Understanding the Ethnobotany, Chemistry, Pharmacology, and Distribution of Genus Hydnora (Aristolochiaceae)

The genus Hydnora (Hydnoraceae) is one of the basal angiosperms in the order Piperales, found in the semi-arid regions of Africa, and the Southern Arabian Peninsula. Plants in this genus play essential roles in communities around the world as revealed by various studies. Currently, there are eight species of the genus Hydnora; seven in Africa and one in the Arabian Peninsula. Notably, Hydnora abyssinica A.Br. and Hydnora africana Thunb. are widely distributed compared to other species. They are widely used for their medicinal and nutritional values. The information on ethnobotany, chemistry, pharmacology, and distribution of genus Hydnora was gathered using phytochemical and ethnobotanical books, electronic sources, and published articles. Preliminary phytochemical screening shows that flavonoids, phenolics, proanthocyanidins, and tannins are the main compounds in H. abyssinica and H. africana. Furthermore, 11 compounds have been isolated from H. abyssinica. The biological activities of H. abyssinica and H. africana have been reported. They include antibacterial, antiproliferative, antioxidant, antidiarrhea, and antifungal potentials. Despite the Hydnora species being practiced in ancient folkloric medicine, their traditional uses and pharmacological value are poorly documented. Based on the available information on ethnobotany, phytochemistry, pharmacology, and distribution, we aim to provide research gaps and challenges for a better understanding of this genus. This may be resourceful in the development of effective phytomedicines, and aid in conservation. The available studies on this genus on some aspects such as phytochemistry, pharmacological activities, and distribution are under-reported hence the need for further research.


Introduction
Hydnoraceae is grouped under the Aristolochiaceae family and consists of parasitic plants characterized by large flowers and lacking leaves [1,2]. They are native and distributed in arid and semi-arid parts of Africa and Asia. This family contains two genera; Hydnora and Prosopanche, named among the oldest parasitic lineages [1][2][3]. Prosopanche is naturally found in Central America and South America, whereas Hydnora is indigenous to Africa, Madagascar, and the Arabian Peninsula [2][3][4].

Materials and Methods
This review employed literature published before December 2020 on the ethnobotanical uses, chemistry, and pharmacological activities of bioactive compounds from the genus Hydnora. To understand the worldwide distribution and other information on the Hydnora species, data were obtained from the following online data sources from February 2020: Regional floras (flora of Tropical East Africa) [11,29,30], world flora online [31], original species descriptions [1,5,7,9], and virtual online databases, e.g., African Plant Database (APD), Global Biodiversity Information Facility (GBIF) [32], International Plant Names Index (IPNI) [33], and Plants of the world online [34]. For some species, circumscriptions were assessed by reviewing the types of specimens on JSTOR Global Plants [35], Kew Herbarium Catalogue [36], World Checklist of Selected Plant Families [37], The Plant List [38], and a community for naturalists [39]. The H. abyssinica specimen was collected from Mt. Kasigau, Taita Taveta County, Kenya (SAJIT-Mkala 0001, −3.1941 • N, 38.4997 • E ( Figure 1). The voucher specimens were deposited at Wuhan Botanical Garden Herbarium, China, and other herbarium collections (e.g., B, K, W, EA, BM, K, NBG, PREUS, GBG, FT, LISU, P, and BM), as part of ongoing research. An extensive search for ethnobotanical uses, pharmacology, and chemistry of Hydnora was made using published articles, journal magazines, Ph.D. and MSc dissertations, conference papers, available data from herbaria, and books published in English. Several papers were obtained from published research articles of the genus Hydnora. Web of Science, Google Scholar, Science Direct, PubMed, and SciFinder databases further facilitated our study using keywords, for example; Hyd-nora, pharmacological activities, phytochemistry, and ethnobotanical, without a specific time limit. Species details were named as per collector, species number, and the herbarium. The plant list and Plants of the world were used in the verification of species names. For the classification of families, we used the Angiosperm phylogeny group classification system (AGP IV) [40]. Chemical structures were drawn using ChemBio Draw Ultra version 14.0. Excel @ 2016 Microsoft Corporation and Adobe Illustrator version (2020-24.1) was used to draw all figures and graphs. Published articles, reviews, and dissertations were searched online using titles and keywords, obtaining a total of sixty-five for Hydnora. Thirty-one papers were retrieved and used in pharmacological activities. The total number of records identified from all citation sources was 172 and 92 after identifying duplicated data. Google Scholar had the highest number of citations with eighty-four, followed by the web of science with twelve.

