Rheum rhaponticum and Rheum rhabarbarum: a review of phytochemistry, biological activities and therapeutic potential

The Rheum genus (Polygonaceae) covers about 60 species of rhubarbs, including specimens with a long ethnomedicinal history in Asia, Europe and other regions of the world. The work reviews available literature (until March, 2020) on phytochemical profile, ethnomedicinal recommendations, biological activities, pharmacological uses and future prospects for therapeutic applications of Rheum rhabarbarum L. (garden rhubarb) and Rheum rhaponticum L. (rhapontic rhubarb). Although the above species are well-known vegetables, scientific interest in these plants is a relatively new issue; most of evidence of their biological activities and therapeutic potential derives from the last 15 years. Rhubarbs contain numerous bioactive substances, belonging to diverse groups of phytochemicals, e.g. stilbenes, anthraquinones and flavonoids. The registered special extract of R. rhaponticum (ERr731®) is administered to alleviate the menopause-related complaints. Furthermore, both ethnomedicinal surveys and recent studies on bioactive substances from rhubarbs indicate that these plants may have significantly broader range of beneficial effects such as antioxidant, anti-inflammatory, antimicrobial and cardioprotective activities.


Introduction
The genus Rheum L. (rhubarb) covers about 60 species of perennial herbs, belonging to the Polygonaceae family. Rhubarbs have been known worldwide (in Asia, in particular) as medicinal and/or edible plants (Agarwal et al. 2001;Cao et al. 2017;He et al. 1992;Pourjabali et al. 2017;Rehman et al. 2014). Scientific interest in chemical composition and biological properties of two of the most popular rhubarb species, i.e. garden rhubarb (Rheum rhabarbarum L./syn. R. undulatum L.-according to www.theplantlist.org and http://www.worldfloraonline.org) and rhapontic rhubarb (Rheum rhaponticum L.; also called Siberian rhubarb) has significantly increased for the last 15 years. Despite the popularity of these species as foods, their phytochemical profiles, bioactive components and pharmacological relevance still remain partly elucidated. Their petioles (stalks) are commonly used for culinary purposes, and the leaves are toxic (Slaughter et al. 2012) due to a high content of oxalic acid. However, contrary to the roots of Rheum officinale Baill., which are predominantly used as a laxative, underground parts of both the garden and rhapontic rhubarb have a broader range of applicability in traditional medicine and contemporary herbal therapies.
While the chemical profile and biological properties of other rhubarb species such as R. australe D. Don, R. palmatum L., R. ribes L., R. officinale Baill., R. tanguticum Maxim. ex Balf. or R. emodi L. have been extensively described and/or reviewed in the literature (Komatsu et al. 2006;Rashid et al. 2014;Rokaya et al. 2012;Wang et al. 2013;Zhang et al. 2015a), there is a lack of a comprehensive look at R. rhabarbarum and R. rhaponticum. The present work is based on a review of available literature dealing with ethnomedicinal data, phytochemical composition, biological activities and prospects for future pharmacological use of these rhubarb species. Since the evaluation of some of biological activities or pharmacological significance of rhubarb-derived compounds and extracts partly remain at the preliminary stage or available data are inconsistent, disputable issues have been critically presented in this work. Our review covers data (until March, 2020) originating from journals indexed in international databases such as Medline/Pubmed, Scopus, Science Direct/Elsevier and Springer Link/ICM. Additionally, local scientific publications, not indexed in the mentioned databases were also taken into consideration.

