Plant secondary metabolites (SMs) are not only a useful array of natural products but also an important part of plant defense system against pathogenic attacks and environmental stresses. With remarkable biological activities, plant SMs are increasingly used as medicine ingredients and food additives for therapeutic, aromatic and culinary purposes. Various genetic, ontogenic, morphogenetic and environmental factors can influence the biosynthesis and accumulation of SMs. According to the literature reports, for example, SMs accumulation is strongly dependent on a variety of environmental factors such as light, temperature, soil water, soil fertility and salinity, and for most plants, a change in an individual factor may alter the content of SMs even if other factors remain constant. Here, we review with emphasis how each of single factors to affect the accumulation of plant secondary metabolites, and conduct a comparative analysis of relevant natural products in the stressed and unstressed plants. Expectantly, this documentary review will outline a general picture of environmental factors responsible for fluctuation in plant SMs, provide a practical way to obtain consistent quality and high quantity of bioactive compounds in vegetation, and present some suggestions for future research and development.

Response of Plant Secondary Metabolites to Environmental Factors.

Centella asiatica accumulates large quantities of pentacyclic triterpenoid saponins, collectively known as centelloids. These terpenoids include asiaticoside, centelloside, madecassoside, brahmoside, brahminoside, thankuniside, sceffoleoside, centellose, asiatic-, brahmic-, centellic- and madecassic acids. The triterpene saponins are common secondary plant metabolites and are synthesized via the isoprenoid pathway to produce a hydrophobic triterpenoid structure (aglycone) containing a hydrophilic sugar chain (glycone). The biological activity of saponins has been attributed to these characteristics. In planta, the Centella triterpenoids can be regarded as phytoanticipins due to their antimicrobial activities and protective role against attempted pathogen infections. Preparations of C. asiatica are used in traditional and alternative medicine due to the wide spectrum of pharmacological activities associated with these secondary metabolites. Here, the biosynthesis of the centelloid triterpenoids is reviewed; the range of metabolites found in C. asiatica, together with their known biological activities and the chemotype variation in the production of these metabolites due to growth conditions are summarized. These plant-derived pharmacologically active compounds have complex structures, making chemical synthesis an economically uncompetitive option. Production of secondary metabolites by cultured cells provides a particularly important benefit to manipulate and improve the production of desired compounds; thus biotechnological approaches to increase the concentrations of the metabolites are discussed.

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Pentacyclic triterpenoids from the medicinal herb, Centella asiatica (L.) Urban.

Antibiofilm activity of essential oils and plant extracts against Staphylococcus aureus and Escherichia coli biofilms

Leaves of Centella asiatica (Centella) were analysed for their triterpene composition and bioactivity such as collagen enhancement, antioxidant, anticellulite and UV protection capacity properties. Triterpenes of Centella were measured using HPLC-PAD on an Excil ODS 5 mm (C18) column for the simultaneous determination of asiatic acid, madecassic acid, asiaticoside and madecassoside. Centella was found to contain significant amounts of madecassoside (3.10 ± 4.58 mg/mL) and asiaticoside (1.97 ± 2.65 mg/mL), but was low in asiatic and madecassic acid. The highest collagen synthesis was found at 50 mg/mL of Centella extracts. The antioxidant activity of Centella (84%) was compared to grape seed extract (83%) and Vitamin C (88%). Its lipolytic activity was observed by the release of glycerol (115.9 µmol/L) at 0.02% concentration. Centella extracts exhibited similar UV protection effect to OMC at 10% concentration. In view of these results, the potential application of Centella in food and pharmaceutical industries is now widely open.

Triterpene Composition and Bioactivities of Centella asiatica

The present study was to optimize the phenolic recovery from Centella asiatica by investigating the effects of ethanol concentration (0-100%, v/v), extraction time (60-300 min) and extraction temperature (25- 65°C) on phenolic extraction using single-factor experiments. Total phenolic content (TPC), total flavonoid content (TFC) and condensed tannin content (CTC) were used for determination of phenolic content while 2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical-scavenging capacity and 2,2'-diphenyl- 1-picrylhydrazyl (DPPH) radical-scavenging capacity were used for measuring the antioxidant capacity of C. asiatica extract. All extraction conditions had significant effect (p<0.05) on the phenolic contents and antioxidant capacities of C. asiatica extract. The optimal conditions for phenolic recovery were 40% ethanol for 60 min at 65 o C, with values of 1203.49 mg GAE/ 100 g DW for TPC, 561.92 mg CE/ 100 g DW for TFC, 181.25 mg CE/ 100 g DW for CTC, 730.37 µmol TEAC/ 100 g DW for ABTS and 1948.30 µmol TEAC/ 100 g DW for DPPH. TFC was found to be positive correlated significantly (0.902, p<0.05) with DPPH under influence of ethanol concentration. However, all antioxidant compound assays (TPC, TFC and CTC) were negatively correlated significantly with ABTS under the effect of extraction temperature.

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Effect of ethanol concentration, extraction time and extraction temperature on the recovery of phenolic compounds and antioxidant capacity of Centella asiatica extracts

Comparative analysis of active constituents in Centella asiatica samples from Madagascar: Application for ex situ conservation and clonal propagation

http://www.farm.ucl.ac.be/Full-texts-FARM/Rangasamy-2007-1.pdf

Screening for anti-infective properties of several medicinal plants of the Mauritians flora

Seeds of Syzygium cuminii (L.) Skeels are collected in Madagascar to prepare Madeglucyl®, a locally used antidiabetic preparation for type 2 diabetes treatment. For ex situ conservation of the Malagasy S. cuminii germplasm and to provide a continuous seed supply, a simple procedure for its in vitro clonal propagation was established. Nodal stem segments from in vitro raised 10-week-old seedlings were cultured on half-strength Murashige and Skoog medium (1/2MS) supplemented with various cytokinins, benzylaminopurine (BA), thidiazuron (TDZ) and N6-(2-isopentenyl) (2iP) adenine, at different concentrations and in combination with an auxin, α-naphthalene acetic acid (NAA). The most effective hormone concentration for shoot multiplication from axillary buds was 4.4 μM BA, providing an average of 7.5 shoots per node after six weeks of culture. In liquid medium, cultures could be maintained without subculturing for more than 4 months and yielded an average of 6.9 shoots per explant. The in vitro-propagated shoots produced adventitious roots when transferred to 1/2MS medium supplemented with either indole-3-butyric acid (IBA) or NAA. The best rooting was obtained in medium supplemented with 0.5 μM NAA with almost 90% of the plantlets developing an average of 5.9 adventitious roots per shoot. More than 70% of the rooted plantlets were successfully established in soil and showed uniform growth. At present, the plants are ready to be transferred for in situ cultivation to produce seeds for commercial purpose. © 2008 Royal Botanical Society of Belgium.

Francisco Rakotoniriana

Papers

Comparative analysis of active constituents in Centella asiatica samples from Madagascar: Application for ex situ conservation and clonal propagation

Screening for anti-infective properties of several medicinal plants of the Mauritians flora

Ex situ conservation and clonal propagation of the Malagasy Syzygium cuminii, an antidiabetic plant