Botanical Features and Taxonomy
Hydnora species are underground holoparasitic herbs and their rhizomes are attached to the host. The rhizomes are 1 cm wide, 4-5 angled, terete, and sometimes flattened. The periderm is well developed, brick-red, apart from the tip of the rhizome. Fresh rhizomes are pink to flesh-red with gummy exudate, which is very bitter and severe. The whole rhizome is enclosed with warty outgrowths of haustoria which can be spread sometimes regularly up to 0.8 mm long or unevenly, and less than 0.5 mm long, except at the tip. The rhizomes are covered with latent and active outgrowths in groups of 2-4. Flowers are 3, 4, or 5 merous. The floral envelope is clear and rests on the ground; sometimes they are not reflexed, and they open flowers by separating the floral envelope. Flowers vary in size from 5 to 25 cm depending on the distance of the rhizome from the ground and pedicel (4-9 cm). The ovary is inferior, unilocular, and highly enfolded pendant placentae. The stigma is sessile with distinctive indentations on the surface. Stamens are attached at the base, and anthers are 2.5-3 × 2-2.5 cm in length. Pollen adhering to anthers is very sticky. The floral envelope is 6-8 cm; some species have "bait bodies" in between the internal margin of the envelopes (lobes) while some contain well-developed petals that are concave in shape. The fruits are fleshy, globose, 10-15 cm wide, with many seeds. The mature pedicel is very short and easily disconnected from the rhizome. The outer skin is scaly, and the internal layer is white. The placenta is similar to the internal layer of the pericarp in texture. Seeds are brown, very hard, irregularly shaped, and oblong to globose [1,4,5]. Their pollination mechanism generally follows the same floral phenology as that of H. africana and H. esculenta [7,8].
The taxonomic placement of the Hydnoraceae family is unresolved and studies have classified it differently using a molecular approach. Regardless of them lacking some morphological features (leaves), the Hydnoraceae family was grouped within Rafflesiales [41]. Another study compared Hydnorales and Rafflesiales to the Aristolochiaceae family [42]. Based on floral characteristics, geographic distributions, and embryology, Hydnoraceae was connected to the Annonaceae and Mistrastemonaceae families [43]. Using the molecular approach, the Hydnoraceae family was grouped in Piperales under the Aristolochiaceae family but did not include Asaroideae [2]. The relationship of families in order Piperales is unresolved in the Angiosperm phylogeny group classification (APG III) system [44]. Some research studies support the placement of Hydnoraceae within the Aristolochiaceae family [45,46]. This has contributed to the placement of Hydnoraceae within Aristolochiaceae due to their past paraphyly APG IV [40]. However, a recent study grouped the Hydnoraceae family as a sister group to the Winteraceae family (Drimys granadensis) because of the long branching problem [45]. Another study indicated that the Hydnoraceae family formed a monophyletic clade with a low bootstrap support value of 13%, which cannot justify the grouping of Hydnoraceae [3]. To date, only H. visseri and H. abyssinica complete genomes have been studied in this genus (5,Mkala et al. in press). In genus Prosopanche, only the Prosopanche americana genome has been documented [3]. More studies on the genomes of the remaining species need to be analyzed. This will enable researchers to match particular characteristics observed in plants to their underlying genetic features.

Habitat and Ecology of Hydnora Species
Most of the Hydnora species are found in the semi-arid and desert regions in Africa and the Arabian Peninsula [1][2][3][4][5][6]. They are obligate parasites on various host plant species belonging to the Fabaceae and Euphorbiaceae families (Table 1).