Rheum rhaponticum and Rheum rhabarbarum in traditional medicine
Rhubarb is known in traditional medicine of Asian, European and other cultures (Agarwal et al. 2001;Pourjabali et al. 2017;Rehman et al. 2014), but a detailed appraisal of ethnomedicinal relevance of individual species or their biological actions is limited by the fact that in some cases, no precise botanical identification (including Latin names) of the used/ described species is given. Even in recent literature, rhubarbs are frequently described generally, as ''Rheum sp.'' or just ''rhubarb(s)''. Nevertheless, available data from ethnomedicinal and ethnobotanical surveys confirm the presence of R. rhaponticum and R. rhabarbarum in medicine from the Middle Ages (Nedelcheva 2009). According to data from one of the oldest Polish books devoted to medicinal plants, i.e. Herbarz Polski'' (Marcin z Urzędowa/Marcin of Urzędów 1595) roots of both these species were used in medicine of that time to cure a wide range of disorders. The author suggested using 1-2 or more ''dragma'' units (1 dragma corresponded to 1/8 of one ounce). Internally, R. rhaponticum (in wine, beer or mead) was used in therapy of gastrointestinal pain, gastritis, liver and spleen disorders, heartache and pain in pericardium, pulmonary system dysfunctions as well as disorders related to the reproductive system, including uterine and breast pains. Drinking of the R. rhaponticum-based mixtures or chewing its root was believed to alleviate indigestion. Externally, a vinegar macerate from this plant was applied to cure skin disorders such as itching and scratches; a water macerate was recommended in the therapy of ulcers. R. rhabarbarum was mainly administered as a purgative agent. Other recommendations for its use included liver, spleen and stomach dysfunctions as well as blood purification. Some fragments of this historical source indicated that this plant was also used to stop bleeding and to alleviate fever as well as to cure injuries, trauma after falls from a height and vein disorders.
Current literature devoted to ethnomedicinal issues also confirms that the roots of R. rhaponticum were an ingredient of different mixtures, administered to cure fever (a sugar syrup), to improve voice (roots boiled in red wine and sweetened with honey), to alleviate heart problems or stomachache (a brandy-based balsam), and a honey-based herbal paste was used in jaundice and distress (Nedelcheva 2012). Furthermore, underground parts of this rhubarb are an ingredient of Turkish folk food and the remedy, i.e. Mesir paste, having over 500 years history of use (Oskay et al. 2010). In Brazilian folk medicine, R. rhaponticum is used to treat gastrointestinal disorders (Cogo et al. 2010), while in Iran, it is one of the herbs traditionally used to treat hyperpigmentation (Ghafari et al. 2017). This rhubarb species has been also mentioned in different types of studies as one of medicinal plants in European and Asian countries (Debnath et al. 2006;Lachumy et al. 2013).
Similarly to R. rhaponticum, also the position of R. rhabarbarum in ethnomedicine of different cultures has been built for centuries. Both species are known in Central Asia and Russia as a natural demulcent, helpful in wound healing and relieving skin problems (e.g. irritations, allergy and dermatitis) (Mamedov et al. 2008). R. rhabarbarum is known in Ayurvedic medicine (Dandekar and Deshpande 2016). In Chinese medicine, this plant has been administered to heal ulcers as well as to treat constipation, jaundice and gastrointestinal hemorrhage (State Pharmacopoeia Committee 2010), or as a natural anti-inflammatory ingredient of different formulations, useful in the therapy of appendicitis, cholecystitis and rheumatoid arthritis (Ma et al. 2009). Furthermore, R. rhabarbarum is a component of Yīn-Chén-Hāo decoction, a traditional Chinese herbal medicine used to treat damp-heat jaundice and hepatic disorders (Li et al. 2017). This rhubarb species was also listed in the Chilandar Medical Codex, a medieval Serbian manuscript, covering documents on European medical knowledge from the 12 th to 15 th century. Recommendations for the use of R. rhabarbarum included blood detoxification, gastrointestinal tract disorders, hydrops, melancholy as well as its administration as a mucolytic, purgative, sedative and antispasmodic preparation (Jarić et al. 2014). It is described as one of ethnomedicinal plants in Hungary (Babulka 1993), depurative herbal medicine in South-Western Germany (Pieroni and Gray 2008) and traditional Korean remedy for gastrointestinal disorders .
