Microbial interference mitigates Meloidogyne incognita mediated oxidative stress and augments bacoside content in Bacopa monnieri L

Microbial interference plays an imperative role in plant development and response to various stresses. However, its involvement in mitigation of oxidative stress generated by plant parasitic nematode in plants remains elusive. In the present investigation, the efficacy of microbe's viz., Chitiniphilus sp. MTN22 and Streptomyces sp. MTN14 single and in combinations was examined to mitigate oxidative stress generated by M. incognita in medicinal plant, Bacopa monnieri. Microbial combination with and without pathogen also enhanced the growth parameters along with secondary metabolites (bacoside) of B. monnieri than the pathogen inoculated control. The study showed that initially the production of hydrogen peroxide (H₂O₂) was higher in dual microbes infected with pathogen which further declined over M. incognita inoculated control plants. Superoxide dismutase and free radical scavenging activity were also highest in the same treatment which was linearly related with least lipid peroxidation and root gall formation in B. monnieri under the biotic stress. Microscopic visualization of total reactive oxygen species (ROS), H₂O₂, superoxide radical and programmed cell death in host plant further extended our knowledge and corroborated well with the above findings. Furthermore, scanning electron microscopy confirmed good microbial colonization on the host root surface around nematode penetration sites in plants treated with dual microbes under pathogenic stress. The findings offer novel insight into the mechanism adopted by the synergistic microbial strains in mitigating oxidative stress and simultaneously stimulating bacoside production under pathogenic stress.

In vitro cultures of Bacopa monnieri and an analysis of selected groups of biologically active metabolites in their biomass

CONTEXT

Bacopa monnieri L. Pennell (Scrophulariaceae) is one of the most important plants in the system of Indian medicine (Ayurveda).

OBJECTIVE

This paper studies the optimal growth of B. monnieri for effective accumulation of metabolites. Biomass growth of this plant could be accomplished in liquid cultures on Murashige & Skoog medium.

MATERIALS AND METHODS

Powdered shoots of in vitro cultures of B. monnieri were extracted by methanol for indole compounds, phenolic compounds and bacosides for RP-HPLC analysis. Fatty acid analysis was performed via gas chromatography. Anti-inflammatory effect of B. monnieri extracts was evaluated in the A549 cells. COX-2 and cPGES expression was analyzed using Western blots.

RESULTS

l-Tryptophan and serotonin were found in biomass from in vitro cultures of B. monnieri on MS medium and in biomass from the MS mediums enriched with the different additions such as of 0.1 g/L magnesium sulphate, 0.1 g/L zinc hydroaspartate, 0.1 g/L l-tryptophan, 0.25 g/L serine, 0.5 g/L serine and 0.5 mg/L anthranilic acid. The content of l-tryptophan and serotonin compounds was significant in biomass from medium with the addition of 0.1 g/L zinc hydroaspartate (0.72 mg/g dry weight and 1.19, respectively). Phenolic compounds identified in biomass from the same variants of MS medium were chlorogenic acid (ranging from 0.20 to 0.70 mg/g dry weight), neochlorogenic acid (ranging from 0.11 to 0.40 mg/g dry weight) and caffeic acid (ranging from 0.01 to 0.04 mg/g dry weight). The main group of fatty acids in biomass was saturated fatty acids (53.4%). The predominant fatty acid was palmitic acid. A significant decrease of COX-2 and cPGES expression was observed in the A549 cells activated with LPS and treated with B. monnieri extracts.

DISCUSSION AND CONCLUSIONS

As far as we know, this is the first analysis of indole compounds and phenolic acids in this plant. The multi-therapeutic effect of B. monnieri is expressed by the activity of bacosides. Information about the presence of indole and phenolic compounds, and fatty acids in this plant is limited, but the content of these compounds might participate in the physiological activity of B. monnieri.

Exiguobacterium oxidotolerans, a halotolerant plant growth promoting rhizobacteria, improves yield and content of secondary metabolites in Bacopa monnieri (L.) Pennell under primary and secondary salt stress

Brahmi (Bacopa monnieri), an integral component of Indian Ayurvedic medicine system, is facing a threat of extinction owing to the depletion of its natural populations. The present study investigates the prospective of exploitation of halotolerant plant growth promoting rhizobacteria (PGPR) in utilising the salt stressed soils for cultivation of B. monnieri. The effects of two salt tolerant PGPR, Bacillus pumilus (STR2) and Exiguobacterium oxidotolerans (STR36) on the growth and content of bacoside-A, an important pharmaceutical compound in B. monnieri, were investigated under primary and secondary salinity conditions. The herb yields of un-inoculated plants decreased by 48 % under secondary salinization and 60 % under primary salinization than the non salinised plants. Among the rhizobacteria treated plants, E. oxidotolerans recorded 109 and 138 %, higher herb yield than non-inoculated plants subjected to primary and secondary salinity respectively. E. oxidotolerans inoculated plants recorded 36 and 76 % higher bacoside-A content under primary and secondary salinity respectively. Higher levels of proline content and considerably lower levels of lipid peroxidation were noticed when the plants were inoculated with PGPR under all salinity regimes. From the results of this investigation, it can be concluded that, the treatments with salt tolerant PGPR can be a useful strategy in the enhancement of biomass yield and saponin contents in B. monnieri, as besides being an eco-friendly approach; it can also be instrumental in cultivation of B. monnieri in salt stressed environments.

