Phytoconstituents and Bioactivity of Plants of the Genus Spiraea L. (Rosaceae): A Review

The genus Spiraea L. belongs to the Rosaceae Juss. family and includes more than 100 species distributed in the temperate zone and subtropical zone of the Northern Hemisphere at the center of species diversity in East Asia. Representatives of the genus are known as ornamental plants with many forms and varieties, are widely used in conventional medicine, and have a high resource potential. This review provides information on the diversity of phenolic compounds (flavonoids, phenolcarboxylic acids, and lignans), terpenoids, alkaloids, steroids, and other classes of secondary metabolites in the species of Spiraea worldwide. The article also presents little-known and hard-to-find data published in Russian concerning Spiraea phytochemistry. The biological activities of extracts and their fractions and of individual compounds having different types of biological activity (e.g., antioxidant, antibacterial, anti-inflammatory, and antifungal) are discussed. Data about biotechnological research on representatives of the genus Spiraea are presented too. The analysis of the literature showed that further chemical and pharmacological studies on Spiraea plants are quite promising.


Introduction
The family Rosaceae Juss. is represented by three subfamilies, 16 tribes, 88-100 genera, and~3000 species [1,2]. It is one of the most economically important families. Rosaceae includes the most common fruit and berry crops (e.g., apple, strawberry, pear, peach, plum, almond, raspberry, and cherry) and essential-oil crops (e.g., rose and almond). In addition to commercial species serving as important food sources, Rosaceae includes many plants with ornamental and therapeutic properties [3]. Species of the genus Spiraea L. are affiliated with these plants.
Spiraea species (spireas) are deciduous shrubs 0.15 to 2.50 m in height, with alternate, simple, estipulate leaves, perfectly bisexual or dioecious flowers that are white, light or dark pink, red to purple, in simple or compound inflorescences, with a cup-shaped or bell-shaped hypanthium, five sepals, five petals, five carpels, usually stamens, and free follicles that dehisce along the inner suture (for more detailed description see refs. [4,5]) ( Figure 1). This taxonomically complex genus belongs to the subfamily Amygdaloideae (formerly Spiraeoideae), tribe Spiraeeae, and contains, according to various estimates, 80 to 120 taxa [1, 6]. Phylogenetic relationships within the genus have not been studied sufficiently, and evolutionary patterns for morphological characteristics remain unclear [7]. Due to high polymorphism, spirea has been subdivided by various authors into subgenera, sections, and series [8][9][10].
Spireas are widespread in the temperate zone and subtropical zone of the Northern Hemisphere. According to O.A. Svyazeva [11], the geographic range of the genus Spiraea The ethnomedicinal uses of spireas have been documented in North America, Russia, and Asia. Spireas are employed as effective therapeutics for inflammation and malaria [14]. In traditional Chinese medicine, young leaves, fruits, and roots of S. japonica and its varieties are used as a diuretic, detoxifier, and pain reliever as well as to treat coughs, headaches, and toothaches [15,16]. Wood oil obtained after burning S. canescens is utilized

Chemical Components and Biological Activity of the Isolated Substances
Various classes of natural compounds have been found in spireas, and flavonoids and their derivatives are the most studied at present. In addition, lignans, terpenes, and other compounds have been detected in these plants. The structures of all the isolated compounds have been determined by chemical and modern spectroscopic methods. As far as we know, more than 80 alkaloids characteristic of the S. japonica complex, six new flavonoid glycosides, one lignan, one neolignan glycoside, four monoterpene acylglycosides, and one megastigmane glycoside have so far been found only in spireas. Most studies on the chemical composition of spireas have been performed on plant samples (leafy shoots, fresh or dry leaves, and extremely rarely, flowers or underground parts) from Siberia and the Far East of Russia as well as Japan and China.

