permeability were determined. The results of the experiment showed that resveratrol destroyed cytomembrane structure， changed its permeability and resulted in leakage of electrolyte. Meanwhile， resveratrol inhibited the production of soluble protein， resulting in decrease of its content. However， no effect of resveratrol on production of DNA was determined.
　　It was also reported that ultraviolet light could reduce the titer of resveratrol， but it has good heat stability within 4-35 ℃ in dark place. The pH value could affect the inhibiting activity of resveratrol on A. solani， and a weakly acidic environment （pH=6） is beneficial to its antifungal activity.
　　The determination of the combination toxicity of resveratrol and four agents including mancozeb showed that resveratrol had a synergistic effect with mancozeb and iprodione， and the mixed application with mancozeb had the most significant synergistic effect when mixed with chlorothalonil， while the mixed application with carbendazim at set ratio exhibited an antagonistic effect.
　　Research Situation and Development Tendency of Resveratrol at Home and Abroad
　　In recent years， many scholars at home and abroad have conducted indepth studies on the biological activity of resveratrol， and it was found that resveratrol as an important phytoalexins， could prevent the oxidation of lowdensity lipoprotein and has the effects of preventing cardiovascular disease and cancer， resisting virus and aging and regulating immunity. Furthermore， America Institute of Natural Medicine （CNN） also found that resveratrol has antiAIDS effect. Currently， the global demand for resveratrol is about 100 t per year， while the production capacity of resveratrol is 50-60 t， so there is a large demand gap. Consequently， resveratrol is a natural substance with very high development value， and is known as a kind of new green anticancer substances following taxol. However， when exploring plant species rich in resveratrol or obtaining new varieties with high resveratrol content through artificial cultivation and breeding methods， in order to enhance the comprehensive development of resveratrol， the extraction rate of resveratrol should be improved by transforming stilbene synthase gene into plant cells or important food crops and economic crops such as rice， peanut， wheat， soybean and potato through genetic engineering technology and plant cell clone technology， combining isolation and purification method. 　　References
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　　Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU