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Without claiming to deal with all aspects of this topic, this editorial article aims to provide a brief overview of the input of plant cell tissue and organ biotechnology in the field of the valorization and conservation of medicinal and aromatic plants and the production of their secondary metabolites. Several textbooks and review articles have been described this topic more comprehensively than is possible in this editorial [1, 2].
Medicinal and aromatic plants ï€¨MAPsï€© are increasing worldwide and continue to attract growing interest for farmers, traders, economists, teachers, professionals, health officials and various industries. The MAPs have always represented important sources of chemicals for the food and pharmaceutical industries through the large numbers of primary and secondary metabolites that they produce. They are natural biological resources that have a great potential to synthesize a huge variety of important secondary compounds, also referred to as natural product, far more than animal and even microorganisms. Are defined as secondary compounds ï‚²metabolitesï‚² all substances specific (characteristic of a species) and which are not synthesized by metabolic pathways of primary metabolism they are common to large groups of plants. Secondary metabolites are rather scarce, their structure is often very complex and their synthesis requires multistep enzymatic reactions. Their levels in plants are low, typically less than 0.1 to 5% of the biomass, and vary depending on numerous factors such as tissue type, plant developmental stage, environmental conditions, etc. These natural compounds are used as pharmaceuticals, nutraceuticals, agrochemicals, cosmeceuticals, functional foods, and natural health products .
The supply of the source plants however, is often limited due to disease, changes in climate, and changes in the economic development in the growing regions.
It is well known that MAPs have been traditionally harvested in the wild state, in only few regions, and they are not used equally worldwide. Till date, these phytoresources have been exploited nearly without any major limitation which may easily result in the exhaustion of plant genetic resources, biotope destruction and loss of wild populations. Thus, also threatening valuable incomes for rural households, especially in developing countries; consequently, the sustainable use of natural resources has become an unavoidable necessity from both environment protection and socioeconomic points of view. Currently, between 4,000 and 10,000 MAP are on the endangered species list and this number is expected.
These problems could be overcome through MAP selection and cultivation under agricultural conditions which could respond to increasing demands in terms of plant security, socio-economic development; biodiversity conservation and sustainable use of genetic phytoresources as basic inputs for the future
Sometimes MAP cultivation is preferable and there is no immediate necessity for any genetic improvement programs in selected species from the wild population. Selection in wild population is the most common method of MAP breeding. Furthermore, breeding research provides the prerequisites of efficiently breeding, for increasing the levels of desired compounds, decreasing the undesired compounds, more resistance, and most homogeneity uniformity, of new varieties. Inheritable traits of MAPs can be adapted by breeding to the special demands of the costumers and of the in the supply chain. Evidence of the increasing importance MAP breeding has been demonstrated by the growing number registered varieties in Germany during the past decade from onlythe growing number registered varieties in Germany during the past decade from only 10 to 66 in 1990 and 2004 respectively.
Cultivated material, which could be easily certified 'organic', is more appropriate for large scale uses such as the production of bioactive compounds by pharmaceutical, cosmeceutical and aromatherapy entities, which require standardized products of guaranteed or known content. The fully traceable MAP at the production chain has become necessary for a quality product. In fact, cultivation decidedly guarantees a steady source of feedstock. Wholesalers and specialized companies could negotiate supply contracts regular and reasonable prices over time with producer and allows controlled post-harvest handling. Therefore, quality controls can be ensured, and product standards can be set with the regulations and consumer preferences [6-8]. Nonetheless, many problems persist including genetics (origin, variation), leaf position and age, harvest, flowering), environment (temperature, photoperiod and light intensity), and finally agricultural practices (nutrition, irrigation, propagation, harvesting and extraction).