Aromatic Compounds
Spireas, just as other flowering plants, produce phenolic compounds, most of which are flavonoids [37]. The flavonoid content of leaves varies within individual species; for example, in S. salicifolia, the concentration ranges from 46.24 to 120 mg/g (dry weight), whereas in S. dahurica, it does not exceed 43.63 mg/g [37,38].
Among flavonols, there are only numerous (more than 25) derivatives of quercetin and kaempferol. The diversity of flavonol structures is mainly determined by the presence of a glycoside residue at position C-3 (extremely rarely at C-4 or C-6 ). The most common carbohydrate residues are glucose and galactose, and less frequently, rhamnose; caffeic acid or glucuronic acid or rutin can also serve as substituents.
S. media and S. humilis are distinguished by a high level (up to 57 mg/g) of other polyphenolic compounds, such as catechins, in leaves and inflorescences [71]; for example, the concentration of catechins exceeds 30 mg/g (of dry matter) in S. dahurica and 18 mg/g in S. salicifolia [38]. Unlike other flavonoids, the catechins of spireas include mainly known compounds (Table 1).
Spiraea species of the section Spiraea have pink inflorescences; accordingly, only in inflorescences of this section's members, anthocyanins (1 mg/g) have been found [71]. All other species of Spiraea have white flowers and do not contain anthocyanins. It has been reported that spireas contain a family of procyanidins: cyanidin and procyanidins B1, B2, and C1 have been detected in flowering shoots of S. salicifolia, whereas procyanidins B1 and B2 (procyanidins) in the shoots of S. hypericifolia [32,52,72,73].
In studies on seasonal dynamics of phenolic compounds in spireas' leaves, it has been noted that the profile of aglycones remains relatively constant during the growth season, with certain changes in the profile of flavonol glycosides. Because the studied sets of morphological characteristics do not fully describe the natural diversity of spireas, as early as 1993, T.W. Kim and Y.M. Lee proposed to study the natural and introduced populations of spireas growing in Korea, in terms of the traits related to the profile and levels of phenolic compounds [74]. The characterization of chromatographic profiles of phenolic compounds is also successfully used to clarify the taxonomy of Spiraea plants growing in Russia and Ukraine [46,48,49,73]. The investigation of intraspecific variability allows to identify the largest number of compounds and to distinguish among them major (constant) compounds and minor ones, i.e., those that are not present in all phases of plant vegetation or not present in all samples [59]. The determination of flavonoid profiles is especially relevant for the controversial species that are classified by most botanists as subspecies or varieties. For example, the present authors have uncovered significant differences in the profiles of phenolic compounds between S. humilis and S. salicifolia, which allows us to regard S. humilis as an independent species; in S. sericea, on the contrary, no signs of species independence have been found [48,75].
Analysis of the variability of the phenolic-compound profile and levels after the introduction of plants into a new environment has shown that after the introduction, there are changes in the profile of phenolic compounds, while there is a relatively stable pattern of phenolic compounds in the specimens growing in the wild. For instance, differences have been registered in high-performance liquid chromatography profiles of phenolic compounds between S. betulifolia subsp. aemiliana specimens from natural and introduced populations. Five new phenolic acids and one flavonol were found in the leaves of the introduced population. After S. betulifolia subsp. aemiliana were transferred from nature (Kunashir Island) to a novel environment (Novosibirsk), there was also a change in the levels of some phenolic compounds. The major phenolic compound is hyperoside (3.36-9.99 mg/g) in leaves from natural populations and quercetin (2.86-5.07 mg/g) in the introduced population [76]. Plant samples collected in introduced populations should be used in taxonomic studies with caution. The chemotaxonomy of spireas and other plants must be investigated using plenty of natural and introduced-population material, taking into account morphological data on the studied plants [49].
The presence of coumarins in studied spireas has been noted by many authors; it has also been established that the level of coumarins is usually higher in inflorescences than in leaves [71]. However, it has not yet been possible to isolate individual compounds of this class, except for a coumarin extracted from S. salicifolia shoots [52].
Phenolic acids are synthesized in shoots, leaves, and flowers of spireas (Table 2). Table 2. Phenolic acids of spireas.  [34], and in S. prunifolia leaves, researchers have [31]. In roots of S. prunifolia var. simpliciflora, investigators have detected phydroxycinnamic acid methyl ester [60,81]. New phenol glycosides have been identified in spireas. For instance, from twigs of S. prunifolia var. simpliciflora, investigators have isolated [82], and from the S. canescens whole plant, 6 -O-trans-p-coumaroyl-(4-hydroxybenzoyl)-β-D-glucopyranoside [29]. The greatest amount of phenolcarboxylic acids accumulates in leaves and flowers; it is also reported that more flavonoids and phenolcarboxylic acids accumulate in (nonlignified) stems of the current year than in lignified stems (second and third years of life) [37,40]. The research on the dynamics of the accumulation of flavonoids and phenolcarboxylic acids among growth season phases has shown that their level is the highest during the flowering period, and it has been demonstrated (using plants growing in Irkutsk Oblast and Buryatia as an example) that the dependence of phenolic compounds' levels on the sites of collection results in only insignificant fluctuations [52].
Among the active substances isolated from Spiraea plants, there are also compounds with high antioxidant activity. Hemiterpene glycosides and dicaffeoyl glycosides from S. prunifolia leaves have a greater DPPH radical-scavenging activity than L-ascorbic acid does (IC 50 = 24.99 ± 0.35 µM), which served as a positive control. 40-Hydroxy-20-methylbutyrate with a substituted hemiterpene moiety showed the highest activity in this assay [60]. It was therefore hypothesized that the carboxyl group of the butyrate stabilizes the free radicals by acting as an electron-donating group. The hemiterpene moiety proved to be important for free-radical-scavenging activities. Superoxide scavenging was measured too to evaluate the antioxidant activities of the compounds isolated from the leaves of S. prunifolia. Compounds bearing two caffeoyl groups exhibited better antioxidant actions than those containing only one. These results suggest that the number of caffeoyl groups in these isolated compounds is important for the elimination of superoxide [60].
The roots of spireas are known as multifunctional plant raw material. Many valuable applications of these species were proved in scientific studies recently. Neolignan glycosides from the roots of S. salicifolia have been subjected to an assay of inhibition of the production of proinflammatory cytokine interleukin-6 in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells, and all the compounds have shown anti-inflammatory effects [95]. Although naturally occurring prunioside A from S. prunifolia var. simpliciflora roots showed no inhibitory effect, its derivatives have inhibitory effects on nitric oxide production in murine macrophagelike RAW 264.7 cells stimulated with LPS and interferon-γ and may be useful for the treatment of inflammatory diseases [97][98][99][100].

