Medicinal Importance and Phytoconstituents of Underutilized Legumes from the Caesalpinioideae DC Subfamily

Abstract

Underutilized legumes are common crops in developing countries with superior dietary potentials that could be useful sources of protein as well as some phytoconstituents. They are more tolerant of abiotic environmental conditions like drought than the major legumes. This makes them more adapted to harsh soil and climatic conditions, which helps to minimize the pressure brought on by climate change. However, despite their potential, underutilized legumes have been greatly overlooked compared to the major legumes due to supply constraints. Underutilized legumes in the subfamily Caesalpinioideae are better suited for use as animal feeds with little or no value as food for humans, and the extracts and infusions of the different parts of plant species in this subfamily are traditionally used for the treatment of different diseases. In addition, underutilized legumes in this subfamily contain phytoconstituents that are of pharmacological relevance, some of which have been isolated, characterized and evaluated for use in the treatment of a variety of disorders. Therefore, this review describes the medicinal activities of some selected underutilized legumes from five genera in the subfamily Caesalpinioideae as well as their phytoconstituents, which could be exploited as lead compounds for drug discovery.

1. Introduction

Over the last five hundred years, with increased contact between dissimilar populations and the development of a global trading system, the use of plants for food has concentrated on about 30 crop species out of an estimated 30,000 edible species, which are now the basis of a considerable amount of the world’s agriculture [1]. This has resulted in the underutilization of many plant species. Legumes are essential food crops that provide high-quality protein and micronutrients that may promote health and livelihoods, particularly in underdeveloped nations [2]. With close to 765 genera and over 19,500 species, the Leguminosae is the third-largest plant family and second in economic importance only to Poaceae [3,4]. Drought, severe heat, salt, and heavy metal stress, among other abiotic factors, have reduced the legume yield by around 50% globally.

Underutilized legumes are often better able to tolerate extreme and unfavourable environmental conditions, such as drought, high salinity, and low soil fertility, compared to other commonly grown crops. This is due to the unique adaptations and traits these legumes have evolved to survive in areas with limited resources. Underutilized legumes are a common crop in developing nations, but their economic impact on international markets is modest. Despite their superior dietary potentials, they are commonly used as animal feed and for other agricultural purposes by resource-constrained farmers. By utilizing these underutilized legumes in agriculture, farmers can increase their crop yields and reduce their dependence on chemical inputs, while also contributing to the preservation of crop diversity and food security. Numerous underutilized legumes have a lot of untapped potential in terms of their nutritive qualities due to their remarkably high protein content. In addition to their nutritional benefits, legumes produce secondary metabolites, some of which are toxic or poisonous, that act as deterrents to predators [5]. Several of these secondary metabolites are being investigated for their potential as pharmacological agents [6]. The metabolites that have been found in the leaves and fruiting parts include flavonoids, alkaloids, terpenoids, nonprotein amino acids, and others.

Underutilized legumes belong to the family Leguminosae (Fabaceae) [7], and the Leguminoseae family was traditionally divided into three well-known, long-recognized and widely accepted subfamilies, Caesalpinioideae DC., Mimosoideae DC., and Papilionoideae DC, each of which has in the past been considered a separate plant family [7,8]. However, because Mimosoideae and Papilionoideae are nested inside a paraphyletic Caesalpinioideae [9], this categorization does not effectively describe evolutionary relationships within the family [10]. Thus, the new Leguminosae classification, based on the evolutionary structure and representing a consensus opinion of the international legume systematics community, recognises six subfamilies, namely Papilionoideae DC., Caesalpinioideae DC., Detarioideae Burmeist., Dialioideae LPWG, Cercidoideae LPWG. and Duparquetioideae LPWG. [2]. The traditionally recognised subfamily Mimosoideae is recognized as a distinct clade nested within the recircumscribed Caesalpinioideae [11]. A diagram of the phylogenic classification of the Leguminosae family and the genera of interest is shown in Figure 1.

The new subfamily Caesalpinioideae, which includes the mimosoid clade, with over 4600 species in 152 genera comprised of trees, shrubs, and lianas, is the second-largest legume subfamily [12]. The subfamily is widespread across the tropical dry, savanna, and wet lowland tropical rainforests. The subfamily Caesalpinioideae is best known for its shade and ornamental species, such as Acacia farnesiana (sweet acacia), Bauhini bartlettii (orchid tree), and Cercis siliquastrum (the Judas tree, or redbud), as well as some of the world’s fastest-propagating problematic invasive weeds, such as Leucaena leucocephala (white popinac). Another important plant, Albizia, is often utilised as a green manure and fodder crop [3]. Both pods and leaves of the Acacia species are used for food for animals (graze) while the stem is used for the production of pulpwood and gum exudates [3]. Leucaena species, including Leucaena leucocephala, have been developed for use as firewood, animal fodder, and building materials, and for their high nitrogen production, which improves depleted soils, particularly in Asiatic tropical regions.

In order to fully harness the potentials derivable from underutilized legumes from the subfamily Caesalpinioideae, there is a need to gather available scientific evidence on their potential pharmacological applications and the phytoconstituents that have been isolated from the plants. This will create awareness about their potential as a source of novel compounds that could serve as a template for drug discovery. Thus, this paper presents an analysis of studies that have been reported in the literature on species from the subfamily Caesalpinioideae.

2. Methodology

A wide range of publications that provided information about the botanical description and ethnobotanical uses, pharmacological studies, and phytochemical constituents of these legumes were reviewed. Relevant pieces of literature were systematically searched online in PubMed, Google Scholar, and Science Direct databases. Articles that reported the use of underutilized legumes were considered. The International Union for Conservation of Nature (IUCN) red list of threatened species was also consulted for the status of the species. The schematic diagram of the methodology adopted is presented in Figure 2.

3. Results

The literature search revealed that several studies have been carried out mainly on five genera from the subfamily Caesalpinioideae, namely Albizia, Acacia, Leucaena, Senna, and Canavalia. This paper presents an analysis of the different studies that have been carried out on these species, highlighting their folkloric uses, pharmacological potentials, and the isolated compounds that could be further developed for the management of diseases.

3.1. Albizia

The genus Albizia, also known as Albizia durazz, contains 150 taxonomically recognized species of rapidly growing subtropical and tropical trees and shrubs [13]. The genus is widespread in Asia, Australia, North and South America, and Africa, where some of the species are weeds in some regions [14]. Due to its umbrella-shaped crown, Albizia, which is a deciduous shrub, is commonly called silk tree, silk acacia, sleeping tree, or umbrella acacia. Since the species are not hardy plants, they are best suited to being grown in pots and require a wind-protected location when growing outdoors. Albizia woods are widely used in Nigeria to carve wooden figures, wooden spoons, mortar and pestles, and food stirrers of various sizes [15]. Gum from the trunk of some Albizia species has been used as an additive in both food and drug formulations [16,17]. The species are essential forage and fuel sources [18]. Africans have long used Albizia species to treat a variety of conditions such as fever, wounds, diabetes, malaria, diarrhoea, and insomnia [14]. A phytochemical study of the species revealed the presence of phenolic glycosides, alkaloids, saponins, flavonoids, triterpenes and triterpenoids [19].

Pharmacological analyses have also revealed that Albizia has analgesic, anti-inflammatory, anti-anxiety, anti-diabetic, and anti-depressant properties [14]. The IUCN red list includes 78 species of this genus, 44 of which are found in sub-Saharan Africa. Some of the well-known but underutilized species of the genus include Albizia zygia, Albizia lebbeck, Albizia ferruginea, and Albizia glaberrima.

3.1.1. Albizia zygia (DC.) J. F. Macbr.

Albizia zygia, known as Nongo in Swahili, Ayin rela in Nigeria (Yoruba tribe), and Nyieavu in Nigeria (Igbo tribe), is native to tropical Africa and has a wide geographic range, primarily in Senegal and Nigeria [16]. A. zygia has been identified as a nitrogen-fixing plant with the ability to improve the pH of acidic soils. In addition to providing shelter and acting as a windbreaker, its ornamental qualities also make it a popular tree in recreation and tourist centres [16]. In Nigeria and other nations, A. zygia is used to treat epilepsy, fever, the flu, and pain. The young leaves of A. zygia are prepared as vegetable soup while its new shoots and leaves are used as feed for livestock. The low protein and amino acid content of the seeds, as well as their low levels of lysine and threonine, demonstrate their poor nutritional value.

Previous studies reported the anti-inflammatory activities of A. zygia in animal models using hydroethanolic leaf and root extracts to ascertain its use for the treatment of fever, pain and inflammation [20,21]. The authors reported significant effects compared to standard reference with ED₅₀ ranging between 30 and 300 mg/kg orally. The alcohol leaf extract of A. zygia was discovered to have antipsychotic activity in an in vivo study with the possibility of alleviating cognitive signs of schizophrenia [22]. The extract also demonstrated antidepressant properties, supporting its ethnomedicinal use for the management of depression [23,24]. Additionally, Koagne et al. [25] reported the anticancer and antiplasmodial activities of compounds isolated from methanol extract from the leaves.

The root, stem, and leaf of A. zygia contain varying amounts of essential oils. As opposed to the root and bark, which have non-terpene derivatives of 27 and 44%, respectively, the leaf oil has 28% oxygenated sesquiterpenes. Major constituents of the leaf are limonene (1), valerianol (2) and 1,8-cineole (3). The stem contains (E)-α-ionone (4), acorenone (5) and 2,6,10-trimethylpentadecane (6) while the root contains limonene, β-caryophyllene (7) and viridiflorene (8). The volatile oil of A. zygia, which was shown to contain hexahydrofarnesyl acetone (9), e-methyl isoeugenol (10) and 2-methyl tetradecane (11), has been reported for its anti-inflammatory potential [26]. The chemical structures of the isolated compounds are presented in Figure 3.

3.1.2. Albizia lebbeck (L.) Benth.

Albizia lebbeck is one of the commonly available species of the genus in Africa. It is commonly called the Siris tree, Igbagbo (Yoruba), Woman’s tongue, Flea tree, or Indian walnut [27]. It is a versatile and rapidly growing species found in rainforests and deciduous monsoon forests, as its natural habitat [28]. The herb has historically been used for the treatment of boils, pain, cough, flu, conjunctivitis, and inflammation [29]. Infusions from the bark and root are ingested to treat many conditions including scabies, itchy eyes, and piles among the Zulus of Southern Africa. In Nigeria, the aqueous extract of A. lebbeck is consumed to treat pain, fever, inflammation, and epilepsy [26,30]. The powdered leaves stem and roots of A. lebbeck are used to treat wounds, toothaches, yaws, and sores, as well as diarrhoea and fever [26,31].

Studies on the methanol and ethanol extracts of the plant have revealed antiprotozoal, antibacterial, antifungal, antidiabetic, and anticancer effects [32,33].

Albizia lebbeck contains non-toxic volatile oils that have been reported for both anti-nociceptive and anti-inflammatory activities owing to the presence of ionones, eugenol derivatives, and other compounds [26]. The main compounds that have been isolated from A. lebbeck are 2-pentylfuran (12), (E)-geranyl acetone (13), (E)-α-ionone (14), and 3-octanone (15) [26]. The chemical structures of the isolated compounds are presented in Figure 3.

3.1.3. Albizia ferruginea (Guill. & Perr.) Benth.

Albizia ferruginea, called Musase, West African Albizia, and Tanga-tanga, is a tree that can reach heights of 6 to 40 m and has a lovely spreading crown. It is a perennial, hermaphrodite medicinal plant that is found and widely used in several African countries, including Nigeria, Senegal, Sierra Leone, Togo, and Uganda. However, it is currently endangered as a result of deforestation [34]. A. ferruginea has been reported for its high protein and crude fibre contents. The tree, which serves as a forage legume for goats, also provides bees with nectar. Its branches are used as firewood for cooking. The bark is a vermifuge and astringent in traditional medicine, and a decoction of A. ferruginea is used to treat pain, fever, dysentery, bronchitis, and jaundice [35]. It can also be used as a wash to get rid of head lice and as a decoction to treat sores and pimples [19]. The infusion of the leaves of A. ferruginea is used to induce labour while the leaf decoctions are used in the treatment of toothaches and malaria by steam inhalation. The plant has been documented to contain some pharmacological activities such as antimicrobial, antioxidant, anti-inflammatory, and anthelmintic [25,36]. Methanol and ethyl alcohol extract of A. ferruginea (Guill) showed growth-inhibitory effects on selected organisms. They also demonstrated dose-dependent contractile effects on isolated uterine smooth muscles from non-gravid and gravid Sprague Dawley rats [34,37]. The ethanol extract showed anthelmintic properties against Pheretima posthuma and Haemonchus contortus [35].

The roots, bark, and leaves are rich in tannins, flavonoids, terpenoids, sterols, saponins, and alkaloids [38]. A new polyhydroxylated flavane was identified from A. ferruginea along with 4′,7-dihydroxyflavan-3,4-diol, julibrosides A1–A3 [38].

3.1.4. Albizia glaberrima (Schumach & Thonn) Benth.

Like other species in the genus, Albizia glaberrima is a deciduous tree, with an umbrella-shaped crown. The majority of tropical African countries where it grows are Kenya, Guinea Bissau, Mozambique, Angola, and Zimbabwe [39]. Its common names include White Nongo (Uganda), Okuro-fi (Ghana), Kolibangban (Ivory Coast), Keke-cama-cama (Guinea Bissau) and Ayunre (Southwest Nigeria) [40]. Different African nations use the wood of A. glaberrima, which is often used as firewood to make furniture, mortar and pestles, stools, farm implements, doors, beds, and carvings. In Nigeria, it is primarily grown for timber production, with massive gum exudates that are typically wasted [17]. Additionally, it is grown in Malawi and Uganda for nitrogen fixation as well as for shade in coffee, tea, cocoa, and banana plantations [41]. Depending on the country, various parts of A. glaberrima have been used to treat different ailments. For instance, in Nigeria, the bark is used topically to treat pyrexia; in Cameroon, the bark of the twig is used to treat blennorrhea, chest pains, and liver disorders; and in Tanzania, aqueous root bark extract is used for the treatment of bilharzia [39,42].

There are relatively few records of pharmacological studies on A. glaberrima based on alleged traditional applications. The aqueous leaf extract of A. glaberrima has dose-dependent anxiolytic/muscle relaxant (low dose) and sedative-hypnotic/anticonvulsant (high dose) effects, which may be mediated by an increase in GABAergic inhibitory actions [39].

Reducing sugars, tannins, terpenoids, steroids, flavonoids, phlobatannins, and phenols have all been found to be present in A. glaberrima. In a neuropharmacological study, saponins, tannins, and phenols were found to be responsible for the plant’s anxiolytic, muscle-relaxant, and anticonvulsant properties. From the root bark of the plant, a new 5-dehydroxyflavan named albiziaflavan B or (+)-(2R, 3S, 4R)-3′,4′, 7-trihydroxy-4-methoxy-2,3-trans-flavan-3,4-trans-diol has been isolated [43,44]. In addition, three existing flavans: (+)-fustin, butin (16), and (+)-mollisacacidin; two steroids: chondrillasterol (17) and chondrillasterone, and a triterpenoid: lupeol (18), have all been isolated from the plant (structures shown Figure 3). All six compounds showed antiproliferative effects on two human cancer cells, HeLa and HL60 [44].

3.2. Acacia

The most important genus of the Leguminosae family, Acacia, was first described by Linnaeus in 1773. Acacia species worldwide are thought to number around 1380 [45,46]. The species are often known as wattles. They are native to the world’s tropical and subtropical regions, including Africa and Australia, where they are recognizable landmarks on rangeland and savanna [47,48]. Several Acacia species are economically significant. Their bark, which is rich in tannins, is used to produce pharmaceuticals, inks, and other products as well as tannins [49,50]. Their unusually small, finely divided leaflets give their leafstalks an appearance of being feathery or fern-like, but they differ depending on the location [51,52]. Acacia species are also known for their tiny, fragrant flowers that grow in globular or cylindrical clusters, and their legume fruits, which vary greatly in appearance among the species [53]. To better reflect the phylogeny of Acacia, the genus has undergone several important taxonomic revisions. As a result, some of the species belonging to the genus are now classified as genera Senegal and Vachellia [45,54]. Acacia plants are extremely important for reforestation, including the restoration of wasteland, and soil improvement. Acacia seeds are utilized in a variety of products, including food.

The majority of the species contain significant amounts of secondary metabolites, primarily condensed tannins, flavonoids, and gums [55,56]. Acacia honey, produced by bees foraging on the Acacia flower, has a delicate floral flavour, a silky-smooth texture, and a glass-like appearance. It is used to sweeten beverages without changing their flavour [57]. Its high fructose content allows it to stay liquid for an extended period before it crystallizes [58,59,60].