Biological activities and pharmacological actions of R. rhapoticum and R. rhabarbarum

Menopausal complaints
To date, the terms ''phytoestrogens'' and ''phytoestrogenic activity'' have been mostly associated with isoflavones from the soya bean (Glycine max L.). On the other hand, the last two decades have brought better understanding of the hormone-like effects of different classes of phytochemicals and identification of various sources of phytoestrogens. Although the soya bean is still considered the main source of phytoestrogens, it has been partly replaced by red clover (Trifolium pratense L.). Furthermore, following the discovery that not only isoflavones display estrogenic properties, the area of studies on hormonal effects of plant metabolites has been significantly extended (Dietz et al. 2016;Franco et al. 2016;Sirotkin and Harrath 2014;Schloss and Steel 2016). Scientific and medical interest in herbal preparations with estrogenic effects are additionally enhanced by data suggesting a beneficial role of phytoestrogens in prophylaxis of different women diseases, including osteoporosis and breast cancer (Obi et al. 2009).

Antioxidant properties
Due to a significant contribution of oxidative stress to the pathogenesis of a variety of disorders, antioxidant activity is one of the most investigated properties of natural, plant-derived extracts or individual substances. However, in the case of R. rhaponticum and R. rhabarbarum, this type of biological activity remains only partly described and evaluated. So far, antioxidant effects of R. rhaponticum-derived extracts have been examined in preliminary radical-scavenging assays (based on the DPPH • and ABTS •? radicals), mostly at concentrations that are significantly higher than physiologically achievable levels of natural substances and their metabolites (Joo et al. 2014;Kalisz et al. 2020;Park et al. 2008;Raudsepp et al. 2008Raudsepp et al. , 2013Won Jang et al. 2018;Wu et al. 2017;Zhang et al. 2007) (for details see the Table 5 in supplementary material). In addition to a limited number of studies, different laboratory protocols, diverse ways of data presentation (including a lack of EC 50 or IC 50 values) and modifications within the methodology significantly hinder unequivocal interpretation of the existing evidence.
What is the current knowledge of antioxidant effects of R. rhabarbarum? Analogously to R. rhaponticum, antioxidant assays of the garden rhubarb were preliminary assessed with the use of non-biological tests, but in this case, the literature offers more information derived from different bioassays ( Table 5 in supplementary material). R. rhabarbarum contains natural antioxidants, capable of combating oxidative stress at different levels of the cellular antioxidant defense. Its bioactive constituents may act both as scavengers of reactive oxygen species (ROS, e.g. nitric oxide, superoxide anion and hydroxyl radical (Kalpana et al. 2012) and as modulators of cell signalling pathways and gene expression. Molecular mechanisms of these effects include the ability of stilbenes to up-regulate functions of the transcription factor Nrf2 (the nuclear factor (erythroid-derived 2)like 2) and the Nrf2-mediated pathways. Six stilbenes (i.e. rhaponticin, rhapontigenin, isorhaponticin, desoxyrhaponticin, desoxyrhapontigenin and resveratrol), isolated from the rhizome of R. rhabarbarum, significantly reduced the intracellular generation of ROS in RAW 264.7 macrophages (Joo Choi et al. 2014). Furthermore, seven compounds from this plant (i.e. piceatannol, resveratrol, rhapontigenin, desoxyrhapontigenin, pterostilbene, (E)-3,5,4 0trimethoxystilbene and trans-stilbene) protected hepatocytes against the arachidonic acid-and iron ionsinduced oxidative stress in vitro. Molecular mechanisms of this beneficial action involved the AMPactivated protein kinase (AMPK) pathway, an important regulator of cellular metabolism and energy homeostasis (Dong et al. 2015).