Host Specificity Concerning Species Distribution
Hydnora species have been reported in South Africa, Madagascar, Uganda, Mozambique, Swaziland, Botswana, Kenya, Sudan, Ethiopia, Tanzania, Angola, Somalia, Namibia, and the Arabian Peninsula ( Figure 2) [1,[6][7][8][9]11,24,28,40,43,44]. Host-based speciation appears to be a dynamic aspect in the evolution of Hydnora and this is reinforced by vicariance and changes in phenology. The wide distribution range of the Euphorbiaceae and Fabaceae families may also have led to the subsequent increase in the distribution range of the Hydnora species. These species are host-specific (Table 1). H. africana, H. longicollis, and H. visseri showed high host-specificity with Euphorbia species. H. abyssinica, H. arabica, and H. esculenta are host-specific to Fabaceae, while H. triceps grow on both Euphorbiaceae and Zygophyllaceae families. The variation of host dependency of these species to their host plants has not been revealed whether they exchange genes that help them to perform other metabolic activities, or they entirely depend on their hosts for nutrients. The distribution range of this genus is increasing due to their apparent host. H. abyssinica and H. africana have shown a higher distribution range compared to the other species ( Figure 2). Furthermore, H. longicollis is not well-known due to its inaccessibility [5]. Recently, this species was collected from Namibia growing on Euphorbia; however, very little information on this species is available in the Kew herbarium (http://specimens.kew.org/herbarium/29047.275 (20 December 2020) [47,48]. H. esculenta is only endemic in Madagascar and mostly found in association with the invasive tree, Pithecellobium dulce. The distribution range of H. esculenta is increasing due to the spread of P. dulce in riparian areas and disturbed habitats in Southern Madagascar [4]. H. triceps is only endemic in Northwestern Cape and Southern Namibia because of the restricted distribution of the host. It depends completely on Euphorbia dregeana. A few samples have been collected since its discovery, and so it remains poorly studied [9]. H. visseri is distributed from Namibia to the Northern Cape Province in South Africa. Its distribution is restricted around the Orange River, where its host plants include Euphorbia gummifera Boiss. and E. gregaria Marloth [5]. H. sinandevu is native in Kenya and Tanzania [11]. H. arabica is distributed from Southern Oman (Dhofar region) to Yemen on Acacia species [1]. H. africana is distributed in Namibia, Swaziland, Nigeria, Kenya, Zimbabwe, Uganda, Madagascar, Saudi Arabia, South Africa, and Ethiopia [8,12,25,43,47,48]. H. abyssinica is the most widely distributed species in Namibia, Northern Botswana, Zimbabwe, Zaire, Tanzania, Kenya, Ethiopia, Somalia, Sudan, and the Arabian Peninsula [11,23,24,29,40,[49][50][51].

Approaches to Conservation Status
Based on the literature, the conservation status of H. arabica in Southern Oman is stated as less threatened; however, it is rare in Yemen and Saudi Arabia [1]. H. abyssinica in South Africa is stated as least threatened, as well as in East Africa Flora [6]. There is a lack of information on H. sinandevu conservation status [29]. H. visseri and H. africana have also been documented as less threatened in the South African online red list database [51]. No documentation is available for H. longicollis, while H. esculenta is stated endemic in Madagascar but there is no information related to its conservation. Similarly, there is no information available on the conservation status of H. triceps. The risk assessment of the Hydnoraceae family is not known in the International Union for Conservation of Nature

Finding the Hydnora Species in the Wild
Hydnora species are rarely found and collected because of their uneven distribution and seasonal flowering. Based on the existing documentation, they are scarce and poorly preserved posing a taxonomic challenge [49]. Hydnora features that are mainly used by gatherers to identify Hydnora species include rhizome (protrude after rain season), and presence of flowers. Most gatherers prefer collecting the Hydnora species in areas they collected them before.

Approaches to Conservation Status
Based on the literature, the conservation status of H. arabica in Southern Oman is stated as less threatened; however, it is rare in Yemen and Saudi Arabia [1]. H. abyssinica in South Africa is stated as least threatened, as well as in East Africa Flora [6]. There is a lack of information on H. sinandevu conservation status [29]. H. visseri and H. africana have also been documented as less threatened in the South African online red list database [51]. No documentation is available for H. longicollis, while H. esculenta is stated endemic in Madagascar but there is no information related to its conservation. Similarly, there is no information available on the conservation status of H. triceps. The risk assessment of the Hydnoraceae family is not known in the International Union for Conservation of Nature (IUCN) and the number of Hydnora species in Herbaria is limited [52][53][54]. There is a need for more efforts to establish a proper habitat and area coverage which has been ignored so far. Effective conservation of Hydnora species relies on the existing knowledge of the genetic structure of natural populations and their habitat protection. Their possibility of being propagated as indicated on H. africana deserves more research [51].