3.2.1. Acacia nilotica (L.) Delile

Acacia nilotica, commonly called the gum Arabic tree, Bagaruwa (Hausa, Nigeria), is widely recognized as a multipurpose tree. It is a medium-sized, prickly, nearly evergreen tree that can reach heights of 20–25 m, but in unfavourable growing conditions, it may remain a shrub. It has yellow flowers and long, grey pods that are constricted between seeds [61,62]. It is among the species used for boosting soil fertility and has been introduced as fuelwood and forage for livestock. Due to its abundance of metabolites, A. nilotica has both nutritional and therapeutic advantages. In many parts of the world, traditional medicine uses all parts of A. nilotica to treat a variety of illnesses. It has been mentioned for the treatment of bone, mouth, and ear cancers as well as STDs. For the treatment of acute diarrhoea, the powdered bark of the plant is used with a small amount of salt.

The pods are believed to be antispasmodic and antidysenteric. Antibacterial properties of the plant have also been demonstrated [63,64]. A. nilotica has been studied for its potential medicinal benefits, including its anti-diabetic, hypolipidemic, anti-leishmanial, anti-plasmodial properties, as well as its molluscicidal property against the schistosomiasis-transmitting snails Bulinus truncates and Biomphalaria pfeifferi, and its cercarial and miracicidal properties against Schistosoma mansoni [65,66]. The extract from the plant has shown activity against S. aureus, E. coli and gonococcus. Gram-positive and Gram-negative bacteria were significantly inhibited by the plant’s methanolic extract, whereas ethanolic extract was more effective against Gram-positive bacteria only. The methanol and aqueous extracts of the bark and pods inhibited HIV-1 Protase replicate activity [67,68]. Additionally, it showed in vitro anti-plasmodial activity against the chloroquine-sensitive 3D7 and chloroquine-resistant and pyrimethamine-sensitive Ddz malaria parasites [69]. The extract significantly increased the reaction times of mice to a hot plate and prevented carrageenan- and yeast-induced pyrexia in rats [70]. Genetic toxicological research has established the antimutagenic and cytotoxic properties of A. nilotica. Studies in biology and epidemiology have shown that oxygen-based free radicals (ROS) interfere with the normal operation of macromolecules and are part of several biochemical processes (DNA).

A. nilotica polyphenolics have recently been proposed as a treatment for arsenic-induced neurotoxicity because of their potent antioxidant potential. The addition of these antioxidants to functional foods can stop the production of oxidative species, ultimately preventing the development of certain diseases [71,72]. A. nilotica demonstrated antimutagenic activity in test strains of Salmonella typhimurium against the direct mutagens 4-nitro-o-phenylenediamine (NPD) and sodium azide (NaN₃) as well as the indirect mutagens 2-aminofluorene (2-AF) [73].

A. nilotica has been reported to contain alkaloids, fatty acid flavonoids, gum, polysaccharides, organic acids, and tannins. The gum yields between 2.7 and 4% of ash that is almost entirely composed of calcium, magnesium, and potassium. It also contains sugars like galactose, L-arabinose, L-rhamnose, 4-aldobiouronic acids, and arabinobioses [64]. The gum contains 12–17% moisture and 2.7–4% ash. The majority of it is made up of arabin, a mixture of arabic acid and calcium. Gallic acid (19), ellagic acid (20), (-)-catechin (21), (-)-epigallocatechin-7-gallate, m-digallic acid (22), (+)-catechin, chlorogenic acid (23), gallolyated flavan-3,4-diol and robidandiol (7,3′,4′5′,-tetrahydroxyflavan-3,4-diol) are among the polyphenols that are abundant in A. nilotica. In addition, it has 44.5% linoleic acid and 29% oleic acid. 3-acetoxy-17-hydroxyandrost-5-ene (24), a steroid with dose-dependent anti-inflammatory activity, has reportedly been found in the aerial parts of A. nilotica, whereas coronaric acid (cis-9, 10-epoxyoctadec-cis-12-enoic) (25) has been isolated from the seed. The species contains the alkaloids 5-methoxy dimethyltryptamine (5-MeO-DMT) (26), N-methyltryptamine (NMT) (27), and dimethyltryptamine (DMT) (28). The chemical structures of the isolated compounds are presented in Figure 4. Crude protein, crude fibre, stearic acid, vitamin C (ascorbic acid), carotene and selenium are all abundant in different parts of the plant [74].

3.2.2. Acacia auriculiformis (L.)

Acacia auriculiformis, commonly referred to as black wattle, Auri, Earleaf acacia, and Earpod wattle, is a species of Acacia that is commonly found in Australia, Papua New Guinea, and Indonesia. It was introduced to Africa about 50 years ago [75]. It is a medium-sized, prickly, nearly evergreen tree that, in its natural habitat, can grow to a height of 20–25 m on favourable sites but may only become a shrub under unfavourable growing conditions [76]. It has been shown that an aqueous extract of A. auriculiformis can be used to dye and finish textile substrates made of both proteins and cellulosic materials simultaneously. Textiles dyed with A. auriculiformis have excellent colourfastness to washing and good light fastness qualities [77].

Acacia auriculiformis is a significant ethnomedicinal plant that has historically been used for the treatment of many health conditions, including itchy skin, rashes, itchy eyes, aches, and rheumatism [78]. The Ibibio people of Nigeria use it to treat malaria while the Australian Aborigines use a decoction of the plant’s root for the treatment of rheumatism and the plant’s bark infusion for the treatment of aches and sore eyes [79,80]. Additionally, the tree’s seeds are employed to treat a variety of skin conditions, including rashes, allergies, and itching [79,80].

