Plant Cell Technology

In recent years, suppliers have introduced plant cell ingredients to the skin and hair care markets, termed plant stem cells, extracts or derivatives. This is, in part, due to the popularity of stem cells in the medical field, which have been used to regenerate tissues including the skin. Therefore, to contribute to skin healing, a wave of excitement and expectation has motivated marketing and R&D departments to look back at plant cell technology as a way to bring stem cell-associated claims to personal care. While plant stem cells initially were marketed for their technological potential, they recently have been identified as an alternative, sustainable mean to produce nature-derived extracts and molecules.

While this technology was initially marketed as “plant stem cell technology,” some scientists consider this terminology confusing and not appropriate since cells derived from callus are not all stem cells but rather a mixture of stem, de-differentiated and partially differentiated cells.1 It is also clear that differences exist between human and plant stem cells themselves,² as well as their final application and scope in dermatology.³

Plant cell cultures have mostly been investigated for the commercial synthesis of high-value, secondary low molecular weight metabolites. Such molecules are synthesized by plants in response to environmental pressure and are essential for survival and adaptation. Attempts to cultivate plant cells began in the early 20th century, but it was not until between the 1940s and 1960s that the technology was optimized, including its industrial scale-up.⁴

Plant cells derived from plant tissues are cultivated under defined physical and chemical conditions in vitro. These conditions are different for each type of plant and tissue, and must be optimized on a case-by-case basis. Explants from leaves, meristems, roots and stems are sterilized and plated in solid growth media with the growth factors and nutrients needed by that species. Explants then proliferate into a callus of non-differentiated cells that also contains stem cells. During this step, the most proliferative explants, i.e., the less differentiated explants, are selected.

Once callus is formed, it can be initially evaluated and screened for specific product/s of interest. Callus can then be transferred in liquid medium to grow a suspension culture. This culture can be collected, filtered, extracted and finally lyophilized in a powder rich in metabolites that will go to further analysis and validation for quality and quantity.

Secondary Metabolites

Produced from primary metabolites such as amino acids, lipids and carbohydrates, secondary metabolites are involved in the plant’s defense mechanisms against herbivores and pathogens. These metabolites often contribute to the plant color, taste and odor, but they do not directly affect the plant’s growth and development. Chemically, they are active molecules such as alkaloids, sterols, phenolic compounds, etc., with essential physiological roles that often enter signaling pathways.^{5, 6}

Usually produced by the plant in small amounts under basal conditions, the number of secondary metabolites can increase and accumulate under stress conditions, i.e., UV exposure, temperature, drought, salinity, mechanical stress, etc.^7-9 Their production also can be increased by optimizing plant cell culturing conditions, selecting the best plant strain, and providing the plant culturing media with primary metabolites, i.e., precursor feeding.⁵ Extensive studies and published work have made it possible to associate the type of physical, chemical or mechanical stress, and the target plant tissue and cells, with the type of molecule produced and sought, allowing the supplier to choose the best model and conditions.⁸

Secondary metabolites are important as drugs for pharmaceutical applications,¹⁰ but they are also used as fragrances, dyes and flavoring agents in the food and cosmetics industry.¹¹ Among the most well-known commercialized secondary metabolites produced in plant cell culture are vanillin, a flavoring agent derived from V. planifolia; artemisinin, an anti-malarial compound from Artemisia annua; morphine, an analgesic from Papaver somniferum; camptothecin, an anti-cancer compound from Camptotheca acuminata; and paclitaxel, an anticancer compound from Taxus spp.

Production through the natural harvest of some of these compounds shows very low yield, often lower than 2% dry weight, as in the case of morphine (Papaver somniferum, 0.05-0.1% dry weight) or artemisinin (Artemisia annua, 1.5% dry weight).¹⁰ In contrast, plant cell culture technology is a valuable alternative to increase yield. Several secondary metabolites have been shown to accumulate at higher levels in plant cell cultures, compared with native plants, by optimizing their culture condition specifically through the subtraction of accumulated molecules and elicitation of specific stressors. Published examples include ginsenosides (Panax ginseng), rosmarinic acid (Coleus bluemei), ubiquinone-10 (Nicotiana tabacum) and berberin (Coptis japonica),⁵ among which, several polyphenols, saponins and phytosterols can be used in personal care applications.

