Plants are capable of producing a vast array of secondary metabolites by circumventing their primary metabolic flux. Primarily a coping mechanism for the plants to ward off biotic and abiotic stresses, these secondary metabolites have multifarious activities, being potent antioxidants, antibiotic, antifungal, antihelminth, antiherbivory agents [9]. Though it is hard to assess the total number of plant secondary metabolites, conservative estimates indicate that around 50,000 have been characterized which accounts for only a small fraction of the total possible trove of phytochemicals [20]. Since the dawn of human civilization, plants have been sourced not only as food, but also as medicine, and as the sources of colorants and perfumes. Trading in plants with medicinal properties flourished as humans travelled to different corners of the globe. For a better part of the three millennia of recorded human history, traditional systems of medicines were almost entirely based on plant extracts. Archeological evidences from the ancient civilizations of Egypt, Mesopotamia, Mayans, etc., show an amazing continuity with the traditional tribal herbal knowledge, which is also codified in the written treatises like the Ayurveda and Nei Ching or Canon of Internal Medicines [1]. With the advent of synthetic chemistry, as identification of individual compounds and their mass production came to be economically more viable than whole plant trade, traditional plant-based remedies lost scientific attention and were relegated as “poor man’s medicine”. Still > 85% of certified drug molecules used as modern medicine are derived from plants. The last century saw an explosion of human ailments, and modern medicine could only offer partial remedy. Driven by the exemplary augmentation of our knowledge of human physiology and metabolism in the last few decades, scientists have realized that a multi-pronged individual-specific strategy is necessary to combat complex diseases like various types of cancer, hepatitis. This has led to a re-alignment of research interest towards the medicinal properties of plant extracts. Buttressed by modern genomic and metabolomic techniques, exploration of medicinal and aromatic plants stands at an exciting juncture. This special issue theme was conceptualized to present the current progress on medicinal and aromatic plant research towards exploring their novel potentials.
Burgeoning human population has fueled a huge demand for medicinal plants. Climate change and environmental degradation have severely depleted wild populations of many such beneficial plants. Though a few economically important medicinal plants were adapted for cultivation, variability in the secondary metabolite production severely limits their economic prospects. Traditional breeding techniques cannot be successfully implemented in medicinal plants, as most of these plants are polyploids, showing a vast range of variations in their economically important quantitative traits [21]. Molecular breeding techniques have the potential of expediting genetic improvement of medicinal and aromatic plants that needs to be validated, especially since genome sequencing has become operationally and economically feasible, as reviewed by Chaturvedi et al. [5]in the current issue.
An in-depth and integrated understanding of the metabolomic, genomic and transcriptomic aspects of medicinal plants is necessary to ascertain the underlying causes of this inherent variability and formulate strategies for judicious exploitation of natural resources [18]. Genetic diversity studies have earlier yielded contra-indicative variability and phylogenetic relationship data. However, they are still relevant in elucidating the extent of genetic and chemosynthetic potentials of plant populations which are used to identify genetically stable, high yielding populations. This is exemplified in the current issue by the genetic diversity analysis of the important Ayurvedic plant, Prishniparni [17].While neutral molecular markers did generate a basic understanding of the extent of genetic variations among and within populations, this data did not reveal whether these variations had evolutionary and/or adaptive implications. Modern techniques like DAMD and ISJ markers or transcriptome derived SSR markers generate crucial information about not only the existing genetic variation, but also indicating whether these variations may impact secondary metabolite profiles as the populations undergo short-term adaptive changes or long-term evolutionary changes. Warlarphih et al. [22] have reported the importance of using DAMD and ISJ markers for species-level discrimination of Hedychium, while Ma et al. [12]have used transcriptome derived SSR markers for DNA fingerprinting and calibrating population-level genetic diversity in Atractylodes chinensis. Compounded with metabolomic data, this genomic dataset yields the blue-print of expected chemical variations, which can then be manipulated using genome modifications and metabolic engineering approaches.
With the establishment of in vitro tissue and cell culture techniques, scientists have developed novel strategies for rapid multiplication and secondary metabolite manipulation of medicinal plants [14]. Biomass and secondary metabolite production has been augmented in a number of medicinal plants through in vitro manipulations and some protocols have been implemented on industrial scale. In vitro rapid propagation of high yielding economically important plants can also help in their conservation deterring arbitrary collections of many endangered plants from the wild. Moreover, stable and increased production of target phytochemicals helps in managing the supply–demand curve for important metabolites catering to the drug industry. However, every plant needs a specialized protocol to maximize biomass and phytochemical turn-over depending on their genotype and tissue-specific metabolite production profiles. Hence, developing efficient and economically viable protocols for important medicinal and aromatic plants needs protracted and consistent investigations. Rapid and efficient propagation protocols for important medicinal and aromatic plants have been reported for Eucalyptus [4], Plumbago auriculata [10] and Glycyrrhiza glabra [2], emphasizing the sustained importance of such endeavors. A novel and exciting extension of in vitro metabolite production involves utilizing microbes as phytochemical production factories. Moving away from the limitations of using prokaryotic bacterial platforms for secondary metabolite production, the basal model eukaryotes, yeasts, have been adopted by researchers for heterologous production of important metabolites. Yeasts are very amenable for genetic transformation with multiple expression constructs, enabling metabolic engineering of key pathways [15]. Still in its infancy, synthetic technologies using yeast-based platforms might emerge as long-term solutions plaguing phytochemical production under in vitro conditions. The review by Bapat and his associates discussed different aspects of this intriguing and futuristic biotechnological intervention [3].
The quest for novel phytochemicals goes on unabated and needs to be expedited not only because humans are facing medical challenges like never before, but also because positioned at the brink of a cycle of great extinction, biodiversity loss is currently occurring at an unprecedented scale. Enquist et al. [6] quantified global distribution of all recorded land plant species, and estimated that almost 40% of these are very rare and endangered. Efficient documentation of bioresources, especially those which have been reported to have medicinal properties in ancient treatises, is the need of the hour. Bioprospecting strategies should include identifying potentially important plants and detailed documentation of their ethnobotanical as well as modern uses, their biochemical profiles and the molecular mode of action of their medicinally important properties. A case in point is the life-long work of Tu Youyou who scoured ancient medicinal texts and succeeded in isolating and establishing the anti-malarial artemisinin from sweet worm-wood Artemisia annua known for eons as a cure for fevers in Chinese medicine [19]. The coveted Indian treatise Auyrveda is a treasure trove of medicinal plants, however, much work is needed to identify and establish these remedies in modern scientific context. Gogoia and Rana [8] and Ghosh et al. [7]have discussed two such plants, whose ethno-medicinal uses have been validated by modern research. Depending on the availability and IUCN status, different aspects of plant life cycle and seed biology need imminent attention to ensure their conservation and judicious exploitation. Along those lines, Krishan et al. [11] have presented an argument in favour of assessment of seed biology of the Indian pokeweed, a little explored Himalayan medicinal herb. The path to identify a specific metabolite in a plant extract for drug development is long and laborious, and more often than not stalls indecisively. Recent improvisations have been implemented in minimizing the cost and time taken for drug development. Nasima et al. [13]have compiled a synopsis of these facets that are accelerating modern day research on drug development from plant natural products. Scientists are using different disease models and mechanisms (DMMs) for testing drug efficacy, including unconventional models like microbes and invertebrates. Pandey et al. [16] have shown how Caenorhabditis elegans model can be used to test the efficacies of phyto-extracts as ageing cognitive booster formulations.
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