Acacia auriculiformis has been researched for a variety of pharmacological activities, including antioxidant, antimalarial, antidiabetic, antimicrobial, anti-filarial, cestocidal, central nervous system depressant, antimutagenic, chemopreventive, spermicidal, hepatoprotective, and wound healing [79,81,82]. The antioxidant activities of different plants part of A. acuriculiformis using DPPH free-radical-scavenging assay activity, hydroxyl radical assay, reducing power assay, linoleic acid emulsion system assay, ABTS assay, metal chelation and antihemolytic activity assays have all been documented [79,81,82,83]. The extracts showed significant dose-dependent radical scavenging power and reducing power for hydroxyl radicals and DPPH [81,83,84]. Organic extract from A. auriculiformis has been shown to have antimutagenic and chemoprotective properties. It has also been shown that the extracts of ethyl acetate and acetone can scavenge free radicals [85]. The effectiveness of the bark and pod extract of A. auriculiformis as a hepatoprotective and anti-diabetic agent was demonstrated in animal models with paracetamol-induced liver injury and alloxan-induced diabetes [79].

Several phytochemicals have been reported from A. auriculiformis including three new triterpenoids and a new triterpenoid trisaccharide discovered by Garai et al. [86]. Botulin, a triterpenoid, was also recently discovered in the stem bark of A. auriculiformis [82]. Two acaciasides isolated from the funicles of A. auriculiformis, acaciaside A and B (33), have significant antifungal and antibacterial activities [87]. At a concentration of about 300 g/mL, the substances prevented the growth of the fungi Aspergillus ochraceous and Curvularia lunata. Teracacidin (29) and 3,4,7,8-tetrahydroxyflavanone (30), two additional isolated compounds (Figure 4), demonstrated strong antifungal activity. Both fruit extracts and a tetrahydroxy flavone isolated from the stem bark of A. auriculiformis have been shown to have protein kinase inhibitory activity. Acaciaside-B (Ac-B), which was isolated from A. auriculiformis, has been granted a patent for use in vaginal contraception and HIV prevention [78]. A combination of two partially triterpenoid saponins (Tg), isolated from the species, has demonstrated sperm immobilizing activity [88]. A. auriculiformis bark contains betulin (36) has been shown to be a multi-target protein kinase inhibitor, an activity that may be related to the plant’s anticancer properties [82].

3.2.3. Acacia senegal (L.) Willd

Acacia senegal, referred to as white gum tree and gum-arabic acacia, is native to the Sahelian and Sudano-Sahelian regions. It is found throughout the drier regions of tropical Africa, Oman, Pakistan, and India, as well as Egypt, Australia, Puerto Rico, and the Virgin Islands [89,90]. Amongst the four varieties of this species; Var. Senegal, Var. kerensis Schweinf., Var. leiorhachis Brenan, and Var. rostrata (Sim) Brenan; Var. Senegal is the most prevalent, and it is the main source of gum arabic (GA). A. senegal is a 15 m-tall deciduous shrub or a small- to medium-sized tree which is used to improve soils, as fuel, and for local architecture [91]. The gum, which is emitted from trees in response to environmental factors such as heat, low soil fertility, drought, or damage, is traded globally. It is useful as a confectionery in the food processing industry, as an emulsifier, as film coating, and for other applications. In ethnomedicine, gum Arabic is used to treat chronic renal failure, digestive discomfort, and so on [92]. Even though acacia gum was once thought to be an inert substance, a recent study has revealed that it has numerous pharmacological and medicinal effects including weight loss, antihypertensive, anti-inflammatory, antihyperlipidemic, anticoagulant, antidiabetic, antibacterial, nephroprotective, and other properties [92]. The gum and seed are used as food and drink; infusions of the gum are used as sedatives; the bark is used for arthritis and rheumatism; the gum, bark and root are used for venereal diseases; the seed is used for leprosy [93,94,95,96,97]. Although A. senegal is best known for its gum arabic, other parts of the plant have also been used in medicines and other applications. A. senegal has historically been used to treat abscesses, boils, trypanosomiasis, stomach aches, abscesses, respiratory tract infections, diarrhoea, haemorrhoids, and sexually transmitted diseases [93].

A. senegal extracts from various parts have been shown in studies to have pharmacological effects against a variety of human diseases [93,94,95]. It has been noted that some Mycobacteria species can be treated with the bark extract of A. senegal to prevent tuberculosis [98]; the leaf extract showed activity against Pseudomonas aeruginosa using the TLC autobiography method [99]; using the agar well diffusion method, the seed extract was significantly effective against one Gram-positive and six Gram-negative bacteria [100]; the hexane extract of the leaves improved the susceptibility of bacteria to phenol antibiotics [96]; and the stem bark extract was significantly active against pathogenic bacteria of the respiratory tract such as Klebsiella pneumoniae, Streptococcus pneumoniae, Pseudomonas aeroginosa, Staphylococcus aureus and Escherichia coli [101]. The crude methanol extract of the leaves possessed a significantly higher total phenol content as it demonstrated antioxidant activity by scavenging ABTS and DPPH radicals and offered promise as a natural antioxidant treatment for oxidative-stress-related diseases [102]. When antimicrobial silver nanoparticles (AgNPs) were synthesized using aqueous leaf extracts, the resulting AgNPs had very potent inhibitory effects on several chosen Gram-positive and Gram-negative bacteria [103]. In addition, the dried gummy exudate of the plant demonstrated cytoprotective, antioxidant, and resistance to hepatic, renal, and cardiac toxicity in experimental rats. In sickle cell anaemia and other conditions marked by oxidative stress, gum arabic also showed strong anti-oxidative properties [104,105].

3.3. Leucaena

Leucaena, known as lead trees, is a genus of flowering plants in the mimosoid clade [106]. They are originally from the Americas and are grown for a variety of uses, and some species have edible fruits and seeds such as Leucaena leucocephala, which is the only species that grows in Africa [107]. Leucaena species are cultivated for a range of applications, including green manure, charcoal production, animal feed, and soil preservation.