Biotechnological Studies on Plants of the Genus Spiraea for Assessing the Ability to Accumulate Phenolic Compounds in In Vitro Culture
Currently, methods for the culturing of plant cells, tissues, and organs are among the tools for basic research into the processes of plant growth and development and have practical importance [101]. In vitro culture is a popular plant propagation technology for the production of valuable secondary metabolites [102,103]. Despite the strong interest in Spiraea members as ornamental plants and sources of secondary metabolites, information on the microclonal propagation of spireas is limited. There are only a few reports on in vitro culture of spireas (about S. bumalda "Anthony Waterer" [104], S. × vanhouttei [105], S. nipponica [106], S. cana [107], and S. crenata [108]). A callus of S. × vanhouttei has been successfully initiated on a sucrose medium [109].
The level of phenolic compounds in representatives of the genus Spiraea in in vitro culture has been estimated only in one species [110]. Micropropagation protocols for S. betulifolia subsp. aemiliana were developed for the first time. This plant from two natural populations-on Sakhalin and Kunashir Islands-was introduced into in vitro culture to assess the ability to accumulate phenolic compounds in in vitro microshoots as compared with intact plants. Active adventitious shoot formation for the Sakhalin plants was implemented on the Murashige-Skoog (MS) nutrient medium supplemented with 0.1 µM 6-benzylaminopurine (18.8 ± 6.0 shoots per explant; 90% regeneration frequency) and for Kunashir plants on MS containing 5.0 µM 6-benzylaminopurine and 1.0 µM 1naphthalene acetic acid (28.8 ± 3.6 and 91%, respectively). Indole-3-butyric acid was found to be effective for the in vitro rooting; the best result was obtained on 1 2 MS supplemented with 0.1 µM indole-3-butyric acid. By high-performance liquid chromatography, it was determined that the profile of phenolic compounds was more diverse in intact plants than in the plants from in vitro culture: 26 versus 13-21 phenolic compounds, respectively. Concentrations of chlorogenic acid, kaempferol, and "phenolic acid 4" were 1.5-2.0-fold higher in the in vitro plants (depending on the cultivation stage) compared to the intact plants, while concentrations of hyperoside, astragalin, and quercetin were higher in the intact plants. Research data indicate feasibility of the use of S. betulifolia subsp. aemiliana plants from in vitro culture as a valuable alternative source of phenol carboxylic acids and flavonoids as well as potential usefulness of further investigation into the accumulation of secondary phenolic compounds in this subspecies in vitro [110].