Ethnobotany
Few studies have been done and documented on the ethnobotanical value of the Hydnora species. H. triceps, H. sinandevu, H. visseri, H. esculenta, and H. longicollis are poorly documented with little information known about them because of their subterranean behavior, and some grow in unsafe zones (H. longicollis in Angola). They are characterized by a unique flowering system and since they are sparsely distributed, it requires a lot of work to be done to identify more of these species. Some of the existing studies indicate that the roots were used in traditional medicine for alleviating diarrhea, possibly due to their high concentration of tannins.

Species Uses
Hydnora plants are a source of food for wild animals, particularly fruits [55]. According to the herbarium label (Bally 7694 (K), rhinoceroses were reported to feed on H. abyssinica. Extensive diggings of H. abyssinica by elephants have also been observed in Etosha Pan National Park [56]. In the Namib Desert, H. africana roots are fed on by wild animals because of their high water content [4,57].
The fruits of H. abyssinica are also edible by humans. Dried roots are used for charcoal preparation and tanning leather in Sudan [19,58]. In Kenya and Uganda, this species is used for food by the Pakwacha and Pokot communities [55]. The reddish-brown subterranean fruit, which resembles fresh lean meat is the edible part. Moreover, it is used to treat various diseases when combined with other medicinal plants such as paralysis, diabetes, hiccups, fever, insomnia, hypertension, measles, hemorrhoids, and diarrhea [54]. In Tanzania, it is used as a remedy for throat inflammations and swollen tonsils, whereas in Angola it is used as a styptic remedy [59]. The flowers were traditionally used as food and to treat gastrointestinal diseases and cancer in Yemen [24].
H. africana has been utilized for food, leather tanning, and fishing nets preservation [14]. Alternatively, H. africana decoction is used to treat dysentery, chronic diarrhea, persistent stomach cramps, and as a coagulant [51]. Infusions have been used as a face cleanser to treat acne [14,51]. Additionally, the root extracts have been used for alleviating inflammation of the throat amongst local communities in South Africa [4].
Both H. abyssinica and H. africana species have been used as antidiarrhea agents because of their high tannin quantities [4,14]. H. arabica is used as food by Jibbali settlers in Oman [1]. H. visseri fruits are edible [5]. H. esculenta was traditionally used as food, and for tanning leather [4,60]. Similarly, since it is rich in tannins, it has also been used as an astringent traditional medicine to treat diarrhea. H. sinandevu is used for the treatment of throat infections [51]. Generally, Hydnora species have been commercialized and they are readily available in the market [8][9][10][11][12]. Similarly, H. triceps and H. longicollis are only used as a source of food [5]. The uses of Hydnora species are summarized in Table 2. H. visseri Food, treatment of diarrhea, hypertension, and diabetes [5] H. esculenta Source of food [4,7] H. arabica Source of food (fruits) [1] H. triceps Food for wild animals [9] H. sinandevu

Summary of Uses
A total of seven Hydnora species (87% of the genus) were found to have ethnobotanical and other usages (charcoal making and fishnet preservation). All species are limited to Africa (seven species of the total eight) and the Arabian Peninsula (two species of the total eight species). Their uses were divided into the following categories (with the corresponding number of species and percentages in brackets), medicinal (four species; 67.39%), food (eight species; 19.56%), and other usages (two species: 13.04%), as shown in Figure 3. The most utilized parts for medicinal purposes are indicated (Figure 4).

Most Used Species
H. abyssinica is widely used for medicinal purposes and food. It has 24 uses followed by H. africana, which has 11 uses ( Figure 3).

Summary of Uses
A total of seven Hydnora species (87% of the genus) were found to have ethnobotanical and other usages (charcoal making and fishnet preservation). All species are limited to Africa (seven species of the total eight) and the Arabian Peninsula (two species of the total eight species). Their uses were divided into the following categories (with the corresponding number of species and percentages in brackets), medicinal (four species; 67.39%), food (eight species; 19.56%), and other usages (two species: 13.04%), as shown in Figure 3. The most utilized parts for medicinal purposes are indicated (Figure 4).