Anti-inflammatory properties
Molecular mechanisms of the anti-inflammatory action of rhubarb-derived substances involve the inhibition of the nuclear factor kappa B (NFjB)dependent pro-inflammatory pathways. In human umbilical vein endothelial cells (HUVECs), an aqueous extract from R. rhabarbarum suppressed the tumor necrosis factor a-induced activation of NF-jB-p65 as well as the expression of adhesion molecules (ICAM-1 and VCAM-1) and the monocyte chemoattractant protein-1 (MCP-1) (Moon et al. 2006). The rhizomederived stilbenes (rhapontigenin, piceatannol and resveratrol), their derivatives (rhaponticin 2 00 -O-gallate, rhaponticin 6 00 -O-gallate) and a naphthalene glucoside (torachrysone 8-O-b-D-glucopyranoside) were found to reduce NO production in macrophages (Matsuda et al. 2000). Other studies on macrophages demonstrated that also aloe-emodin was able to suppress the pro-inflammatory response (Hu et al. 2014). According to available reports, stilbenes isolated from rhizomes of this plant may impair an allergy-induced inflammatory response (Matsuda  (Kim et al. 2000). Desoxyrhapontigenin displayed anti-inflammatory properties in vitro and in vivo. It may be also a natural regulator/ stimulator of activity of the hemeoxygenase-1 enzyme (HO-1), an important anti-inflammatory, antioxidant and cytoprotective enzyme (Joo Choi et al. 2014). It is believed that the anti-inflammatory and anti-osteoporosis effects of desoxyrhapontigenin are through its inhibitory action on the receptor activator of NF-jB ligand (RANKL) (Tran et al. 2018).
Outcomes of in vitro examinations of anti-inflammatory properties of the isolated compounds (mainly stilbenes and their derivatives) and extracts from the R. rhabarbarum plant have been summarized in the Table 4. Besides aforementioned works, anti-inflammatory properties of extracts derived from R. rhabarbarum have been also found in animal and clinical studies. The suppression of toll-like receptor 4 pathway by rhein (100 mg/kg b.w.) was demonstrated in an animal model of the lipopolysaccharide (LPS)induced intestinal injury during sepsis (Zhang et al. 2015b). In a randomized clinical trial involving 120 patients with appendectomy, the therapy (twice a day) with an ointment containing 1 mg/g of R. rhabarbarum rhizome extract (methanol/water; 1:1, v/v) significantly reduced inflammation and improved the sutures healing process. Complementary experiments in this study indicated on HUVECs indicated on proangiogenic activity of the examined rhubarb extract (at concentrations of 5 and 10 mg/ml) (Li et al. 2016).
Aloe-emodin RAW 264.7 macrophages Reduction of NO generation as well as interleukin-6 and -1b (IL-6 and IL-1b) synthesis and secretion Hu et al. (2014) Cardioprotective properties In most cases, cardioprotective effects of plantderived substances are primarily attributed to their antioxidant and/or anti-inflammatory actions, even though, the biological activity of many natural substances evidently goes beyond these mechanisms. Due to a complex and multifactorial character of etiology and pathophysiology of cardiovascular disorders, research on modern prophylactic and therapeutic strategies encompasses a wide range of aspects related to these diseases. In addition to antioxidant and anti-inflammatory effects, the cardioprotective properties of natural substances include antiplatelet and anticoagulant effects, vasorelaxation and improvement of the blood lipid profile. A hypolipidemic activity of R. rhaponticum fibre was evidenced in animals maintained on the high-cholesterol diet (Basu et al. 1993) and in hypercholesterolemic human subjects in studies involving 10 hypercholesterolemic men, consuming a daily dose of 27 g of rhubarb stalk fibre, for 4 weeks. The fibre had also an ability to bind bile acids (Goel et al. 1997) and displayed stimulatory effects on the expression of the cholesterol 7ahydroxylase gene and excretion of bile acids in mice (Goel et al. 1999). Although the hypolipidemic action of rhubarb was originally attributed to the presence of pectins based on their well-known cholesterol-lowering properties (Fernandez 1995;Fernandez et al. 1992), later works demonstrated that also stilbene compounds, i.e. rhapontin and rhapontigenin might improve the lipid profile. In rats fed a high-cholesterol diet, an intake of these compounds (1-5 mg/kg/day) resulted in a dose-dependent decline of the serum lipid level and a considerable increase of the high-density lipoprotein (HDL) cholesterol level. Additionally, pathological changes in the degenerating fatty liver were significantly reduced by a treatment with either rhaponticin or rhapontigenin (Jo et al. 2014). However, no such promising effects of the rhubarb-fibre diet were found in an experimental model of diabetic rats (treated with 50 g of the rhapontic rhubarb stalk fibre/kg b.w.) (Cheema et al. 2003).