Nitrogen Compounds
In one of the first chemical studies regarding nitrogen-containing substances in spireas, isoamylamine was found in S. media flowers [111]. Subsequent studies (mainly on S. japonica and its varieties) have led to the isolation and identification of more than 80 alkaloids belonging to the atisine, hetisine, and atisane families. Chemotaxonomy has been proposed based on the structures described for diterpene alkaloids. It is believed that the Southwest of China is the center of modern differentiation, probably the original center of the S. japonica complex. The features of these alkaloids, as well as S. japonica chemotaxonomy on the basis of the isolated substances are reviewed in more detail by X. Hao et al. [25]. After the publication of that review, from the aerial part of S. japonica var. acuminata Franch, Y. Ma et al. [112] have isolated 11 diterpene alkaloids of atisine and atisane types (spiramine A, B, C, C-2, D, F, P, and Z), spiramilactone B and G, and spiramine C, previously found in other varieties of S. japonica. The atisine type also includes spiramilactams A and B isolated from the aerial part of S. japonica var. ovalifolia [113], whereas the atisane type includes new compounds spiramilactone E and spiratisanins A-C from the aerial part of S. japonica var. acuta [114,115] and spiraeosides A and B from S. japonica var. ovalifolia [116]. The hetisine type includes spiraqine, 6-hydroxyspiraqine, spiradines A-C, and spirasines II, III, V, and VI from the aerial part of S. japonica var. fortunei [117,118]. In the aerial part of S. japonica var. ovalifolia, in addition to diterpene alkaloids of the atisine type, new compounds from the class of diterpene glucosides of the atisine type have been found for the first time, which are extremely rare in plants [116]. Studies on alkaloids of the S. japonica complex, for example, have confirmed the validity of the subdivision of spireas' alkaloids into three types and have shown that the S. japonica complex is a special and relatively independent group within spireas. A hypothesis has been advanced that the complicated S. japonica complex has originated in the Southwest of China and has an evolutionary tendency from west to east; this hypothesis has been proven successfully [117].
In addition to S. japonica, alkaloids have been detected in some other spireas: in S. salicifolia, alkaloid spiradine F has been identified, whereas in the leaves of S. koreana, spirajine has been found [119]; in branches of S. formosana, spiraeaine A has been registered [34,120], and in the roots of S. fritschiana var. parviflora, spirafines II and III and spiradine D have been detected [121]. Screening of Spiraea species for alkaloids revealed that extracts of S. crenata, S. nipponica, S. × vanhouttei, S. × billardii, and S. media do not contain alkaloids, and these compounds have been detected only in the roots of S. chamaedryfolia [122]. Nevertheless, in that study, it was not possible to isolate alkaloids from this species either, because of the low stability of these compounds; at the same time, it was determined that alkaloids accumulate in the secondary cortex and secondary xylem of the S. chamaedryfolia root, while alkaloids are absent in the core [122]. A nitrogen-containing compound called aurantiamide acetate has been found in an ethanolic extract of S. formosana branches [34]. Some alkaloids of spireas have been successfully synthesized artificially [123].
Research conducted in recent years focused on biological activity of alkaloids from S. japonica extracts. Some alkaloids from spireas have been shown to possess substantial bioactivity, including anti-inflammatory, anti-platelet aggregation, and neuroprotective effects [25]. Spiramine Q, an atisine-type diterpene alkaloid from S. japonica var. incisa, has been shown in vitro and ex vivo to decrease mouse mortality caused by intravenous injection of arachidonic acid [124]; in the study just cited, spiramine Q was more active than aspirin. Spiramine T, an atisine-type diterpene alkaloid from S. japonica var. acuta, exerts protective effects against cerebral ischemia-reperfusion injury in gerbils, and its mechanism of action may be related to the inhibition of calcium overload, antiperoxidation effects, and modulation of an endogenous antioxidant [125]. Spiramine C1 from S. japonica var. acuta concentration-dependently inhibits the platelet aggregation induced by platelet-activating factor, adenosine-5 -diphosphate, and arachidonic acid with IC 50 of 30.5, 56.8, and 29.9 µM, respectively; these data are suggestive of a nonselective anti-platelet aggregation action. The inhibitory influence of spiramine C1 on arachidonic acid is reported to be as potent as that of aspirin [126]. Spiramine N-6 inhibits platelet aggregation in vitro and in vivo, decreases a serotonin release from platelets, and suppresses the binding of activated platelets to neutrophils [127]. Compounds isolated from the S. japonica species complex possess an anti-tobacco mosaic virus activity. Moreover, diterpene alkaloids with an imine group at position C-20 and an acetyl group at position C-6 have more potent anti-tobacco mosaic virus activity (protective effect) than other compounds do [112]. Atisine-type diterpene alkaloids from the ethanolic extract of S. japonica at a concentration of 100 µg/mL can inhibit the infection by tobacco mosaic virus and show in vivo curative properties, with the cure rate of 55.4% for hsp-X26, 53.2% for hsp-X31, 58.1% for hsp-X35, and 60.3% for hsp-X40, which is better (p < 0.05) than that of a positive control, ningnanmycin (55.2%). The diterpene compound hsp-X40 showed the best inhibitory activity (78.1%). The antitobacco mosaic virus activity of atisine-type diterpene alkaloids from S. japonica is related to downregulation of the tobacco mosaic virus coat protein [128]. Chinese scientists [129,130] have revealed an anticancer activity of diterpene alkaloids from S. japonica. A spiramine derivative (with a β-unsaturated ketone group) isolated from Spiraea species, is a novel anticancer agent capable of inducing apoptosis in cancer cells.
Megastigmanes belong to the class of terpenes, a subclass of sesquiterpenes, but due to their specific C13 skeleton, they got a separate name: isonorterpenoids or megastigmanes. Megastigmanes are found in many plants, such as Cucumis sativus and Juniperus communis [138]. From the aerial part of S. prunifolia var. simpliciflora, a new megastigmane glycoside named simplicifloranoside (81, Figure 2) has been isolated [31].
In most species (S. humilis, S. beauverdiana, S. salicifolia, S. latifolia, and S. media) from the living collection of the Botanical Garden at the Institute of Biology, Komi Scientific Center (the Ural Branch of the Russian Academy of Sciences) and from the natural population, the highest level of saponin-containing extractive substances was found in inflorescences (1.8% to 8.1% of dry weight), while the highest level of these substances was found in inflorescences of S. corymbosa (8.1%) and in leaves and inflorescences of S. betulifolia (7.1% and 7.2%, respectively) [139]. In leaves of all the tested species, except for S. betulifolia, saponin-containing substances were found [0.8% (S. salicifolia) to 3.0% (S. media)]. By thinlayer chromatography and high-performance liquid chromatography, among the obtained saponins, five to six individual compounds have been identified, including diosgenin: the genin of most of spirostanol glycosides [139].

Polysaccharides
Spireas are distinguished by a high concentration (up to 20.7%) of polysaccharide complexes (more than 69% in flowers), which include mono-and disaccharides and uronic acids [71,141].

Higher Fatty Acids
The fatty acid composition of S. salicifolia and S. hypericifolia shoots has been studied and is now known to include palmitic, stearic, linoleic, and linolenic acids; besides, in S. hypericifolia, investigators have found myristic, pentadecanoic, and palmitoleic acids [142], whereas in S. salicifolia, they have detected arachidic, behenic, tricosanoic, or lignoceric acid as well as higher aliphatic carbohydrates: hexylheptadecane, octadecane, pentacosane, eicosane, heneicosane, docosane, tricosane, and tetracosane [52]. Among the lipids of S. media leaves and inflorescences, palmitic and linolenic acids are predominant. Saturated fatty acids dominate among lipids in leaves, significantly exceeding the level of unsaturated fatty acids. There are rather high concentrations of such rare unsaturated acids as vaccenic and gadoleic and saturated acids: behenic and lignoceric [143]. Spiraeamide, a novel sphingolipid, has been found in a methanolic extract of the S. brahuica aerial part [135].

