Forest trees provide valuable resources for economic, environmental, and industrial development. Indeed, these plants sustain human life directly or indirectly, which supply a wide range of goods and ecological services essential for survival and prosperity. The medicinal plants used in traditional medicine all over the world are a potentially rich source of therapeutic compounds. Population increase along with rapid technological advances are putting tremendous pressure on natural genetic resources, especially in developing countries, where such resources are rapidly declining, and more species face extinction [1]. The more than 70 species in the pantropical genus Pterocarpus (Fabaceae) are also faced with development pressure [2]. The use of different plant parts in Pterocarpus spp. to treat illnesses since ancient times has been well documented [3,4,5,6,7,8]. The species has received much attention in experimental studies because of the growing evidence of potential bioactivities. The increasing demand for wood has led to unsustainable harvesting from wild sources of three species of Pterocarpus, namely Pterocarpus marsupium, P. santalinus, and P. indicus, which are now recognized as threatened species [9]. Besides the pharmaceutical value of Pterocarpus species, the wood is also valuable for bridge and boat building, as well as small-scale construction materials, plywood, veneer, and specialty wood for musical instruments [10].

The increase in public awareness of phytochemical-based drugs and the rapid growth of plant-based pharmacological industries have led to a greater demand for, and overexploitation of, natural flora. An increase in subsistence or non-commercial harvesting, as well as recent climatic changes have harmed many plant species, including Pterocarpus spp. One possible biotechnical response to the decline of some species is micropropagation, which capitalizes on the totipotent nature of plant cells [11]. In vitro propagation through tissue culture plays an important role in mass multiplication, plant improvement, plant breeding, regeneration of elite or superior clones, exchange of planting materials, secondary metabolites production, and germplasm conservation [12,13]. However, conventional propagation practice is time-consuming and labor-intensive and requires the availability of many plants for plantation or afforestation. Notwithstanding these challenges, plant cell, tissue, and organ culture techniques will become increasingly important in the cultivation of medicinal, as well as aromatic, plants by providing healthy and disease-free planting stock that can be widely utilized in commercial propagation and reforestation programs [14,15]. The information on actual genetic structure and the cryptic number of the differentiated genetic resources are valuable aids not only for developing in vitro regeneration protocols but also for the conservation of these medicinal plants. Many adulterants of Pterocarpus plant materials are available in the market, and they affect the efficacy of the drug; in some cases, these adulterants might be toxic and can prove to be lethal. A DNA barcoding technique can be effectively utilized to characterize and authenticate Pterocarpus and the detection of adulterants [16]. This technique is an alternative for rapid and robust species identification of genuine plant materials for the herbal drug industry [17].

P. marsupium propagates only by seed; the germination rate has been reported to be less than 30%, apparently because of the hard fruit coat coupled with poor viability and pod setting [18]. The mature fruits are harvested from the trees in April and May or before they drop to the ground. Pathogenic infections of fallen fruit also affect the germination rate under natural conditions [19]. Ahmad [20] recommended freshly collected seeds as a good planting source for obtaining healthy plantlets. The oleo-resin exudates of this species contain several unique active constituents, including vijyayosin, pterosupin, marsupsin, and pterostilbene, all of which show a wide range of pharmacological activity [21]. In addition, the National Medicinal Plant Board (NMPB) of India has estimated that the annual trade value of P. marsupium is approximately 300–500 metric tons per year and that each mature tree (10–15 years) produces approximately 0.5–0.6 tons of dry heartwood, valued at US $ 1200–1500 for each mature tree in the international market. Due to the aforementioned increased interest in recent years in the pharmaceutical, as well as the economic value of P. marsupium, the government of India has begun to encourage programs for large-scale cultivation and conservation of this species. The high demand for oleo-resin and wood, unsustainable harvesting practices, anthropogenic threats, and lack of regeneration, have together resulted in the rapid decline of natural populations. Furthermore, illegal harvesting of oleo-resin by damaging or wounding the wood can cause the trees to be susceptible to pests or diseases. Thus, uncontrolled extraction of oleo-resin could lead to adult mortality in combination with fragmentation and a low regeneration rate, threatening the persistence of the species.

Despite the enormous ethnobotanical value of P. marsupium, there is limited information on its genetic structure. Data on intraspecific relatedness is vital for selecting the best genotypes for plant breeding, effective population management, and conservation of germplasm. Given that genetic diversity allows populations to adapt to changing environments, the investigation of the genetic diversity of P. marsupium is not only important for species conservation, but also for the development and utilization of germplasm for improvement of this valuable but threatened medicinal plant. P. marsupium is a highly valuable medicinal plant but is a threatened species in India. Efforts for the conservation and propagation of this tree species will ultimately lead to the development of policies to make the species available for general use at a low cost. In view of these facts, it is of paramount importance to develop biotechnological techniques that ensure rapid propagation, multiplication, and authentication of the species for optimal germplasm conservation.

2. Phytochemistry and Therapeutic Values of P. marsupium

2.1. Active Constituents

A large number of important phytochemicals, such as glucosides, sesquiterpene and vijayoside (Table 1) have been isolated from aqueous extract of heartwood of P. marsupium [30]. The extract of heartwood contains pterostilbene (Figure 2A, pterosupin) (Figure 2O, marsupsin (Figure 2M), and liquiritigenin (Figure 2N), (−)-epicatechin) (Figure 2H) [31,32]. Bark extract contains several reputed phytochemicals such as 3-o-methyl-D-glucose (Figure 3C), n-hexadecanoic acid (Figure 3D), 1,2-benzenedicarboxylic acid (Figure 3E), tetradecanoic acid, (Figure 3F), 9,12-octadecadienoic acid (Z,Z) (Figure 3G), D-friedoolean-14-en-3-one (Figure 3H), and lupeol (Figure 2J) [33]. At least eleven bioactive compounds, namely pterocarposide (Figure 2P), 2,6-dihydroxyphenyl glucopyranoside (Figure 2Q), pteroside (Figure 3I), vijayoside (Figure 3J), pterosupol, marsuposide, epicatechin, quercetin (Figure 3M), vanillic acid (Figure 3N), formononetin (Figure 3O), and naringenin have been extracted from the heartwood of P. marsupium [21]. The most important bioactive compounds extracted from P. marsupium are presented in Table 1. Structures of bioactive compounds of P. marsupium are shown in Figure 2 and Figure 3.

Figure 2. Molecular structure of bioactive compounds extracted from Pterocarpus marsupium Roxb.