Similarly to lipid-lowering properties of rhapontic rhubarb, also R. rhabarbarum showed a cardioprotective potential. The current state of art in this field includes results from in vitro and animal studies, and most of the available data complement each other. It has been found that this plant contains natural inhibitors of the soluble epoxide hydrolase (sEH) enzyme, which is one of the important molecular targets for the therapy of cardiovascular diseases. Preliminary analyses employing a recombinant human sEH demonstrated the following degrees of its inhibition: 49.8%, 107%, 30% and 39%, by the methanol extract, n-hexane, chloroform, and butanol fractions from R. rhabarbarum (at 25 lg/ml), respectively. Further experiments revealed that the individual rhubarb-derived compounds modulated the sEH activity by diverse mechanisms. While piceatannol 3 0 -O-b-D-glucopyranoside was a competitive inhibitor of the enzyme, resveratrol, desoxyrhapontigenin, desoxyrhaponticin, rhaponticin, isorhapontin, astringin, chrysophanol-8-O-b-D-glucopyranoside and aloeemodin acted as mixed-type inhibitors. Rhapontigenin and emodin were noncompetitive inhibitors of sEH. The strongest inhibitory effect was found for astringin (IC 50 = 2.5 ± 0.5 lM), and the weakest one of the inhibitors was desoxyrhaponticin (IC 50-= 53.2 ± 4.4 lM) (Jo et al. 2016). The anti-obesity and hypolipidemic effects of R. rhabarbarum-derived substances were found in vitro and in vivo. A hot water extract from this plant was found to inhibit protein tyrosine phosphatase 1B (PTP1B), an important regulator of the insulin signalling . Studies in high-fat diet-fed mice revealed the metabolism-regulatory activity of rhubarb extracts and its anthraquinone components, i.e. chrysophanol and physcion. The extract, at one daily dose of 100 mg/ kg of b.w., administered for 8 weeks modulated the lipid metabolism in animals (including a stimulation of adiponectin synthesis) and significantly reduced the increase of body weight. In additional experiments, the isolated anthraquinones (30 lM) inhibited PTP1B activity and enhanced the insulin sensitivity in serumstarved 3T3L1 cells (Lee et al. 2012b).
Some basic information on anti-thrombotic activity of R. rhabarbarum stilbenes is available as well. Desoxyrhapontigenin and rhapontigenin (100-200 lM) inhibited the arachidonic acid-or collagen-induced aggregation of blood platelets in vitro, but no antiplatelet action was recorded for piceatannol (Ko et al. 1999). Besides the aforementioned effects, anti-diabetic and vasorelaxant activities of R. rhabarbarum are postulated. An in vitro screening of the a-glucosidase inhibitory activity of extracts from twenty four plants, suggests that aerial parts of R. rhabarbarum contain moderate inhibitors of this enzyme. The methanol extracts from peel and the peeled stalks inhibited the enzyme with IC 50 values of 0.013 and 4.94 mg/ml, whereas for the ethyl acetate extracts, the IC 50 amounted 0.24 and 0.21 mg/ml, respectively. For a comparison, the strongest inhibitor in this study (i.e. the methanol extract from Cinnamomum zeylanicum) was characterized by the IC 50-= 0.009 mg/ml (Kongstad et al. 2015). However, the latest study on a-glucosidase inhibitory activity of phytochemicals isolated from R. rhabarbarum has provided diverse results. The IC 50 values for the most of the isolates exceeded 100 lM; however, the inhibitory actions of d-viniferin (IC 50 = 0.5 lM, i.e. 0.23 lg/ml) and rhapontigenin (IC 50 = 15.4 lM, i.e. 3.97 lg/ml) exceeded the efficiency of acarbose (IC 50 = 126.8 lM, i.e. 81.8 lg/ml), a well-known inhibitor of this enzyme (Ha et al. 2020).