Organic Acids
The profile of organic acids in the flowers and leaves of S. salicifolia is known: researchers have found acetimidic, butyric, succinic, fumaric, and 3-ketoglutaric acids [52,141]. The amino acid composition of the above-ground and underground parts of S. hypericifolia has been researched in detail, and the levels of macro-and microelements in S. hypericifolia were determined too. The major amino acids in S. hypericifolia are glutamate, aspartate, and alanine [142]. The accumulation of ascorbic acid has also been studied in leaves of 22 Spiraea species introduced into the Belgorod region. For most species of Spiraea, there is a tendency toward a decrease in the concentration of ascorbic acid during the growth season. Leaves of S. × rubella and S. canescens collected in the first month of summer and leaves of S. ussuriensis collected in the last month of summer have been proposed for therapeutic use. It was demonstrated that S. × rubella contains the highest concentration of ascorbic acid among the studied species [144]. Sanleng acid has been isolated from S. japonica var. ovalifolia [93].

Elemental Composition
In the leaves and flowers of S. salicifolia from Eastern Siberia, nine macro-and 54 microand ultramicroelements have been detected. Among macronutrients, there is a high concentration of potassium, calcium, magnesium, and phosphorus; among trace elements, high concentrations of manganese, zinc, and copper are present. Flowers accumulate more iron and copper than leaves do [52,145]. In the aerial and underground organs of S. hypericifolia, there are 11 macro-and microelements according to multielement atomic emission spectral analysis, and major elements among them are Ca, K, and Mg [142]. The levels of trace elements have been studied in S. dasyantha too [146].
The essential oils of spireas have been tested for various types of activity. Antifungal in vitro activity has been documented for S. alpina essential oil [26]. In particular, in one study, this essential oil at a concentration of 10 µg/mL suppressed the growth of Ralstonia solanacearum, and at a higher concentration (up to 125 µg/mL), suppressed the growth of R. solani, Fusarium graminearum, and E. turcicum [26]. The essential oil from the leaves and branches of S. mongolica neutralizes free radicals of DPPH (IC 50 = 900 µg/mL); on the other hand, this essential oil is reported to be inactive against Bacillus subtilis, Aspergillus flavus, and Candida albicans [27]. Nonetheless, its individual components such as 1β-hydroxy-6,9-diene-8-oxoeremophil-11-nor-11-ketone and 3-(4-methoxyphenyl)-propanal possess antibacterial and antifungal activities against the above microbes [27].

Biological Activity of Extracts and Their Individual Fractions from Plants of the Genus Spiraea
Spireas have been repeatedly studied worldwide not only regarding their chemical composition but also on the subject of various useful properties.

Anti-Inflammatory Activity
The attention of scientists from various countries is attracted by the anti-inflammatory activity of spireas (Table 3). Young leaves, fruits, and roots of S. prunifolia var. simpliciflora are used in Oriental medicine to treat pyretic and emetic diseases. A fever is induced as part of the acute phase response: a generalized reaction to infection and inflammation [22,97]. Anti-inflammatory effects of extracts from S. prunifolia var. simpliciflora have been studied repeatedly in vitro and in vivo. A methanolic extract from the roots of this plant is reported to significantly inhibit the formation of nitric oxide and superoxide in RAW 264.7 cells. This work showed low cytotoxicity of the extract from the plant's roots: the methanolic extract did not affect the viability of RAW 264.7 cells even at 200 µg/mL. Overall, the antipyretic effects of the methanolic extract from the roots of S. prunifolia var. simpliciflora may be explained by direct inhibition of nitric oxide and a reduction in superoxide production [22]. An aqueous extract of S. prunifolia var. simpliciflora roots strongly suppresses the production of nitric oxide during an LPS-induced inflammatory response, without cytotoxicity. Furthermore, the extract decreases H 2 O 2 cytotoxicity by enhancing cell viability and significantly reduces the intracellular level of reactive oxygen species. The cytotoxicity of the aqueous extract has been tested. The extract is reported to be nontoxic to RAW 264.7 cells at 250 µg/mL [80]. Lee et al. [24] have demonstrated that a methanolic extract of S. prunifolia var. simpliciflora leaves has a good potential for the treatment of acute lung injury. The extract of the leaves decreased the number of inflammatory cells and the levels of tumor necrosis factor, interleukin (IL)-1β, and IL-6 in bronchoalveolar lavage fluid and suppressed inflammatory-cell infiltration in lung tissue. The extract effectively inhibited airway inflammation and reactive oxygen species-mediated oxidative stress; this effect is closely related to its ability to induce activation of nuclear factor erythroid 2-ƒrelated factor and to inhibit the phosphorylation of mitogen-activated protein kinases and subunit p65 of nuclear factor κB. A methanolic extract from S. prunifolia leaves has also been shown to be nontoxic at 100 µg/mL [24].
Choi et al. [148] have examined the inhibitory effects of a methanolic extract of S. fritschiana on LPS-induced nitric oxide production in RAW 264.7 cells. The nitric oxide inhibition rate was 90% at 200 µg/mL methanolic extract. At the same concentration, the expression of proinflammatory genes, such as inducible nitric oxide synthase and cyclooxygenase, decreased too. The extract at 10-200 µg/mL does not affect cell viability [148].
The anti-inflammatory activity of S. media and S. salicifolia was confirmed in an in vivo experiment. A 50% aqueous-ethanolic extract from S. media leaves when administered intragastrically at a dose of 100 mg/kg had an anti-inflammatory effect at all stages of the inflammatory process. In rats, the extract reduced the degree of tissue alteration and enhanced the recovery of physiological processes at the site of inflammation caused by subcutaneous injection of 9% acetic acid into the back with simultaneous administration of a dextran solution. The area of tissue damage on days 9 and 29 was smaller by 14.43% and 25.71%, respectively. In this context, chamomile infusion had a weaker effect on the alterative phase of inflammation [23]. V.M. Mirovich and colleagues [149] have developed a method for obtaining a dry extract from S. salicifolia shoots that has well-pronounced anti-inflammatory, diuretic, and antioxidant activities. In that study, the dry extract of S. salicifolia at a dose of 100 mg/kg had an anti-inflammatory effect, as evidenced by a decrease in the degree of tissue alteration and an enhancement of recovery of physiological processes at the site of inflammation. The area of tissue damage on days 9 and 29 was smaller by 14.9% and 19.9%, respectively. The tested dose of the dry extract of S. salicifolia had an antiexudative effect, reducing the swelling of the animals' paw by 36.5% as compared to a control group [149].