Leucaena leucocephala (Lam.) de Wit

Leucaena leucocephala is commonly called white lead tree, horse tamarind, white popinac, Leucaena, or wild Tamarind (English) [108,109,110,111,112]. Since the time of the Mayans, almost the entire plant has been used for food and traditional medicine, and in some regions of the world, it is regarded as an exotic species [113,114]. Young leaves, flowers, and pods of L. leucocephala are consumed in soups and salads in Central America, Thailand, and Indonesia, while in the Philippines, the seeds are roasted and used as an alternative for coffee or popped like popcorn, and the young pods are eaten as a vegetable [114,115]. The seeds are regarded as non-traditional sources of protein [114,115,116] and the bark is consumed to treat internal discomfort. A decoction of the bark and root is prepared in Latin America and used for hair removal and as a contraceptive [117]. L. leucocephala has been used for construction poles, firewood, and a source of shade in long-term plantations [118], or the manufacture of pulp and paper [119,120], production of wood [118,121], green manuring [122,123], phytoremediation and revegetation of contaminated areas including fly ash basins [124,125], prevention of slope failure [126], and as a source of energy and chemicals using autohydrolysis [127].

L. leucocephala has various medicinal uses and its nematicide, pesticide, hypocholesterolemic, hepatoprotective, diuretic, antimicrobial, anthelmintic, antibacterial, antidiabetic, anticancer, anti-inflammatory, antitumor, antihistaminic, antiandrogenic, antioxidant, and anti-proliferative activities have been demonstrated [114,116,128]. Alopecia, cataracts, goitre, growth restriction, decreased fertility, and mortality in non-ruminants are all brought on by the poisonous, non-protein amino acid found in Leucaena, mimosine (31). It has been hypothesized that the mechanism of mimosine’s toxicity involves blocking the metabolic pathways of tryptophan and aromatic amino acids, chelating metals, counteracting vitamin B6′s effects, inhibiting DNA, RNA, and protein synthesis, negatively impacting collagen biosynthesis, and interfering with the metabolism of some amino acids, primarily glycine [129]. Other pharmacological activities attributed to the leaf extract of L. leucocephala include antibacterial (against Staphylococcus aureus and Escherichia coli), antidiabetic, anti-inflammatory, anticancer (particularly for oral cancer), and larvicidal effects [130,131,132,133,134,135]. The seed extracts, on the other hand, have been found to exhibit antioxidant and anthelmintic activities [135,136,137].

Mimosine is only one of many toxic and anti-nutritional components in L. leucocephala; other components also increase the toxicity of the species. Ruminants, because of the presence of rumen microbes, can frequently convert toxic substances into a usable form that is not toxic, thus causing mimosine and tannins not to be as deleterious as they normally would have been [138]. The seed typically contains more mimosine than the leaf [139]. However, Yadav and Yadav [140] reported the highest mimosine content in young shoots of L. leucocephala followed by seeds and leaves [141,142].

The major components that have been isolated from the leaves are 1,2-benzenedicarboxylic acid (32), betulin, lupeol, 9,12,15-octadecatrienoic acid (33), betamethasone (34), and β-sitosterol (35) while those from its fruits are β-sitosterol, 1,2-benzenedicarboxylic acid, lupeol, betulin, stigmasterol (36), and campesterol (37) [116]. The chemical structures of the isolated compounds are presented in Figure 5. Studies have shown that L. leucocephala leaf extract at doses of 0.54 g, 0.72 g, and 1.08 g demonstrated analgesic effects comparable with paracetamol in male white mice [135].

3.4. Senna

Senna is a genus of roughly 250–300 shrubs and trees endemic to temperate regions of all continents, including Australia and South America [143]. The genus is commonly called popcorn cassia. Many species of the genus Cassia are included in this genus. Some of the species are used as ornamental plants for parks and gardens. Many species of Senna are underutilized with about 95 species of the genus captured on the IUCN Red List of threatened species. Most members of the genus can be found in temperate and sub-tropical regions of the world [144]. The taxonomy of Senna as a group of plants has been described as puzzling because of extreme morphological variability and unclear limits between the taxa. The genus contains a wide range of anthraquinones, flavonoids, anthraquinone glycosides, and flavonoid glycosides, which vary from one species to the other [143].

3.4.1. Senna didymobotrya (Fresen) Irwin & Barneby

Senna didymobotrya (Synonym: Cassia didymobotrya Fresen.) is a native African flowering shrub popularly known as African Senna [145]. Its common names are African wild sensitive plant and Candelabra tree. It grows well in rich, moist and well-drained soil with direct sunlight. S. didymobotrya is an elegant, fast-growing shrub with evergreen leaves that are deciduous in cold weather and give off the smell of roasted corn if rubbed. It has beautiful yellow flowers that bloom twice a year [146]. It is frequently employed as a laxative and purgative in East Africa, owing to the presence of anthraquinones and their glycoside derivatives. When given as a root or leaf decoction, S. didymobotrya promotes lactation and induces uterine contractions and abortion [147].

Phytochemicals like alkaloids, saponins, tannins, steroids, terpenoids, flavonoids, and anthraquinones have all been reported from different plant parts [148,149]. Isreal et al. [150] reported the isolation of a resveratrol derivative; 2, 6, 4′- trihydroxy-trans-stilbene (38) and a phenyl anthraquinone derivative; 4-(2′-oxymethylene-4′-hydroxyphenyl) chrysophanol, which was not only identified in S. didymobotrya for the first time but also in the genus.

3.4.2. Senna hirsuta L.

Senna hirsuta is a shrub that grows widely in South India’s Eastern and Western Ghats. It is a common plant in Gabon, Congo, and Nigeria, and the boiled seeds are generally eaten in India and Africa [151]. To prevent plaque and cavities, the powdered seeds are massaged into the teeth and gums [152]. The root has been used for treating elephantiasis in Congo [153] while the leaf decoction has been reported for treating hepatic diseases in Gabon [154]. In Nigeria, it has a variety of uses including haematuria, rheumatism, dysentery, stomach troubles and abscesses [155]. The seeds of S. hirsuta have been reported to contain crude protein, lipid, fibre, and nitrogen-free extractives, which make them a good source of calories.