In order to overcome some of these various problems, the agrobiotechnological approach by utilizing plant cell tissue and organ culture (PCTOC) technology has been expected to be an efficient and useful tool for the breeding (selection) of high-quality MAPs [9-11] and for the preservation of endangered species as well [10, 11].while providing an alternative source of controlled production of phytochemical products [11, 12]
Potential of plant cell tissue and organ culture biotechnology
There is a widespread interest in the application of PCTOC techniques and biotechnological approaches to the production of MAPs and isolation of secondary metabolites. PCTOC systems have been successfully developed for many MAP species especially endangered species [9, 10], offer certain advantages over traditional agricultural methods of propagation, including: the rapid production and propagation of high quality disease-free and uniform planting material. The MAP propagation can be done under a controlled environment, anywhere, independent of climatic changes and soil conditions, on a year-round basis. For slow-growing and sparsely distributed species, poor production of seeds or seeds with poor germination rate, rapid multiplication is possible. The easy selection of desirable traits directly from the vitro culture, thus reducing spaces needed for field testing. The reduction of the selection cycle required in the MAP is possible no need to wait for the lifecycle of seed development. It allows for the avoidance diseases of plants by careful selection and the use of sterilization techniques. It allows the preservation of the gene pool by storing pollen, organs, tissues and cells (as a seed bank). It enables the reduction of deposits for a large number of viable plants. It guarantees national and international trade and exchanges of diseases-free plant material (Quarantine is not required) In addition, use of PCTOC systems offers a tremendous tool for dissecting the physiological, biochemical and molecular regulation of plant development and stress response phenomena. Furthermore, they are extensively utilized for clonal propagation, as a gateway for genetic engineering of a vast majority of valuable economic plants and as an economical and large scale production platform for various pharmaceutics, drugs, flavors colors, enzymes and medicinal compounds under controlled conditions. [9-14].
PCTOC biotechnology is a corpus of techniques designed for the growth and multiplication of plant cells tissues and organs using nutrient solutions under aseptic and in a defined physical and chemical environments in vitro. Provided that an appropriate phytohormone regime is chosen, explants such as plant meristems, buds, leaves, stems, roots, etc cultivated in vitro can undergo coordinated division and development resulting in the formation of various and complex structures such as cotyledons, shoot tips, hypocotyls, anthers, internodes, leaf disks, roots, stem and thin cell layers and ultimately complete plants. PCTOC is based on the principle of totipotency, which states that every cell within the plant has the potential to regenerate into a complete plant.
Briefly, PCTOC is now a proven technology for the rapid clonal propagation  preservation of endangered species and valuable germaplasms, regeneration and multiplication of genetically modified fertile clones [10, 11, 15, 16] enzymes production  and economically valuable chemicals [11, 16-18].
The empirical approach used extensively in several studies on PCTOC has shown that success is principally dependent on following factors explant choice, medium composition of nutrient medium, especially phytohormone or synthetic growth combinations and control of in vitro environment. The optimization of the composition of the medium remains the main task for the establishment of successful PCTOC techniques. This biotechnology has been developed to such a level that any MAP species can be in vitro regenerated through one of the following methods (culture, anther culture, callogenesis, somatic or asexual embryogenesis, and organogenesis. The choice of either method depends on the species, success rate of the method for producing plants at a realistic cost, and local production conditions
Many plant species rich in high-value products are refractory to conventional cultivation and the chemical syntheses of these compounds is often not feasible at reasonable cost du to the complexity of their structures. The PCTOC of MAPs can provide an alternative way of consistent medicinal chemical isolation from plants materials.
After approximately four decades of the study and numerous attempts by several laboratories around the world, it has been demonstrated that PCTOC systems will produce many unusual secondary metabolites. Compared to conventional techniques for the production of plants, advantages of a PCTOC systems areï€º
ï¶ Phytochemical compounds can be produced by PCTOC under controlled physical and chemical environments independent of climatic changes or soil conditions.
ï¶ Organ, tissue and cell culture would be free diseases.
ï¶ The cells of any plants could easily be in vitro cultivated to produce their specific metabolites.
ï¶ Organ, tissue and cell can be selected for high production of many compounds.
ï¶ Organ, tissue and cell can be stored for long periods
ï¶ Plant cells can be grown in automated bioreactors, containing thousands of liters of medium, with improved productivity and reduced labor costs.
ï¶ Cultured plant cells often produce different quantities with different profiles of secondary metabolites and this production might be more manageable reproducible and reliable.
ï¶ Harvest the product can be quick, clean, concise and accurate compared to the extraction of complex whole plants.
ï¶ Organ, tissue and cell cultures may provide a source of defined standard phytochemical in large amounts.
Furthermore, In addition, new molecules which have not previously been found in the intact plants or have been synthesized by chemical means have been produced by tissue and cell cultures biotechnology. Thus, this technology is a true path of producing new molecules.
Furthermore, new molecules which have not been found previously in plants or have even been synthesized chemically have been produced by cell cultures. Thus this technology constitutes powerful tool of achieving production of novel secondary metabolites with new and improved biological activities that will be of great value for the pharmaceutical industry. Thus, the discovery and development of novel, more efficient, and safer phyto-therapeutics that will contribute to cure, at lower costs to the healthcare system, a variety of human diseases. In this context, the development of bioprocess for industrial scale production of plant secondary metabolites is thus of first importance. Several attempts were undertaken to produce secondary metabolites of major importance, through the use of cell or tissue cultures as an alternative to whole plant extraction with limited success because of the slowness and low yield of this culture process. 12, 17, 18].