Most Used Species
H. abyssinica is widely used for medicinal purposes and food. It has 24 uses followed by H. africana, which has 11 uses (Figure 3).

Chemical Composition and Biological Activities
Hydnora species serve as food for both humans and animals. Local communities use the roots, rhizomes, and whole plants for medicine preparations [58]. Despite their use in folkloric medicine, their chemical profiles and biological significance are not yet well established. Generally, there is limited documented evidence on this genus as some of its species are hardly studied.

Traditional Medicine
Four species were used, both separately and as part of the mixture to treat 20 different diseases. H. abyssinica dominated the literature with 19 medicinal uses, followed by H. africana (nine uses), (Figure 3). Diseases are categorized according to the body parts affected. The plants are prepared and used as decoctions, infusions, and powders to treat intestinal, metabolic, respiratory disorders, reproductive, skin, urinary, cancer, paralysis, and styptic diseases ( Figure 5).

Chemical Composition and Biological Activities
Hydnora species serve as food for both humans and animals. Local communities use the roots, rhizomes, and whole plants for medicine preparations [58]. Despite their use in folkloric medicine, their chemical profiles and biological significance are not yet well established. Generally, there is limited documented evidence on this genus as some of its species are hardly studied.

Chemical Composition and Biological Activities
Hydnora species serve as food for both humans and animals. Local communities use the roots, rhizomes, and whole plants for medicine preparations [58]. Despite their use in folkloric medicine, their chemical profiles and biological significance are not yet well established. Generally, there is limited documented evidence on this genus as some of its species are hardly studied.

Extraction Methods Used in Hydnora Species
Selecting an appropriate method for extraction depends on the bioactive compounds being targeted [61]. To enhance the extraction process, plant samples should first be pulverized to increase the surface area of sample contact with the solvent method. Moreover, solvents of varied polarities are used in the extraction process. Some of these solvents include chloroform, ethanol, water, methanol, dichloromethane, ethyl acetate, and n-butanol. For example, a previous study conducted by Koffi et al. [62] discovered that ethanol was more effective in the extraction of many phenolic compounds from walnut fruits as compared to methanol. Compounds tend to dissolve in these solvents depending on their polarity [61]. Currently, commonly used extraction methods include ultrasound-assisted, Soxhlet, percolation, and pressurized solvent. In this regard, the main extraction procedures used for the extraction of Hydnora plant samples are soaking, Soxhlet, ultrasound-assisted extraction, and maceration [28,63,64].

Phytochemistry
Understanding the phytochemical composition and content of Hydnora species is necessary to substantiate their ethnopharmacological uses. Moreover, characterizing their chemical components would facilitate the proper dispensing of their crude preparations. Currently, only a few studies are available on the pharmacological significance of the Hydnora species and their phytochemical composition. H. abyssinica is the only species that has been widely explored in this genus.
Gas chromatography coupled with mass spectrometry (GC-MS) is an ideal analytical technique reported to analyze less polar and volatile compounds [70]. Only a few compounds of pharmacological importance have been characterized in H. abyssinica using this technique. They include myristic acid, oleic acid, palmitic acid, sabinene, acetic acid, γ-Terpinene, D-limonene, stigmasterol, and α-Terpinene [20,24,71] (Table 3, Figure 6). Furthermore, 11 compounds have been characterized and their structures elucidated using NMR (Table 3, Figure 7). They include katsumadin, rhodioloside, catechin, tyrosol, cirsiliol, oleic acid, trans 3 ,5-dihydroxy-4 ,7-dimethoxyflavonol, benzoic acid, 3, 4, dihydroxy-, ethyl ester, 2-hydroxyhexadecyl ester, vanillin, and protocatechuic acid. Additionally, it is worth noting that there has been no liquid chromatography-mass spectrometry work performed on all Hydnora species. Similarly, no phytochemical studies have been conducted on the other six Hydnora species, which is an opportunity for further exploration.      Crude drugs consist of multiple chemical species but only traces of these species contribute to their beneficial or harmful effects as well as potency. The content/yield of compounds in medicinal herbs and plants is believed to influence their biological effects [72]. Thus, estimation of compounds yield in these species is important. In this regard, total flavonoids, tannins, phenolics, anthocyanidins, saponins, and alkaloids contents have been estimated and reported in some Hydnora species. Elhassan et al. [64] reported high TFC, TTC, and TPC of 1163.43 mg QE/mg, 276.6 mg TAE/mg extract, and 5061.13 mg GAE/mg, respectively, in rhizome ethanol extract of H. abyssinica. A high TPC yield of 662.10 ± 14.4 and 661.58 ± 43.9 mg tannin acid/g was reported in water and ethanol extracts of H. abyssinica roots, respectively [73]. A recent study reported low yields of flavonoids, polyphenolics, and tannins in methanol, methanol-dichloromethane, and water fractions of H. abyssinica rhizomes [28]. A similar study was conducted on H. abyssinica roots which reported that total phenolics were the most abundant compounds ranging from 14.40 to 15.50 mg/g while alkaloids were the least abundant, ranging from 0.19 to 0.26 mg/g [26]. The slight variation in the yield contents may be linked to the difference in geographical areas where the plant samples were collected, and different extraction methods used.