Antioxidant Activities
Polyphenols of Spiraea are promising natural antioxidants for the food and pharmaceutical industries. Information on the antioxidant effects of Spiraea is compiled in Table 3. The investigation into the antioxidant potential of most spireas has been based on the DPPH method. Trolox [150,151] is used more often as a comparison sample; ascorbic acid [148] and n-propyl gallate are employed less often [29]. In some research articles, the antioxidant activity of a reference compound is not specified [80]. An assessment of the cytotoxicity of extracts or their fractions from spireas has been conducted only in the studies where, in addition to the antioxidant activity, an anti-inflammatory effect was evaluated [24,148].
A crude extract of S. canescens and its partitioned fractions (ethyl acetate and butanolic) have been found to exert potent actions in radical-scavenging and superoxide anion-scavenging assays, while a moderate activity was found in a Fe +2 -chelating assay [29]. The activity of the extract and fractions is comparable to the activity of the standard [n-propyl gallate: DPPH radical-scavenging activity (90.13%) and superoxide anion-scavenging activity (91.04%)]. In addition, pure compounds were isolated from S. canescens, and their antioxidant activity was evaluated. The assay results revealed a higher scavenging activity of quercetin and cyclolignans [(+)-isolariciresinol 3a-O-b-Dglucopyranoside and (+)-lyoniresinol 3a-O-b-D-glucopyranoside] toward the DPPH and superoxide anion radicals. Among them, quercetin showed a potent activity against superoxide anions (IC 50 = 68.11 µM), which was stronger than that of the standard compound n-propyl gallate (IC 50 = 106.23 µM). In addition, quercetin has been found to be a potent scavenger of DPPH radicals (IC 50 = 50.30 µM); these findings point to its strong antioxidant properties [29].
A methanolic extract of S. fritschiana has an antioxidant activity, with RC 50 of 76.61 µg/mL [148]. The antiradical activity of this extract is approximately 14-fold lower than that in the control (ascorbic acid, RC 50 = 5.37 µg/mL). As a result of measuring the reducing ability of a spirea extract at 25-500 µg/mL, it was confirmed that the absorption value (0.13-1.8) increased significantly as the concentration of the sample increased. The extract showed approximately 50% reducing capacity as compared to butylated hydroxytoluene (absorbance = 0.77) used as a control at 100 µg/mL. Considering that only a crude extract from the spirea aerial part was tested above, it can be said that its antiradical activity is relatively high. Furthermore, a high total level of phenolic compounds (212.78 µg gallic acid/mg) and flavonols (66.84 µg quercetin/mg) was found [148].
Seeds of native and naturalized plants currently growing in the Mississippi River Basin of the United States have been evaluated as potential new sources of antioxidant activity [151]. Antioxidant levels of 158 studied plants ranged from 2.40 to 261.38 µM Trolox/100 g. The screening of extracts from the plants' seeds for antiradical activity showed that a methanolic extract from S. tomentosa seeds (141.31 µmol Trolox/100 g) has a high antioxidant activity [151].     [157] Total antioxidant levels in extracts from leaves and inflorescences of nine representatives of spireas growing in the Far East of Russia have been investigated by the operative amperometric method [152]. In that study, among the Far Eastern representatives of this genus, S. betulifolia (2.79 mg/g in an aqueous extract from inflorescences) and S. beauverdiana (2.54 mg/g in an aqueous extract from leaves and 2.11 mg/g in an aqueous-ethanolic extract from leaves) (section Calospira), S. humilis (1.61 mg/g in the aqueous extract from inflorescences) and S. salicifolia (0.82 mg/g in the aqueous extract from leaves) (section Spiraria), S. pubescens (0.72 mg/g in the aqueous-ethanolic extract from leaves), and S. media (0.59 mg/g in the aqueous extract from leaves) (section Chamaedryon) were found to be promising antioxidant materials. Spireas probably contain water-soluble antioxidant compounds of the phenolic type because the antioxidant activity of aqueous extracts from the leaves and inflorescences is higher (0.16-2.79 mg/g) than that of water-alcohol extracts (0.06-2.54 mg/g). The antioxidant activity in the leaves of spireas is generally higher than that in inflorescences. A significant positive correlation was observed between the antioxidant activity of aqueous extracts from the organs of Spiraea and the level of oxycinnamic acids [152]. In a study on antioxidant activity of aqueous and aqueous-ethanolic extracts from S. media leaves and inflorescences collected in a natural population (Ust-Kulomsky District, Komi Republic), it was found that aqueous extracts of S. media leaves have a lower radical-binding activity than do the aqueous-ethanolic extracts [150]. At the same time, the radical-binding activity of the aqueous-ethanolic extract of leaves was greater than that of inflorescences. In that study, the aqueous-ethanolic extract of leaves showed the greatest radical-binding activity among S. media extracts. Its IC 50 was 4.1-fold higher than that of a standard (Trolox, IC 50 = 1.18 mg/mL). A strong correlation between the radical-binding activity and concentration of polyphenols was found in the extracts (r = -0.80, p < 0.05). In the same work, the assay for the ability to chelate Fe 2+ revealed that all the extracts are 77.0-133.7-fold less active than the chelator dipyridyl (EC 50 = 0.13 mg/mL). The aqueousethanolic extract of inflorescences (13.75 mg/mL) had greater chelating activity than did the aqueous extract [150].
The high antioxidant potential of S. prunifolia var. simpliciflora has been confirmed many times. The extract has been found to have a high activity similar to that of superoxide dismutase and a high total level of phenolic compounds (56.7 µg gallic acid/mg) and flavonols (15.1 µg rutin/mg) [80]. In a study by Lee et al. [24], a methanolic extract of S. prunifolia var. simpliciflora leaves showed a potent DPPH radical-scavenging activity and suppressed reactive oxygen species production in tumor necrosis factor-stimulated NCI-H292 cells. In addition, S. prunifolia var. simpliciflora effectively attenuated lipid peroxidation and restored glutathione concentration in the lung tissues of a mouse model of LPS-induced acute lung injury and in tumor necrosis factor-stimulated NCI-H292 cells [24].