Plant Cell Technology vs. Plant Cultivation

Among the advantages of plant cell technology are: a lack of environmental conditions that create plant variations; independence from seasonal availability; species abundance; and increased plant growth rate. Also, while natural cultivation is often linked to the use of pesticides, contaminants, heavy metals, micro-organisms, insects, etc., plant cell cultures lacks these. As noted, it is also possible to use a specific stress to increase a specific molecule or family of molecules in different plant cell species.⁸ In culture, this stress can be controlled and optimized, which is not the case with plant cultivation where a larger variability exists.

On the other hand, limitations of plant cell technology exist, slowing down its broader introduction to the market, especially for the mass production of molecules where cost is an important factor. Such limitations have mainly been associated with: slow growth of the cultures, also due to some plant cell variability; low yield; the specific optimizations required; and contamination. These problems can be resolved but in some cases, optimization is costly and laborious, especially if sophisticated techniques are introduced. Due to the lack of sufficient industrial plants to process plant cell cultures, and the limitations outlined, natural cultivation and wild harvest are still the main sources for most natural products, if chemical synthesis and bioengineering are not possible or too costly.

Sustainability

In a sustainable model, wild harvest and/or natural cultivation should be conducted without depleting resources, allowing plants to regenerate and balance consumption with availability. Plant endangerment has been caused, for example, by producing the anti-cancer drug paclitaxel, which requires 340,000 kg of Taxus spp. bark (or 38,000 trees) to produce the 25 kg/year demand.¹² It also has been caused by the massive use of Prunus africana bark to produce pygeum. Many plants have been classified as endangered species by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) due to their excessive use and high demand. Thus, the alternative use of cell plant cultures to produce the metabolites in demand could reduce or rebalance the depletion of such endangered species, protecting local use and maintaining a bio-diverse ecosystem, specifically if the system is not sustainable anymore.

Plant cell culturing also considerably reduces water use and controls land use. In relation, associating and developing plant cell culture technology in conjunction with sustainable crop and plant land cultivation could increase the product’s availability. Moreover, cell plant technology could reduce pesticide and contaminant use, as well as the amount of solvent used to refine the finished product. Finally, plant cell technology optimizes the amount of a given plant used, in turn eliminating unwanted biomass. And since a specific part of a plant is used to replicate in culture, the rest of the plant remains intact, avoiding plant shortages due to massive harvest.

Formulation and Cosmetic Use

Plant cell technologies have entered cosmetic formulations in recent years to enrich extracted cells for active molecules such as growth factors, peptides, polyphenols, sugars, etc. An example is shown in Table 1, where the active molecules in Apium Graveolens (celery) extract and celery plant cell culture are compared. Personal care formulations are not suitable to maintain living cells. Therefore, intact plant cells are not incorporated into formulations or on the skin. Rather, plant cell lysates or extracts, i.e., the cellular content, are incorporated into finished formulas at 0.1–1.0% depending the molecule’s titer, to increase the availability in the skin.

Plant cell-derived extracts also have several formulation advantages over their natural plant extracts. Since the concentration of their active components is very high, they can be used at lower concentrations in the final formulation, which drastically reduces their effect on color and odor, compared with standard extracts. This is not the case for the natural plant extracts—even at low concentrations, especially if they are rich in polyphenols.

Moreover, natural plant extracts have less active content than plant cell extracts, unless they have been enriched through extract fractionation,¹³ which necessitates additional technology and costs. When developing a specific formulation for cosmetic application, formulators can use plant cell extracts enriched for a given molecule¹⁴ or family of related molecules,¹⁵ but also to eliminate unwanted molecules, as in the case of the plant cell extract from celery shown in Table 1. Here, the plant cell culture does not carry toxic secondary metabolites such as the furanocoumarins found in the natural plant extract, and it contains an increased amount of selected antioxidant molecules. Finally, some of these extracts derived from plant cell technology have proven effective for specific skin benefits, such as barrier repair,¹⁶ and have been “molecularly tuned” for specific applications. These are starting to enter the cosmetic market.

Conclusions

It is possible that if plant cell technology is further developed for cosmetic applications, it will not suffer the limitations observed in the pharmaceutical industry when greater amounts of ingredients are demanded. Also, the volumes required for personal care are not the same as for pharmaceuticals, and the production guidelines, although rigorous, are not as strict. So with the gradual introduction of plant cell technology into the cosmetic market, the scale-up and industrialization needed will grow in parallel with evolving regulatory requirements and market demand. Plant cell technology ultimately provides natural extract suppliers with a novel process to create natural, sustainable and highly technological products specifically tuned to a desired application.

References
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