Significant amounts of total free phenolic compounds with potential anti-oxidant and enzyme inhibition properties in type II diabetes were discovered in the methanol extract of S. hirsuta seeds [151]. The species has been shown to have cytotoxic activity against human lung fibroblast (MRC-5) cell lines and antiprotozoal activity against Plasmodium falciparum, Trypanosoma brucei, and Trypanosoma cruzi [153].

The nutritional components of S. hirsuta have been extensively discussed in the literature, but very limited reports regarding its chemical constituents have been carried out. S. hirsuta seeds also contain iron, calcium, manganese, sodium and magnesium, which are important micro-nutrients [156]. In addition to these, the seeds of S. hirsuta have also been found to contain some antinutritional substances, including tannins, 3,4-dihydroxyphenylalanine (L-dopa, a toxic amino acid that is not found in proteins) (39), and phytohaemagglutinins (lectins). A triterpenoid, 3β,16β,22-trihydroxyisohopane; and a new bianthraquinone, 4,4′-bis (l,3,8-trihydroxy-2-methyl-6-methoxy anthraquinone); have been isolated from the seeds of S. hirsuta [151].

3.4.3. Senna podocarpa (Guill. & Perr.) Lock

Senna podocarpa (Synonym: Cassia podocarpa Guill. et Perr.), called Asuwon (Yoruba) and Agelo-Oglala (Igbo), is one of the six medicinally useful species of Senna. It is a smooth almost hairless shrub that grows up to five metres high and is widely distributed in West Africa, especially in the Savanna forest area of the region [157]. The plant is readily available in Nigeria in ancient farmlands in the western and northern parts. S. podocarpa has traditionally been used to treat several health conditions including skin diseases including, eczema, scabies and ringworm. It has also been used in folklore as a labour inducer, purgative, anti-gonorrhoea medicine and guinea worm expeller. The leaf infusion or decoction is also given as a mild laxative while a large dosage of it acts as a purgative [158].

Isaac et al. [157] produced an emulgel formulation of the plant extract, which offered superior pharmacological activity in comparison with the S. podocarpa poultice used traditionally for wound healing. The 7.5% emulgel formulation demonstrated an optimal effect in the inhibition of oedema and burns in addition to promoting wound healing. In addition to its wound-healing property, S. podocarpa has also been reported for its antimicrobial, antiviral and anti-inflammatory activities [159].

Senna podocarpa contains phytochemicals such as saponins, tannins, anthraquinones, phlobatannins, phenolics, flavonoids and alkaloids, among others [160]. Some chemical compounds identified from the leaves and pods of S. podocarpa include chrysophanol, rhein (40), emodin (41), sennoside A (42) and sennoside B (43) [161]. The chemical structures of the isolated compounds are presented in Figure 6.

3.4.4. Senna occidentalis L.

Senna occidentalis (Synonym: Cassia occidentalis) is also a small shrub that grows to about three feet tall. Although it is indigenous to the tropics of America, it has been recognized as a native species in Australia, Africa, and the southern and eastern United States. It is a weed that thrives in fields where cereals like corn and soybeans are grown as well as in pastures close to fences. All the plant parts are used in herbal medicine.

The plant contains different classes of phytoconstituents including alkaloids, flavonoids, tannins, saponins, terpenes and glycosides, which are believed to be responsible for its pharmacological effects. S. occidentalis has been reported for its anti-inflammatory, antioxidant, immunosuppressive, antihepatotoxic, antibacterial, antifungal and antiplasmodial activities [162,163]. Despite the reports on the pharmacological potentials of this species, information on the active constituent for specific functions is limited.

3.4.5. Senna obtusifolia (L.)

Senna obtusifolia, commonly known as Cassia or Java bean, is an annual or perennial viable woody shrub. It is a common plant that grows wild in many parts of Nigeria and generally in Africa, with the exception of Madagascar [164]. As a weed, S. obtusifolia is widespread and has been ranked as one of the most pervasive weeds in major crops such as maize, peanut and soybean [165]. The young leaves of S. obtusifolia have been regarded as a native leafy vegetable eaten by the rural population in much of Africa. Frequent or excessive consumption of the older leaves has been found to cause watery stool. This could also be related to its laxative effects, which also extend to the seeds and roots [164]. The leaves are used for fever and the treatment of scorpion stings and gingivitis [166]. S. obtusifolia also serves as a valuable substitute for locust beans, and the mucilage from its seed has a variety of uses in the industry.

The extracts of S. obtusifolia have phytoconstituents including saponins, terpenoids, steroids, phlabathannins, tannins, alkaloids and flavonoids depending on the solvent used for extraction (Doughari et al., 2008). The nutritional composition of the leaves and seeds include protein, fat, carbohydrate, fibre, sodium, copper, manganese and calcium [164]. In addition, the nutritive value of seeds has been reported to be higher than the leaves, and this further supports their use as food, feedstuff and a supply of important nutrients for livestock.

Obtusifolin (44), an anthraquinone extracted from the seeds of S. obtusifolia, has been reported to inhibit high-glucose-induced mitochondrial apoptosis, attenuate memory impairment, and suppress breast cancer metastasis to bone [167]. Additionally, it also inhibited the NF-κB pathway in airway epithelial cells [167]. Pang et al. [168] reported the isolation of a new anthraquinone analogue, obtusifolin-2-O-β-d-(6′-O-α, β-unsaturated butyryl)-glucopyranoside, and a new eurotinone analogue, epi-9-dehydroxyeurotinone-β-d-glucopyranoside, from the seeds of S. obtusifolia, with the new anthraquinone analogue exhibiting a stronger specific inhibitory effect on human organic anion transporter 1 (OAT1).