Antiviral Activity
An in vitro study of antiviral activity was performed on 14 plant species of the genus Spiraea growing in Asian Russia (Table 3) [153]. In that study, the inhibition of the replication of human influenza virus A/Aichi/2/68 (H3N2) and avian influenza virus A/chicken/Kurgan/05/2005 (H5N1) in MDCK cell culture was evaluated for extracts from the aerial part, at their highest tolerable concentrations. It was revealed that all aqueousethanolic extracts of the tested Spiraea species have an antiviral effect of various sizes. Virus neutralization indices (NIs) for these samples ranged from 0.5 to 4.25 lg toward both strains of the influenza virus. In this project, of all the tested samples, the aqueous-ethanolic extract of S. hypericifolia had the greatest antiviral effect (NI was 4.0 lg for human influenza virus and 4.25 lg for avian influenza virus), and this species is in the group of the least toxic Spiraea plants (highest tolerable concentrations for cultured MDCK cells: 0.5 mg/mL). The following plants were less potent but promising against avian influenza A/H5N1 virus: S. alpina and S. crenata (NI for these samples was 3.25 lg) as well as S. dahurica, S. aquilegifolia, S. betulifolia, S. media, S. salicifolia, S. pubescens, and S. elegans (NI = 2.25 to 2.75 lg), whereas against human influenza virus A/H3N2, the promising species were S. pubescens, S. betulifolia, S. media, S. salicifolia, and S. dahurica (NI = 2.0 to 3.0 lg) [153].

Antimicrobial and Antibacterial Activities
To date, all studies on the antimicrobial and antibacterial activities of spireas have been conducted in vitro (Table 3). A methanolic extract from S. tomentosa seeds has an inhibitory effect on four microorganisms: Staphylococcus aureus, Pseudomonas aeruginosa, E. coli, and C. albicans [151]. Nevertheless, in that report, the antibacterial activity of the S. tomentosa seed extract was lower than that of an antibiotic called ticarcillin (inhibition zone of 47 mm against S. aureus, 32 mm against E. coli, and 25 mm against P. aeruginosa), which served as a positive control. Furthermore, the antifungal activity of the extract against C. albicans was less than that of the antifungal mixture of essential oils "RC" from Young Living (inhibition zone of 15 mm), which was used as a positive control. It should be pointed out that S. alba manifested no antimicrobial activity [151]. Because the researchers have not detected a high correlation between antimicrobial and antioxidant activities in these plants, it is likely that the antimicrobial action is exerted by nonphenolic substances that are recognized as antioxidants.
Moderate antibacterial activities have been found in extract fractions (of different polarity) from the aerial part of S. chamaedryfolia. Three fractions showed an antibacterial activity against gram-positive and gram-negative bacterial strains, and one fraction exerted an antibacterial activity against methicillin-resistant S. aureus (MRSA). Solvent-soaked paper discs served as negative controls. In the fractions from the aerial part of S. chamaedryfolia, alkaloids were found that are possibly responsible for the antibacterial activity of this species [122,154]. Because these studies did not assess the antibacterial activity of alkaloids and other Spiraea species (S. crenata, S. media, S. salicifolia, S. nipponica, S. × vanhouttei, and S. × billardii), which do not contain alkaloids as shown in this project, it is not possible to draw a conclusion about the antibacterial activity of S. chamaedryfolia alkaloids.
In an ethanolic extract from the leaves of S. thunbergii J. Lee et al. [89] have revealed a high antimicrobial activity against E. coli, and the same was done for a butyrolactone [S-(-)-tulipalin B] isolated from the leaves of S. thunbergii. The researchers stated that the methylene group possibly is key for the antibacterial activity of this compound, and the hydroxyl group may have a synergistic effect with the methylene group [89].