Pharmacology of Hydnora Species
Both modern and traditional formulations complement each other in that they have a specific active component/s that take part in physiological effect once ingested or in bioassays. The presence of phytocompounds in crude preparation modulates biological activities by interacting synergistically with other test components, thus lowering any ad-verse reactions. Antiproliferative, antioxidant, anti-fungal, antidiarrhea, and antibacterial potentials have been reported in H. abyssinica and H. africana. Folkloric medicinal practices are currently being recognized in the search of new drugs as most natural products and other botanicals are regarded safe with minimal adverse effects if any [75].
In modern medicine, biological assays have been conducted to evaluate the antioxidant, antifungal, antiproliferative, antidiarrhea, and antibacterial potentials of these species.

Antibacterial and Antifungal Activities
Hydnora species are ascribed with good antibacterial and antifungal activities as shown by various experiments. Particularly, H. abyssinica has been extensively studied for antibacterial properties. Solvents of different polarities (water, methanol, dichloromethane, and ethanol) have been used to determine fungal and bacterial growth inhibition. Micro dilution, cup-plate diffusion, and agar disc diffusion were the most used antibacterial techniques. Both Gram-positive and Gram-negative are commonly used in most experimental tests. Some of the frequent bacteria strains used include Escherichia coli, Staphylococcus aureus, Aspergillus aureus, and Bacillus subtilis. Additionally, activity results were given in minimum inhibitory concentration (MIC). In plants, antibacterial activity is low if the minimum inhibitory concentration is greater than 625 µg/mL, high when the MIC is less than 100 µg/mL, and moderate if the MIC is between 100 µg/mL and 625 µg/mL [76]. This criterion is normally applied in deducing both antibacterial and antifungal activities. The antifungal and antibacterial activities of Hydnora species are reported in Table 4. Table 4. Antibacterial and antifungal activities of H. abyssinica and H. africana.

Plant/Part Investigated Assay Method Results References
H. abyssinica flowers Agar diffusion All extracts showed low activity on yeast, Gram-positive, and Gram-negative bacteria [24] H. abyssinica rhizome Agar disc diffusion/well diffusion In both assays, the methanol and methanol-dichloromethane extracts exhibited moderate to high activities against bacteria tests. Their highest activity was against Candida albicans. Water extract showed relatively low activity. Generally, the inhibition activities were dose-dependent [29] H. abyssinica rhizome Cup-plate agar diffusion This sample showed promising antibacterial activity against the four bacteria and fungi strains assayed; however, it showed no inhibition against Sesbania leptocarpa [67] H. abyssinica rhizome Disk diffusion The crude extract had the highest activity of >20 mm minimum inhibition diameter [63] H. abyssinica root Disk diffusion The methanol extract exhibited low antibacterial activity. No activity was observed at 6.5 and 12.5 mg/mL sample concentrations on Bacillus subtillis [21] H. abyssinica root Cup-plate agar diffusion Methanol, chloroform, and petroleum ether exhibited partial to high antibacterial activity on all strains tested except on Pseudomonas aeruginosa where no activity was observed. High inhibition zones were observed in methanol extract. The activities were dose-dependent [66] H. abyssinica root Cup-plate agar diffusion The extracts' activity increased with an increase in concentration. The water extract exhibited higher inhibition against all fungi and bacteria strains with >16 and >6 mm, respectively. A weak activity was observed in chloroform [64]