Cell suspension cultures are the most culture type in the research and production of secondary metabolites. Unfortunately, in many cases low quantities of the desired compounds accumulated by undifferentiated cell cultures and often the biosynthesis of a certain compound is not produced in cell culture and remains organ or tissue specific[19, 20].
In many cases the biosynthesis of a certain compound is organ or tissue specific. In such cases the organ cultures, i.e. shoot or root cultures, may provide a better alternative than undifferentiated c ell cultures. For example, cardenolide biosynthesis was achieved in cultures regenerating shoots or somatic embryos more than undifferentiated cells of Digitalis sp [21, 22]. Fritillaria unibracteata can be rapidly propagated, directly from small cuttings of the bulb by the technique of organ culture. The growth rate of cultured bulb was about 30-50 times higher than that under natural wild growth conditions. The content of alkaloid and beneficial microelements in the cultured bulbs was higher than found in the wild bulb .
Otherwise, the pharmaceutical industry is highly interested in the secured availability and biotesting of natural but also of novel phyto-molecules for the discovery of new drugs. In particular, demand is high for new families of phyto-molecules displaying improved properties, including lower toxicity and higher solubility in water, and for feeding the many new molecular targets resulting from the intensive research in genomics and proteomics. These new trends in the biopharmaceutical industry have stimulated research interest in various areas for the supply of new chemo-diversity including genetic transformation to manipulate specific genes involved in secondary metabolism biosynthetic pathways using Agrobacterium rhizogenes.
Once produced, hairy roots can be excised from the explant and placed on a medium containing a suitable antibiotic to free them from the bacteria. It is characterized by high growth rate, genetic stability and growth in hormone free media. Transformed hairy roots mimic the biochemical machinery present and active in the normal roots, and in many instances transformed hairy roots display higherproduct yields. The greatest advantage of hairy roots is that their cultures often exhibit approximately the same or greater biosynthetic capacity for secondary metabolite production as compared to their mother plants . They can be a promising source for the continuous and standardized production of secondary metabolites under controlled conditions without losing genetic or biosynthetic stability .
In addition, whatever in vitro systems used for secondary metabolites production or harvest and depending on the usage in the in vitro studies, different approaches and strategies are being used to obtain higher yields of secondary metabolites in cultures through biotic and/or abiotic elicitors ï€¨methyl jasmonate, salicylic acid, chitosan, autoclaved pathogen phytoalexins, heavy metal ions, osmotic stress, ultraviolet irradiation, or gamma irradiations, high salinity,…ï€©, gene technology, media manipulation, adsorption of the metabolites to overcome feedback inhibition, permeation of metabolites to facilitate downstream processing, phytohormone combinations, precursor feeding, plant cell immobilization, biotransformation, and bioconversion, many of them are completely interactive. The basic idea behind the usage of elicitors focuses on the stress that they induce upon administration in the cell cultures, which concomitantly affects the yield and quality parameters of the secondary products accumulated [11, 12, 17, 18].
Attempts have been made to manipulate pharmaceutically important medicinal plants for their secondary metabolic pathways by using transgenic technique. The development of transgenic plants is the result of integrated application of rDNA technology, gene transfer and PCTOC techniques. These technologies have enabled the production of transgenic plants in more than 150 species, which include MAPs and most major valuable economic crops. Examples of direct DNA transfer methodology to engineer medicinal plants and cultures have been reviewed [26, 27]. Direct genetic transformation, a technology that is progressing fast in other areas of plant improvement, can only be exploited for secondary metabolite formation, if we have information about the gene(s) controlling the synthesis of the desired product or genetic markers associated with it. Knowledge of the genetics of biosynthetic pathways and their regulation is thus of crucial importance to bypass the low yield of various secondary metabolites in plant cells. Since secondary products however are often biosynthesized in multi-step enzymatic reactions in specifically differentiated cells, manipulations of such pathways to alter metabolic production is complex, complicated and unpredictable.
Scale-up of in vitro mass production of secondary metabolites
Large-scale PCTOC is found to be an attractive alternative approach to traditional methods of plantation as it offers a controlled supply of phytochemical compounds independent of plant availability.