Protistocidal Activity
The protistocidal effect of spireas has been assessed in vitro. There are data only on five species of Spiraea (Table 3). Extracts from the branches of S. aquilegifolia and S. dahurica have a protistocidal effect, causing almost instantaneous death of protozoa (the S. media extract does this within 5 min, and the S. salicifolia extract within 35 min). It was also revealed that the extracts from the roots of S. aquilegifolia and branches of S. chamaedryfolia do not have a protistocidal activity. Experiments intended to identify the active substances responsible for the protistocidal action are not mentioned in that article [155].

Antifungal Activity
Spireas also show antifungal activity. Antimycotic activity has been found in S. prunifolia (Table 3). Extracts from the leaves of S. prunifolia at various concentrations are reported to be effective at reducing the growth of the mycelium of seven fungi that cause the rot of tomatoes and eggplant fruits [156]. In that work, a high concentration (7 mg/mL) was found to be more effective than a low concentration (2 and 5 mg/mL). An ethanolic extract of S. prunifolia showed the highest antimycotic activity against R. solani and the weakest inhibitory effect against the growth of Alternaria alternata. An aqueous extract of S. prunifolia showed the strongest mycelial inhibition in Aspergillus niger and the weakest inhibitory effect on A. alternata [156]. Extracts from S. prunifolia can be used as a new natural fungicide for the management of fungal rot pathogens. Nonetheless, further studies on S. prunifolia extracts are needed regarding active ingredients and possible activities against other pathogenic fungi.
It has been demonstrated that different fractions of the ethanolic extract from S. alpina leaves inhibit mycelium growth of the fungus causing late rice sheath blight and the fungus causing corn leaf blight (Table 3). In one study [88], the ethyl acetate fraction was found to be most effective against R. solani and E. turcicum (dose 0.3 mg/mL, inhibition rates 47.4% and 40.8%, respectively). As the extract was purified, the scientists were able to isolate highly polar substances that possess the highest antifungal activity [88]. Further research by the scientists [85] led to the isolation of a new antifungal compound. The major fungitoxic ingredient of S. alpina turned out to be a new diacylated sugar, structurally identified as 6-O-(3 ,4 -dihydroxy-2 -methylenbutyryl)-1-O-trans-cinnamoyl-β-D-glucopyranose. This compound at 0.1 mg/mL can inhibit the growth of R. solani and E. turcicum by 87.6% and 63.2%, respectively. In that study, pseudomycin A served as a reference compound. The antifungal activities against R. solani were similar between the isolated compound and the reference substance [85]. It is possible that this compound and extracts from S. alpina also have an insecticidal activity because the molecule of the isolated compound contains a relevant moiety, namely, a derivative of α-methylene-γ-butyrolactone [83]. Similar compounds isolated from S. thunbergii have shown insecticidal activity.

Other Beneficial Properties of Spiraea
Spireas also have other useful effects. Ethanolic extracts of S. alpina leaves have an anticancer effect [157] (Table 3). A methanolic extract of S. cantoniensis protects soft steel against corrosion [158]. The ability of spireas to tolerate increased concentrations of pollutants under urban conditions has been demonstrated using S. × vanhouttei as an example [159]. Spireas are resistant to high soil salinity [160]. S. trilobata leaves hold promise as an insect repellant of plant origin [161]. After an analysis of the chemical composition of flowers, leaves, and branches of 11 spireas growing in China, recommendations have been made regarding their use as tonic drinks of various quality categories, and the timing has been clarified for phenological periods most suitable for harvesting these plants [162,163].

Conclusions
The present review shows that spireas have been a subject of various scientific studies. Chemical analysis has resulted in the isolation and identification of both known compounds and previously unknown secondary metabolites in the aerial part, leaves, inflorescences, and stems of spireas. Pharmacological studies confirm the ethnomedical significance of spireas and have revealed new beneficial properties of these species. The identification of biological activities of individual compounds from spireas is also suggestive of strong effects of the whole plant. It would be worthwhile to search for species with pronounced antitumor, anti-inflammatory, neuroprotective, antiviral, and other activities among representatives of spireas and to seek their further adoption by conventional medicine. Additionally, phytochemical studies on spireas are beginning to address their in vitro culture, which has great prospects for scaling up the synthesis of some desired substances. Thus, the above information on the chemical composition and biological activities of spireas points to a fairly strong therapeutic potential. It is likely that studies on other species of this genus will yield new results that are practically useful.
Author Contributions: V.A.K. and N.V.P. contributed equally to this work. Both coauthors read and approved the final manuscript. All authors have read and agreed to the published version of the manuscript.

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