Antioxidant Activity
Pro-oxidants such as reactive oxygen species (ROS) are generated from pollutants from environs and also intracellularly due to stress and physiological activities in mitochondria [77]. Polyphenolics ubiquitously generated in plants are regarded as natural antioxidants. Antioxidants react with the free radicals, suppressing their deleterious effects and thus establishing cell homeostasis. Some of the mechanisms by which extracts from plants exhibit antioxidant properties include peroxidation of lipids, hydroxyl, oxygen radical scavenging, and metal chelation [78]. 2, 2-diphenyl-1-picrylhydrazyl (DPPH) and 2, 2 -azino-bis 3-ethylbenzthiazoline-6-sulfonic acid (ABTS) assays have been frequently used to evaluate the antioxidant potential of these species.
Onyancha et al. [27] assayed rhizome methanolic extract of H. abyssinica using DPPH assay. This fraction showed higher activity of IC 50 value of 26.7 µg/mL compared with the ascorbic acid (standard) which had an IC 50 of 29.3 µg/mL. The radical scavenging potential was concentration-dependent. Moreover, the antioxidant potential of three extracts of H. africana (water, methanol, and acetone) were evaluated [25]. In both ferric-reducing antioxidant potential (FRAP) and DPPH assays, methanol extract showed high activity while acetone displayed the lowest. In nitric oxide (NO) scavenging, water extract depicted a higher activity than the control standard (gallic acid). Methanol and acetone extracts had good activities as reported in the ABTS assay. The radical scavenging and antioxidant reducing potential depicted by this species can be wholly or partially linked with the high content of phenols, flavonoids, and tannins that were earlier reported in these species. Additionally, the polarity of the solvents might have influenced the results since the compounds dissolved in them differently according to their polarity.

Antiproliferative Activity
Globally, cancer is a major health concern [79]. The search for an effective cancer cure is still on course since there is no specific chemotherapeutics for this dreadful disease. Stem cell treatment, radiotherapy, surgery, and chemotherapy are some of the commonly used cancer treatments [80]. They are linked with adverse effects such as excessive bleeding and non-specificity, as well as being ineffective and, more so, expensive [81]. This fact has influenced and propelled the medicinal research fraternity to look for alternatives to curb the high mortalities and morbidity associated with this disease. Herbs and other botanical products have been used to cure such diseases as they are considered safe and easy to administer with minimal chances of causing adverse reactions.
H. abyssinica rhizome was assayed using the 3-(4,5-dimethylthiazol-2-yl)-diphenyl tetrazolium bromide (MTT) assay on breast cancer (HCC) cells in vitro [65]. The methanol fraction depicted moderate anticancer activity of IC 50 value 27.20 ± 1.1 µg/mL with high selectivity of SI = 3.68. Water extract showed low activity of IC 50 value 499.3 ± 1.3 µg/mL. Water and ethanol extract of H. abyssinica root displayed low cytotoxicity against human KB cell lines with IC 50 values > 50 µg/mL [65]. No antitumor activity was observed in the isolated compounds. Similar findings were reported in other studies [20,24,29]. Wintola et al. [82] performed an in vivo study and established that the safe dose of H. africana root extract was 5000 mg/kg body weight in both male and female laboratory rats when administered orally. This conclusion was drawn after 4 weeks of treating the animal models with the same dose with no reported cases of adverse reactions.