Desoxyrhapontigenin, emodin and chrysophanol isolated from rhizomes of R. rhabarbarum influenced the glucose metabolism in vivo. At oral doses of 0.21 (desoxyrhapontigenin), 0.45 (emodin) and 0.18 (chrysophanol) mg/kg b.w., these compounds reduced the postprandial hyperglycemia in animals by 35.8, 29.5, and 42.3%, respectively (Choi et al. 2005). Among seven stilbene-type compounds from the rhubarb rhizome (i.e. desoxyrhapontigenin, piceid, piceatannol, resveratrol, rhapontigenin, rhaponticin and e-viniferin), the strongest vasorelaxation of the isolated rat aorta was observed after the treatment with piceatannol (EC 50 = 2.4 lM) (Yoo et al. 2007). It is suggested that molecular mechanisms of the piceatannol-induced vascular relaxation involve the endothelium-dependent nitric oxide signalling pathway, including the activation of the large conductance, Ca 2? -activated K ? channels (BKCa) ). In vitro vasodilatory properties (also mediated by the endothelium-dependent NO/cGMP pathway) in rat aorta preparations were observed for an aqueous extract of R. rhabarbarum rhizomes (Moon et al. 2006). Moreover, both the vasorelaxant and antiinflammatory effects of this extract were confirmed in vivo, in atherogenic diet-fed rats. The treatment with rhubarb extracts resulted in significant antiatherogenic effects, including a reduction of lowdensity lipoprotein-cholesterol and an increase of the HDL-cholesterol in blood plasma. The extracts suppressed the atherogenic diet-induced expression of the vascular NF-jB-p65, adhesion molecules (ICAM-1 and VCAM-1) and E-selectin in the examined animals (Moon et al. 2008). Inhibitory effects of R. rhabarbarum stilbenes and their derivatives on functions of the cell adhesion molecules (CAMs) were also described in other papers, which hypothesized that the biological activity of rhubarb-derived substances may be a base for development for new therapeutic strategies (Lee et al. 2012a;Spelman et al. 2011).

Anticancer activity
Both the stilbene (Sirerol et al. 2016) and anthraquinone (Ö zenver et al. 2018) components of plant extracts display chemopreventive and antitumor properties; however, the anticancer action of the rhubarbderived extracts has been evaluated hitherto only in vitro. A methanol extract from R. rhabarbarum (at concentrations of 70-350 lg/ml) mediated cancer cell death through the activation of the intrinsic (mitochondria-dependent) apoptotic pathway in human adenocarcinoma gastric (AGS) cells (Hong et al. 2015). The exposure of these cells to aloe-emodin or chrysophanol 1-O-b-D-glucopyranoside increased the poly(ADP-ribose)polymerase (PARP) cleavage and induced downregulation of the anti-apoptotic protein, Bcl-2. Additionally, chrysophanol 1-O-b-D-glucopyranoside slightly stimulated the expression of proapoptotic proteins such as Bid and Bax (Trinh et al. 2019). In the MCF-7 human breast cell line, aloeemodin and rhapontigenin induced the mitochondriaindependent apoptosis (mediated by the caspase-8 pathway), while chrysophanol 1-O-b-D-glucopyranoside, acted by the mitochondria-dependent apoptotic pathway ). In the same experimental model, the induction of cancer cell apoptosis was also found for desoxyrhapontigenin. Its anticancer effect was through a dilation of endoplasmic reticulum (ER) and up-regulation of the expression of ER stress markers such as a chaperone protein GRP78, inositol-requiring kinase 1a (IRE1a), eukaryotic translation initiation factor 2 (eIF2a), C/EBP homologous protein (CHOP) as well as JNK and p38 kinases (Venkatesan et al. 2016). Furthermore, a hexane extract from R. rhabarbarum (20-60 lg/ml) significantly decreased the growth and viability of cancer cells and induced apoptosis in the HN22 and SCC15 oral cancer cell lines (Choi et al. 2011).
