Elicitation is a possible aid to overcome various difficulties associated with the large‐scale production of most commercially important bioactive secondary metabolites from wild and cultivated plants, undifferentiated or differentiated cultures. Secondary metabolite accumulation in vitro or their efflux in culture medium has been elicited in the undifferentiated or differentiated tissue cultures of several plant species by the application of a low concentration of biotic and abiotic elicitors in the last three decades. Hairy root cultures are preferred for the application of elicitation due to their genetic and biosynthetic stability, high growth rate in growth regulator‐free media, and production consistence in response to elicitor treatment. Elicitors act as signal, recognized by elicitor‐specific receptors on the plant cell membrane and stimulate defense responses during elicitation resulting in increased synthesis and accumulation of secondary metabolites. Optimization of various parameters, such as elicitor type, concentration, duration of exposure, and treatment schedule is essential for the effectiveness of the elicitation strategies. Combined application of different elicitors, integration of precursor feeding, or replenishment of medium or in situ product recovery from the roots/liquid medium with the elicitor treatment have showed improved accumulation of secondary metabolites due to their synergistic effect. This is a comprehensive review about the progress in the elicitation approach to hairy root cultures from 2010 to 2019 and the information provided is valuable and will be of interest for scientists working in this area of plant biotechnology.

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Bacopa monnieri has been used as a reputed drug in the Indian traditional ayurvedic system for centuries. This medicinal herb with important phytopharmaceuticals has been popularly known as “Brahmi”. In recent years, B. monnieri has been extensively studied for its bioactive constituents, constituents responsible for memory enhancing effect, and also its diverse other useful effects. It possesses many pharmacological activities such as antioxidant, gastrointestinal, endocrine, antimicrobial, anti-inflammatory etc. The plant has been also used for the treatment of neurological and neuropsychiatric diseases. Due to its multipurpose therapeutic potential, micropropagation using axillary meristems and de novo organogenesis has been extensively studied in the species and is being reviewed. High frequency direct shoot organogenesis can be induced in excised leaf and internode explants in the absence of exogenous phytohormones and the rate of induction is enhanced in the presence of exogenous cytokinins, supplements, growth regulators, etc. Using explants from tissue culture raised plants, direct shoot regeneration leading to production of more than 100 rooted plants/explant within 8–12 weeks period with 85%–100% survival in the field after acclimatization can be expected following optimized protocols. Bioreactor based micropropagation was found to increase the multiplication rate of shoot cultures for the commercial propagation of B. monnieri plants. The maximum content of bacosides has been recorded in shoot biomass using an airlift bioreactor system. Further studies for the biosynthesis of bacosides and other secondary metabolites need to be conducted in the species utilizing untransformed shoot cultures in bioreactors.

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Bacopa monnieri (Linn.) Wettst. (family Scrophulariaceae), a therapeutically important perennial herb, was transformed to study the stability of the effects associated with the insertion of a gene encoding b-cryptogein, a proteinaceous elicitor, either alone or in addition to the T-DNA genes in long-term in vitro culture as well as after transfer to the greenhouse. Plant lines BmAr-IXcrypt obtained via transformation by LBA 9402-crypt (pRi1855?pBin19??crypt) showed integration and expression of rolA, rolB, rolC, and rolD genes after 4 years of maintenance in vitro. The plant line BmAt-ncrypt obtained via transformation with A. tumefaciens strain LBA4404-crypt (harboring pBin19??crypt) as well as plant line BmAr-IXcrypt showed stable integration and expression of nptII gene and crypt gene even after transfer to greenhouse. The clones of Ri-crypt-transformed plants differed morphologically from Ri-transformed plants in that the typical ''hairy root syndrome'' was not observed in plants expressing crypt gene in the presence of rol genes. Except plant line BmAr-IXcrypt, none of the other transgenic plant lines showed any alteration in floral morphology and onset of flowering. The crypt-transformed plant lines (BmAr-IXcrypt and BmAt-ncrypt), maintained under long-term culture, showed significantly higher (p B 0.05) amount of bacoside production in vitro (1.66-to 2.05-fold, with respect to non-transformed plants). In addition, accumulation of all the four bacosides was significantly higher (p B 0.05) in crypt-transformed plants as compared to non-transformed plants grown for 1 year in greenhouse. Thus, the crypt-transformed plant lines of B. monnieri may be considered as the potential source for sustainable bioproduction of bacosides.