Exiguobacterium oxidotolerans, a halotolerant plant growth promoting rhizobacteria, improves yield and content of secondary metabolites in Bacopa monnieri (L.) Pennell under primary and secondary salt stress

Brahmi (Bacopa monnieri), an integral component of Indian Ayurvedic medicine system, is facing a threat of extinction owing to the depletion of its natural populations. The present study investigates the prospective of exploitation of halotolerant plant growth promoting rhizobacteria (PGPR) in utilising the salt stressed soils for cultivation of B. monnieri. The effects of two salt tolerant PGPR, Bacillus pumilus (STR2) and Exiguobacterium oxidotolerans (STR36) on the growth and content of bacoside-A, an important pharmaceutical compound in B. monnieri, were investigated under primary and secondary salinity conditions. The herb yields of un-inoculated plants decreased by 48 % under secondary salinization and 60 % under primary salinization than the non salinised plants. Among the rhizobacteria treated plants, E. oxidotolerans recorded 109 and 138 %, higher herb yield than non-inoculated plants subjected to primary and secondary salinity respectively. E. oxidotolerans inoculated plants recorded 36 and 76 % higher bacoside-A content under primary and secondary salinity respectively. Higher levels of proline content and considerably lower levels of lipid peroxidation were noticed when the plants were inoculated with PGPR under all salinity regimes. From the results of this investigation, it can be concluded that, the treatments with salt tolerant PGPR can be a useful strategy in the enhancement of biomass yield and saponin contents in B. monnieri, as besides being an eco-friendly approach; it can also be instrumental in cultivation of B. monnieri in salt stressed environments.

Genetic transformation of Bacopa monnieri by wild type strains of Agrobacterium rhizogenes stimulates production of bacopa saponins in transformed calli and plants

We have developed an efficient transformation system for Bacopa monnieri, an important Indian medicinal plant, using Agrobacterium rhizogenes strains LBA 9402 and A4. Transformed roots induced by strain LBA 9402 spontaneously dedifferentiated to callus while excised roots induced by strain A4 spontaneously showed induction of shoot buds within 10 days. PCR and RT-PCR analysis revealed the presence and expression of the rolAB and rolC genes at the transcription level in pRi A4 transformed cultures indicating that the TL-DNA was integrated retained and expressed in the A4-Ri transformed shoots. Transformed calli showed the presence of rolAB or rol A, TR and ags genes. Transformed plants showed morphological features typically seen in transgenic plants produced by A. rhizogenes. Growth and biomass accumulation was significantly higher in the transformed shoots (twofold) and roots (fourfold) than in the non-transformed (WT) plants. In pRi A4-transformed plants, the content of bacopasaponin D, bacopasaponin F, bacopaside II and bacopaside V was enhanced significantly as compared to WT plants of similar age while bacoside A3 and bacopasaponin C content was comparable with that of WT plants. Significant increase in content of five bacopa saponins could be detected in pRi 9402-transformed callus cultures. There is an overall stimulatory effect on accumulation of bacopa saponins in transformed plants and cells of B. monnieri establishing the role of endogenous elicitation by Ri T-DNA of A. rhizogenes.

Effect of bavistin on in vitro plant conversion from encapsulated uninodal microcuttings of micropropagated Bacopa monnieri (L.)--an Ayurvedic herb

A reliable and reproducible protocol for contamination free plant recovery system from alginated encapsulated uninodal microcuttings of micropropagated Bacopa monnieri L. have been developed after storage at 18 degrees C for 45 days. Node segments excised from freshly micropropagated plants were encapsulated as single explant beads with 3.0% sodium alginate and 80 mM CaCl2 2 H2O. To find out the optimal concentration of fungicide bavistin for efficient plant recovery different concentrations of bavistin (1.0 - 15 mg l(-1)) were incorporated in to the encapsulation medium. 3.0mg l(-1) bavistin showed no reduction in plant conversion and generated maximum number of shoots (45.6 +/- 1.69) at high frequency with out any contamination after storage up to 45 days at 18 degrees C. At high concentrations (13 and 15 mg l(-1)), rupturing of calcium alginate coats after 8 - 9 days and gradual decline in the number of shoots indicates the toxic effect of bavistin on plant conversion. Encapsulated node cuttings stored up to 45 days regenerated shoots (5.2) and multiple shoots (45.6) in MS basal and hormone medium respectively. Maximum shoot length (8.2 +/- 0.37 cm) was observed from encapsulsted node cuttings incorporated with 3.0 mg l(-1) bavistin on MS basal medium. 90% of the recovered plantlets were hardened off and successfully established in soil.