The environment in witch plants cells tissues and organ grow usually changes when cultures are scaled-up from shake flasks to bioreactors, and this may result in reduced productivity. The bioreactors are the key step towards commercial production of secondary metabolites by plant biotechnology. However, this technology is still being developed and despite the advantages, there is a variety of problems to be overcome before it can be adopted for the production of useful plant secondary metabolites. With the ultimate aim of implementing an industrial-scale process, the behavior of cell and tissue culture in bioreactor has received much attention. Engineers are currently developing improved and appropriate bioreactors for the improvement of production systems by adopting techniques of growth cells and metabolite production.
It is noteworthy to point out that PCTOC have proven successful for the commercial production of several important plant constituents. Scale up of the process from laboratory scale, through pilot to industrial scales was successful, adopting either continuous or semicontinuous cultures and using stirred bioreactors with a capacity of up to 75000 liters, with full automation of medium preparation and sterilization, transfers, aeration, stirring, growth monitoring and harvesting . This has been successfully achieved for the production of immunostimulant polysaccharides by Echinacea purpurea cultures  and several other plant constituents by commercial or semi-commercial large scale procedures [29, 30].
It should be mentioned that literature reports provide only an incomplete picture of the actual commercial progress, since current industrial investigations fall well under the realm of proprietary and patentable research ï›30ï. Briefly, in 1983, the anti-inflammatory drug shikonin was produced by plant cell cultures of Lithospermum erythrohizon on an industrial scale for the first time by Mitsui Petrochemical Industries Ltd. . Screening for clones derived from individual high-producing cells, and the use of two state cultures-one for growth and another for production has resulted in the commercial production of the antihypertensive drug, ajmalicine by Catharanthus roseus cultures [31, 32]. Other examples include the production of rosmarinic acid from cell suspension cultures of Coleus blumei [33, 34], digoxin by cell cultures from Digitalis sp.  and ginsenosides from Panax ginseng cultures . The successful industrial production of paclitaxel, an anti-cancer drug originally extracted from the bark of 50-year-old Pacific yew trees, Taxus brevifolia, by plant cell cultures  will trigger research into other plant-based chemotherapeutics such as podophytoxin and comptothecin etc
Concluding remarks and Outlooks
Even with the advances in microbial and chemical productions process, plants remain an indispensable source for a number of chemical substances that are difficult to synthesize chemically owing to their complicated structures. They represent a vast source of untapped chemicals, with a wide variety of proteins and secondary products that have already been isolated from plant tissues.
Nowadays, most of MAPs are still harvested from the wild in a no sustainable way. Such practice can lead to over-exploitation of endangered and vulnerable species as well as to biotope destruction. Therefore, cultivated material is more suitable for large scale uses and breeding ï€¨selectionï€© of MAPs is undoubtedly linked to the future success of the exploitation.
Cultivation of medicinal plants is not only a means for meeting current and future demands for large volume production of MAPs and their secondary products, but also a means of relieving harvest pressure on wild populations. Otherwise, due to the structural complexity of natural products, total synthesis does not present an economical option for industrial production for many of these metabolites. Thus, PCTOC is a power biotechnological tool that provides a renewable, easily scalable source of natural product. PCTOC has been established for a wide variety of species, a particularly useful strategy for secondary metabolites production from endangered or rare plants.
Despite the enormous progress achieved in secondary metabolites production into PCTOC systems, several important challenges remain. These include the low productivity, only a small fraction of plant natural products can be expressed at industrially useful levels in culture, recalcitrance of some plant genotype or cell line systems to express their biosynthetic potential, knowledge of the biosynthetic pathways of desired compounds in plants as well as of cultures is often still rudimentary, and strategies are consequently needed to develop information based on a cellular and molecular level. Most notable are cases where some of the new compounds are formed in cells cultures that are not present in whole plants into the same species. This raises a very simple question how is biosynthetic pathways for the production of various secondary compounds in plants? How many of these compounds produced are genetically controlled and how many of them are being attributed to environmental factors?
There has been remarkable progress in understanding of the biochemistry of MAPs due to the development of new analytical tools such as metabolomic, proteomics, and genomics. These newly designed techniques would not only provides data on phenotypic variation, caused by in vitro environmental factors, but also yield data on the genes involved in the biosynthesis metabolic pathways. As well, these new tools could facilitate discovery of new compounds in MAPs, and more interestingly, may identify compounds of medicinal efficacy in yet unexploited in untapped plant species. Progress in this way will open new avenues for high volume production of pharmaceuticals, neutraceuticals, and other beneficial substances.