Antidiarrheal Activity
This is the frequent passage of semi-solid loose stool. It is characterized by dehydration and loss of minerals from the body. Some of its causes are bacterial infection, food/water contamination by parasites, medication side effects, and food poisoning [83]. Some antidiarrhea drugs bind to the receptors of pathogens with high affinity reducing their virulence. Other drugs coat the gastrointestinal (GIT) wall, minimizing the flow of fluids and ions in the gut and intestines, allowing absorption to take place [84]. Plants could provide a long-term solution for different ailments such as diarrhea, especially H. abyssinica.
As mentioned earlier, the Hydnora species have good antibacterial activities, implying their capability to minimize diarrhea which is implicated as an opportunistic disease of bacterial infection such as Escherichia coli [69]. An in vivo study conducted using albino rats demonstrated that indeed H. abyssinica root extract has antidiarrheal activity [85]. Firstly, diarrhea was induced in the animals by feeding them with castor oil. Later, they were treated using different sample doses ranging from 200 to 400 mg/kg. Those that received 400 mg/kg dosage showed the highest antidiarrhea inhibition of 74% and 60% after 4 and 6 h, respectively. The activity was concentration-dependent, but not time-dependent. This activity was linked to the inhibition of prostaglandins and other diarrhea inducers implicated in cytoprotective ability as previously reported [85]. Moreover, since not all pharmacological activities of this genus have been fully explored as well as the clinical evaluations of the crude preparations and isolated compounds, this creates an avenue for more research.

Conclusions and Future Perspectives
This review pointed out the benefits of Hydnora species in the traditional medicinal applications among different communities in Africa and Asia especially in treating various diseases. Previous studies have reported different classes of bioactive compounds isolated from only H. abyssinica which contain several pharmacological properties. No such studies have been conducted on the other species of this genus which remain hardly studied.
Regardless of these species commonly used as a remedy for gastrointestinal, diarrhea, and throat inflammations, the knowledge on their morphological features in the past was unknown, for instance, rhizomes resembling H. abyssinica ("mavumbule") were traded in Mozambique markets. However, the traders had inadequate information on the appearance of the flowers and where the plant grew [86]. Therefore, the selected parts of the Hydnora species used for curing various ailments should be scrutinized to avoid further confusion and wrong administration.
A previous study indicated that H. abyssinica was used together with other medicinal plants to increase its efficacy in curing several ailments. For example, when mixed with Justicia exigua S. Moore leaves, it is used to cure paralysis. When combined with Microglossa angolensis Oliv. and Hiern (= Conyza pyrrhopappa Sch. Bip. ex A. Rich.) roots, it is used to treat diabetes. Further, when combined with Passiflora edulis Sims leaves, it is used to treat hemorrhoids [56]. However, the clinical application of these combinations has not been confirmed. Additionally, the pharmacological action mechanisms underlying the synergistic effects of these species and their medicinal material applications are yet to be explored and also their therapeutic properties remain unknown. Thus, it is highly recommended to test the clinical applications of Hydnora when combined with other medicinal plants in future research. The information available on Hydnora species indicates that they are less toxic. Therefore, dosages administered should be carefully determined to avoid adverse effects. Furthermore, most of the studies conducted on H. abyssinica and H. africana focused majorly on their rhizomes. Consequently, future studies should be conducted to evaluate and analyze the biological activities of their leaves, seeds, and stems.
Moreover, no specific comprehensive studies on the cultivation, processing, quality, and management of Hydnora species. Only H. africana could be cultivated once outside its native area on Euphorbia caput-medusae in California. Additionally, trials to cultivate H. africana in the University of Bristol Botanic Garden (UK) since 2008 using seeds planted on Euphorbia tirucalli L. have been unsuccessful [12]. Hence, more trials to cultivate these plants will enhance understanding of the life history of this important genus and conservation purposes. This will be helpful to populations, which are endemic to endangered environments, for example, H. arabica in the Arabian Peninsula and H. esculenta in Madagascar.
Hydnora being the oldest in the parasitic lineage provides a gap for further research in studying the evolutionary origin of their parasitic nature and more molecular research work is encouraged. Recently, molecular studies on this genus have indicated a spotlight using plastome to understand evolution in parasitic plants. Moreover, more research work is necessary to understand the life history and biology as well as the diversity of this genus.
In conclusion, this review paper summarized the past and current research studies on the ethnobotanical uses, phytochemistry, pharmacological activities, and distribution of the genus Hydnora. However, further research studies are encouraged to explore the efficacy of the identified bioactive compounds, and understanding the toxicity of these medicinal plants as well as confirming their safety for clinical use. Additionally, as an essential medicinal holo-parasitic plant in both Africa and the Arabian Peninsula, the ethnobotanical identities of the six remaining species of Hydnora and their pharmacology, toxicology, and phytochemistry are underexplored, thus requiring more research attention.