3.5. Canavalia

Canavalia is regarded as belonging to the third-largest family of flowering plants, which consists of about 50 species of tropical vines that are found throughout tropical and subtropical parts of the world [169,170]. Due to their nutritional value to humans, they have a long history of being consumed as food. Similar to other edible legumes, they have been documented to be a rich source of carbohydrates, proteins, fibre, essential amino acids, and fatty acids. Canavalia seeds have nutritional qualities that are on par with or better than those of other food grains like rice, wheat, and soybean [171,172]. According to reports, the high nutritional value of Canavalia species sprouts, which can be further improved with Molybdenum ion (Mo) treatment to increase their nutritional and pharmacological values, makes them extremely suitable for commercial use [173]. The utilization of some species within the genus is, however, low given the knowledge of their nutritional advantages.

3.5.1. Canavalia ensiformis (L.) DC.

Canavalia ensiformis is commonly called Jack beans as well as Sese nla (Yoruba, Nigeria) [174]. Despite being native to South America and the West Indies, where it is commonly used as green fertilizer, it is found throughout the tropics [175] Due in part to its high levels of anti-nutritional factors, it is one of the legumes that is underutilized in the production of animal feed. In traditional medicine, C. ensiformis seed powder or decoction has been used as an antibiotic and antiseptic. Western nations use it as a cover crop while the roasted seeds are pulverized to make a coffee alternative [176]. In Indonesia and China, heat-treated seeds and pods are used as medicine [177].

Due to the presence of toxic and antinutritional substances like concanavalin A and B, the C. ensiformis bean has been underutilized as a food despite its high adaptability to unfavourable conditions [178,179]. However, germination and chemical treatment, as well as heating and non-heating processes, can effectively lessen the content of these toxic components [180]. C. ensiformis seeds contain several bioactive compounds with specific applications in tissue markers, immunostimulation, and blood grouping. Despite the few reports that have been made on C. ensiformis, it is among the legumes that have not been fully exploited.

C. ensiformis seeds have saponins, flavonoids, and alkaloids along with some polyphenols that have antioxidant properties and may lower the risk of diabetes and other chronic diseases [181,182]. The predominant jack bean polyphenolics, which are thought to have antioxidant and other bioactivities, were identified and quantified by Suteda et al. [183]. One of the four kaempferol glycosides found was kaempferol 3-O-α-l-rhamnopyranosyl (1→6)- β-d-glucopyranosyl (1→2)-β-d-galactopyranosyl-7-O-[3-O-o-anisoyl]-α-l-rhamnopyranoside, a novel compound. When compared to acarbose, this new substance effectively inhibited α-glucosidase better.

3.5.2. Canavalia gladiata (Jacq.) DC.

Canavalia gladiata, commonly called Sword bean, is an underutilized edible bean widely cultivated in Africa, where it is used mostly as a cover crop, forage, green manure and ornamental plant [184]. C. gladiata is a strong perennial climber that is frequently grown as an annual crop, primarily for its seeds and seed pods [185,186]. Additionally, it is grown as a decorative climber on fences and houses [187,188]. Although C. gladiata is a lesser-known and underutilized edible bean, it has been consumed and used medicinally in China for many decades. It contains a variety of nutrients, including proteins, carbohydrates, vitamins, and minerals [189]. The dry, fully mature seeds are poisonous, but the young, green pods are widely consumed in Asia and Africa and served as a boiled green vegetable. In Korea, C. gladiata is used to treat inflammatory conditions and swelling, epilepsy, schizophrenia, asthma, obesity, stomach aches, dysentery, coughs, headaches, intercostal neuralgia, and kidney-related diseases [190,191]. The fruits and seeds of the plant are sources of nutrients, such as amino acids, fatty acids, proteins, fibres, and functional substances with noteworthy nutraceutical properties [192].

The pharmacological properties of C. gladiata, including its anti-inflammatory, hepatoprotective, anti-angiogenic, and antimicrobial effects, have been reported in the literature [189,193,194]. According to reports, C. gladiata is comparable to soy and black beans in terms of its high total phenolic and flavonoid content [195].

Several studies have revealed the presence of some bioactive substances in C. gladiata, including ent-kaurane-type diterpene glycoside (canavalioside), gallic acid derivative, and various flavonol glycosides (gladiatosides) [196,197,198]. According to An et al. [199], eighteen compounds were isolated from C. gladiata, out of which there were two novel compounds, namely kaempferol-7-O-α-l-dirhamnopyranosyl(1 → 2;1 → 6)-O-β-d glucopyranosyl(1 → 2)-O-α-l-rhamnopyranoside and (Z,1R,7S,9S)-7-hydroxy-11,11-dimethyl-8-methylenebicyclo [7.2.0]undec-4-ene-4-carboxylic acid. Some of the compounds identified include trilinolein (45), dillenetin (46), canavalioside, and lupeol, among others. Similarly, Dinda [200] reported the isolation of gladiatin (47) and a phenolic compound, methyl gallate (48), from the methanolic extract of C. gladiata. The chemical structures of the isolated compounds are presented in Figure 7.

4. Conclusions

The review of the underutilized legumes in the subfamily Caesalpinioideae showed that most of the legumes in this subfamily have little or no potential for use as food for humans. They are more suitable as fodder for animals. In addition, extracts of the different plant parts are used for the treatment of various diseases, and studies have shown that they contain phytoconstituents of medicinal importance, some of which have been isolated and characterized. The medicinal properties that have been reported for the legume species include anti-inflammatory, hepatoprotective, anti-angiogenic, and antimicrobial effects, etc. Therefore, phytoconstituents derived from plants in the Caesalpinioideae subfamily might serve as a source of lead compounds for the development of new drugs for the treatment of a wide variety of diseases. There is, however, a need for the full characterization, quantification, biological assay and toxicity analysis of the phytoconstituents to explore the legumes in the Caesalpinioideae subfamily for health benefits.