In the case of R. rhaponticum, the available literature comprises a small number of reports strictly related to anticancer action of the extracts, including a few mechanistic studies on this issue. The vast majority of data derives from studies on estrogenic properties of this plant; the hormone-like action is believed to be a molecular basis of cancer-preventive or anticancer effects of R. rhaponticum. A R. rhaponticum extract and its components were found to bind to ERb in various highly specific cancer cells such as human bone osteosarcoma (U2OS) and human endometrial adenocarcinoma (HEC-1B) cell lines (Kaszkin-Bettag et al. 2008a, b;Möller et al. 2007). In contrast, neither the extract nor its compounds and their metabolites (resveratrol and piceatannol) were able to activate the ERa receptor (Kaszkin-Bettag et al. 2008a, b;Möller et al. 2007). Additional source of data may be works on anticancer action of individual compounds (not isolated from R. rhabarbarum), but occurring also in these plants. Among others, such studies were described for piceatannol (Banik et al. 2020).

Antimicrobial and antiviral activities
It is assumed that R. rhaponticum and R. rhabarbarum contain antimicrobial substances that are active, at least, towards some pathogens (for details, see the Table 6 in supplementary material). The antiviral properties of rhubarbs have been described to a much lesser extent (Nurbaulina et al. 2009). However, despite some encouraging results, the pharmacological significance of antimicrobial action of rhubarbs is difficult to estimate, and main concerns are related to the lack of in vivo examinations. Furthermore, in many cases, beneficial effects were observed only at high, physiologically unachievable concentrations. On the other hand, it is known that various plant preparations are used externally, e.g. onto wounds, burns or ulcers. In these cases, also higher doses of bioactive substances may be clinically useful, if only, the necessary in vivo tests are done.
A comparative assessment of antibacterial actions of the rhizome and roots extracts, prepared from R. rhaponticum, R. palmatum and R. rhabarbarum demonstrated that all of the examined crude extracts (96% ethanol; v/v), were more efficient against the Gram-positive bacteria than those Gram-negative. The most effective one was the preparation from R. rhabarbarum (the minimum inhibitory concentration (MIC) = 125-250 lg/ml), but its activity was significantly weaker, compared to gentamicin and cefuroxime. In the case of these antibiotics, the MIC values for reference strains of Staphylococcus spp. ranged from 0.12 to 0.49 lg/ml and from 0.49 to 0.98 lg/ml, for gentamicin and cefuroxime, respectively (Kosikowska et al. 2010). Even an infusion from R. rhaponticum was found to display antimicrobial effects, though, its activity was weaker than chloramphenicol, a reference bacteriostatic agent (Raudsepp et al. 2013). A screening of antimycobacterial activities of R. rhaponticum root extract and its bioactive components such as rhaponticin, desoxyrhaponticin, resveratrol, barbaloin, aloe-emodin and chrysophanol revealed that the anthracene compounds were the most efficient (the MICs ranged between 32 and 64 lg/ml). However, no antimicrobial effects were found for the petiole extract (Smolarz et al. 2013b). Another study evidenced antibacterial properties of the crude, ethyl acetate and aqueous extracts originating from rhapontic rhubarb (Ziad et al. 2011).
Tests employing seventeen bacterial and one fungal strain ( Table 6 in supplementary material) revealed antimicrobial activity of the ethanolic extract from the underground organs of R. rhabarbarum (Canli et al. 2016). An extract from the petioles was found to possess bacteriostatic, but not bactericidal properties against ATCC strains of S. aureus, S. epidermidis, K. pneumoniae, P. aeruginosa and E. coli (the MICs ranged from 700 to 900 lg/ml) (Pájaro et al. 2018). Surprisingly, in other comparative studies, based on antimicrobial actions of various extracts and freshly pressed plant juice from different species, the R. rhabarbarum preparations had moderate or weak antibacterial properties, except the water extract, which was an efficient inhibitor of growth of two Gram-positive bacteria, i.e. Bacillus subtilis and Bacillus cereus (Krisch et al. 2008).