Shoot bud regeneration from different explants of Bacopa monniera (L.) Wettst. by trimethoprim and bavistin

A mass in vitro propagation system devoid of growth regulators for Bacopa monniera (L.) Wettst., a traditional Indian medicinal plant, has been developed. Direct shoot bud regeneration was induced by culturing internode and leaf explants on Murashige and Skoog's (MS) medium supplemented with an antibiotic (trimethoprim) or a fungicide (bavistin). Bavistin showed a marked cytokinin-like activity, as evident from high number of shoot buds induced in node, internode and leaf explants. Optimum adventitious shoot buds induction occurred at 300 mg/l bavistin from internode explants. In vitro regenerated shoots were elongated and rooted before transferred to field with 85% survival. The regeneration protocol developed in this study illustrates the usefulness of additives for mass propagation and germplasm conservation of B. monniera.

Cadmium accumulation and its influence on lipid peroxidation and antioxidative system in an aquatic plant, Bacopa monnieri L

Bacopa monnieri L. plants exposed to 10, 50, 100 and 200 microM cadmium (Cd) for 48, 96 and 144 h were analysed with reference to the accumulation of metal and its influence on various enzymatic and non-enzymatic antioxidants, thiobarbituric acid reactive substances (TBARS), photosynthetic pigments and protein content. The accumulation of Cd was found to be increased in a concentration and duration dependent manner with more Cd being accumulated in the root. TBARS content of the treated roots and leaves increased with increase in Cd concentration and exposure periods, indicating the occurrence of oxidative stress. Induction in the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and guiacol peroxidase (GPX) was recorded in metal treated roots and leaves of B. monnieri. In contrast, a significant reduction in catalase activity in Cd treated B. monnieri was observed. An increase was also noted in the levels of cysteine and non-protein thiol contents of the roots of B. monnieri followed by a decline. However, in leaves, cysteine and non-protein thiol contents were found to be enhanced at all the Cd concentrations and exposure periods. A significant reduction in the level of ascorbic acid was observed in a concentration and duration dependent manner. The total chlorophyll and protein content of B. monnieri decreased with increase in Cd concentration at all the exposure periods. Results suggest that toxic concentrations of Cd caused oxidative damage as evidenced by increased lipid peroxidation and decreased chlorophyll and protein contents. However, B. monnieri is able to combat metal induced oxidative injury involving a mechanism of activation of various enzymatic and non-enzymatic antioxidants.

Phytochelatin synthesis and response of antioxidants during cadmium stress in Bacopa monnieri L

The phytotoxicity imposed by cadmium (Cd) and its detoxifying responses of Bacopa monnieri L. have been investigated. Effect on biomass, photosynthetic pigments and protein level were evaluated as gross effect, while lipid peroxidation and electrolyte leakage reflected oxidative stress. Induction of phytochelatins and enzymatic and non-enzymatic antioxidants were monitored as plants primary and secondary metal detoxifying responses, respectively. Plants accumulated substantial amount of Cd in different plant parts (root, stem and leaf), the maximum being in roots (9240.11 microg g(-1) dw after 7 d at 100 microM). Cadmium induced oxidative stress, which was indicated by increase in lipid peroxidation and electrical conductivity with increase in metal concentration and exposure duration. Photosynthetic pigments showed progressive decline while protein showed slight increase at lower concentrations. Enzymes viz., superoxide dismutase (SOD, EC 1.15.1.1), guaiacol peroxidase (GPX, EC 1.11.1.7) ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) showed stimulation except catalase (CAT, EC 1.11.1.6) which showed declining trend. Initially, an enhanced level of cysteine, glutathione and non-protein thiols was observed, which depleted with increase in exposure concentration and duration. Phytochelatins induced significantly at 10 microM Cd in roots and at 50 microM Cd in leaves. The phytochelatins decreased in roots at 50 microM Cd, which may be correlated with reduced level of GSH, probably due to reduced GR activity, which exerted increased oxidative stress as also evident by the phenotypic changes in the plant like browning of roots and slight yellowing of leaves. Thus, besides synthesis of phytochelatins, availability of GSH and concerted activity of GR seem to play a central role for Bacopa plants to combat oxidative stress caused by metal and to detoxify it. Plants ability to accumulate and tolerate high amount of Cd through enhanced level of PCs and various antioxidants suggest it to be a suitable candidate for phytoremediation.

In vitro studies of Bacopa monnieri--an important medicinal plant with reference to its biochemical variations

Micropropagation of Bacopa monnieri was achieved on MS and B5 medium supplemented with BAP and NAA using leaf explants and nodal segments. Best results were found on MS medium in both the explants with BAP (2.0 mg/l) showing higher percentage of regeneration. Besides that the biochemical parameters, like chlorophyll, carbohydrate, protein, of leaves both in vivo and in vitro have also been carried out in order to establish the sustainability of plants.

Synthesis of a novel plant growth promoter from gallic acid

Gallic acid has been modified to naphthophenone derivatives with esterified fatty acid side chain. Compound 12, an ethyl crotonate ester of naphthophenone derivative has shown potent auxin like growth promoter activity. This is the first example of naphthophenone derivatives with plant growth promoting activity.