Since the therapy of gastrointestinal disorders is one of the ethnomedicinal recommendations for use R. rhaponticum, extracts from this plant were tested in terms of anti-Helicobacter pylori activity. Nevertheless, literature dealing with antimicrobial properties of this plant has not confirmed its pharmacological efficiency in combating H. pylori (Cogo et al. 2010). It seems to that the use of rhubarb-derived substances for disease-preventive purposes, e.g. for maintaining oral hygiene and dental health, can be more promising. Traditionally, the R. rhabarbarum root-based preparations have been used for the treatment of dental diseases, and current studies on its inhibitory action on the dental plaque formation or glycolytic acids production by Streptococcus mutans and Streptococcus sobrinus confirmed a protective effect of this plant in dental hygiene (Kim et al. 2011). Rhein, one of the anthraquinone components of R. rhabarbarum and R. rhaponticum roots, displays an ability to combat Porphyromonas gingivali, a Gram-negative periodontopathogen, and thus, it is considered potentially useful in maintaining of oral health (Chinsembu 2016). Moreover, it has been demonstrated that rhein may act synergistically with a nitroimidazole antibiotic, metronidazole (Azelmat et al. 2015).

Other biological activities
Hydroxystilbenes isolated from the roots of R. rhaponticum displayed hepatoprotective effects in mice exposed to ethanol inhalation. Animals were treated with trans-resveratrol (20 mg/kg b.w.) or received a hydroxystilbene-containing R. rhaponticum extract (air-dried root in ethanol, 1:10 w/v), administered to an equivalent of the trans-resveratrol dose of 20 mg/kg b.w. .
A methanolic extract from R. rhaponticum rhizome inhibited the tyrosinase activity in UV radiationstimulated human epidermal melanocytes at very low concentrations. According to the authors, inhibitory effects of the examined rhubarb were comparable to the efficiency of kojic acid (IC 50 = 0.06 lg/ml and 0.02 lg/ml, for the extract and kojic acid, respectively). Additionally, a significant reduction of both the UV-induced secretion of cytokines (IL-1a and TNF-a) and the alpha-melanocyte stimulating hormone (a-MSH) synthesis was found (Silveira et al. 2013).
Advances in studies on R. rhaponticum and R. rhabarbarum: current state of art The described rhubarb species contain bioactive phytochemicals with some health-promoting or therapeutic potential. However, only the estrogenic activity of R. rhaponticum has been well established in preclinical studies and clinical trials (data summarized in the Table 3), while other activities of this plant have been mainly evidenced by basic studies. Thus, most of the other curative uses of rhapontic rhubarb is still based on ethnomedicinal recommendations. Similar concerns are related to R. rhabarbarum-also in this case, results from in vitro works prevail and are supported only by few animal studies. A cross-search in scientific sources and medical databases such as NIH U.S. National Library of Medicine (i.e. Clini-calTrials.gov and PubMed) and SpringerLink indicated that at least several clinical trials including a word ''rhubarb'' have been registered, but no records containing ''rhababarum'' or ''undulatum'' were found. In other scientific resources, only one clinical study on wound-healing and anti-inflammatory properties of R. rhabarbarum was found (Li et al. 2016).
One of recent trends in research on rhubarb extracts is their use as pharmacophores. Modern strategies are particularly focused on ecofriendly nanosize materials, produced during the greener synthesis/the green chemistry processes (Kharissova et al. 2019). Such attempts have been successfully undertaken with other substances of plant origin Saw et al. 2019). The R. rhabarbarum-based silver nanoparticles were found to possess significant antibacterial activity both against E. coli (CCM 4517) and Staphylococcus aureus (CCM 4516). Additionally, these particles displayed a dose-dependent anti-cancer effect in human epithelial carcinoma (HeLa) cell line (Palem et al. 2016(Palem et al. , 2018.

Conclusions
Rheum rhaponticum and Rheum rhabarbarum are popular edible plants, commonly cultivated in different regions of the world. A wide range of biological activities of these species has been evidenced by numerous in vitro and several in vivo studies. Both of these species constitute interesting sources of biologically active compounds with health-promoting or therapeutic potential. However, the current knowledge of physiological effects and pharmacological efficacy of these rhubarbs remains incomplete. Therefore, further, more advanced studies (including clinical trials) on pharmacological activity of these rhubarb species are needed.
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