Vidhi Chaudhary

Rediscovering cyanobacteria as valuable sources of bioactive compounds (Review)

Cyanobacteria are a simple, but primitive and diverse group of microorganisms, with characteristics in common to both bacteria and algae. Their success as a group in a wide range of habitats has been attributed to their unique physiological characters and high adaptive ability under a wide range of environmental conditions. The potential of cyanobacteria as a source of a variety of compounds such as polysaccharides, lipids, proteins, vitamins, sterols, enzymes, pharmaceuticals and other fine chemicals is well recognized, and their demand is now on an increasing trend. This compilation reviews the salient advances in the discovery of bioactive compounds from cyanobacteria and their significance in agriculture and industry.

Evaluation of fungicidal activity of extracellular filtrates of cyanobacteria--possible role of hydrolytic enzymes.

A set of seventy axenised and unicyanobacterial isolates belonging to the genus Anabaena were evaluated for biocidal activity against a set of phytopathogenic fungi. Among them, 35 Anabaena strains showed zone of inhibition against one or more fungi. The extracellular filtrates from 4 and 8 weeks old cultures of these Anabaena strains were further evaluated in terms of hydrolytic enzymes, proteins and IAA employing standard methods. Significant differences were also observed among the strains in terms of their FPase, chitosanase and xylanase activity, while low and relatively similar values of CMCase, cellobiase and protease activity were recorded in the strains analyzed. IAA production was also observed in all the strains. Comparative evaluation of activity of hydrolytic enzymes and antifungal activity revealed that such enzymes may contribute to the fungicidal activity of the cyanobacterial strains, besides other bioactive compounds, including IAA, which are established promising traits for biocontrol agents. This study is a first time report on the production of hydrolytic enzymes by these oxygenic photosynthetic prokaryotes, which can be potential candidates for the development of biocontrol agent(s) against selected phytopathogenic fungi. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cyanobacteria mediated plant growth promotion and bioprotection against Fusarium wilt in tomato

Cyanobacteria - phytopathogenic fungi - tomato plant interactions were evaluated for developing suitable biological options for combating biotic stress (Fusarium wilt) and enhancing plant vigour. Preliminary evaluation was undertaken on the fungicidal and hydrolytic enzyme activity of the cyanobacterial strains (Anabaena variabilis RPAN59, A. laxa RPAN8) under optimized environmental/nutritional conditions, followed by amendment in compost-vermiculite. Such formulations were tested against Fusarium wilt challenged tomato plants, and the Anabaena spp. (RPAN59/8) amended composts significantly reduced mortality in fungi challenged treatments, besides fungal load in soil. Cyanobacteria amended composts also led to an enhancement in soil organic C, nitrogen fixation, besides significant improvement in growth, yield, fruit quality parameters, N, P and Zn content. The tripartite interactions also enhanced the activity of defence and pathogenesis related enzymes in tomato plants. A positive correlation (r = 0.729 to 0.828) between P content and pathogenesis/defense enzyme activity revealed their role in enhancing the resistance of the plant through improved nutrient uptake. Light and scanning electron microscopy (SEM) revealed cyanobacterial colonization, which positively correlated with reduced fungal populations. The reduced disease severity coupled with improved plant growth/ yields, elicited by cyanobacterial treatments, illustrated the utility of such novel formulations in integrated pest and nutrient management strategies for Fusarium wilt challenged tomato crop.

EVALUATING THE ESTABLISHMENT AND AGRONOMIC PROFICIENCY OF CYANOBACTERIAL CONSORTIA AS ORGANIC OPTIONS IN WHEAT–RICE CROPPING SEQUENCE

Cyanobacteria represent promising organic inputs in rice–wheat cropping system, as they contribute towards accretion of N and C, besides secreting growth-promoting substances which influence plant productivity and soil fertility. The present study focused towards using a combinatorial approach for evaluating field-level colonization of cyanobacteria in soil and their effect on soil microbiological and plant parameters, employing agronomic and molecular tools. A consortium of cyanobacterial strains (BF1, Anabaena sp., BF2, Nostoc sp., BF3, Nostoc sp. and BF4, Anabaena sp.) was employed in different three-and four-member combinations along with 75% N + Full dose of P and K fertilizers. A significant enhancement in microbial activity and plant growth/yields and savings of 25% N in the wheat–rice cropping sequence were recorded, especially in treatments involving 75% N + Full dose of PK+BF1+BF2+BF4 and T5, i.e. 75% N + Full dose of PK+BF1+BF2+BF3. Such treatments were significantly higher or statistically at par with fertilizer controls – 75% N + Full dose of PK fertilizers. The use of DNA-based markers further helped to establish the colonization of the inoculated cyanobacteria, especially BF2 and BF3 strains. Our study clearly illustrated the establishment of inoculated cyanobacterial strains and their role in enhancing the crop productivity and soil health of the rice–wheat cropping system.

Deciphering the biochemical spectrum of novel cyanobacterium-based biofilms for use as inoculants

An investigation was done using cyanobacterium Anabaena torulosa as a matrix for developing novel biofilms, with anti-grazer traits against microfauna/pathogenic fungi, through co-inoculation of agriculturally important bacteria and fungi. The biofilms generated were evaluated after 4, 6 and 9 weeks of incubation for the activity of hydrolytic enzymes and fungicidal activity against phytopathogenic fungi. The activity of β-1,3-glucanase, in general, showed a gradual increasing trend up to 9 weeks, while endoglucanase activity was highest after 6 weeks of incubation, and a 40–50% reduction in chitosanase activity was recorded by the end of 9 weeks of incubation. Observations revealed that the fungus–cyanobacterium biofilms, especially Anabaena–Aspergillus awamori, exhibited the highest activity of β-1,3-glucanase and ranked second in terms of chitosanase activity. Fungicidal activity was recorded up to 9 weeks in most of the biofilms, and the highest values were recorded in cyanobacterium–Bacillus and cyanobacterium–fungus biofilms. Such biofilms were also tested against selected nematodes in microcosm experiments, which revealed no significant deleterious effects. The biocontrol activity of such biofilmed preparations against phytopathogenic fungi, but not towards selected nematodes, illustrates their promise in agriculture as potential inoculants that can effectively establish in soil.

Cyanobacteria-PGPR Interactions for Effective Nutrient and Pest Management Strategies in Agriculture

Soil microorganisms are known to play an active role in increased crop yields and soil fertility through a diverse array of mechanisms and such organisms are termed as PGPR (Plant Growth Promoting Rhizobacteria). This enhancement has been attributed to their involvement in the cycle of nutrients like carbon and nitrogen or in the decomposition of the organic matter, or production of allelopathic metabolites or enzymes influencing the pathogenic flora/fauna which indirectly promotes plant growth. Cyanobacteria are a ubiquitous group of organisms which have been relatively less investigated as PGPR, although their role in nitrogen dynamics of paddy based cropping systems is well investigated. Cyanobacteria are known to produce compounds with a wide range of activities, including phytohormones, biocidal metabolites or nutraceuticals. The interactions between agriculturally useful heterotrophic bacteria and autotrophs such as cyanobacteria can be effective and environment friendly options as biocontrol agents and biofertilizers. Plant-microbe partnerships are increasingly being focussed for not only nutrient management, but also for improving biomass production and remediation of polluted/inhospitable environments. This compilation provides an overview of the developments on this aspect and projections for the future.

Influence of phosphorus and pH on the fungicidal potential of Anabaena strains

The genus Anabaena is known to be a rich source of bioactive metabolites, but the biocontrol potential of this genus, mediated through hydrolytic enzymes is less investigated. In our investigation, five Anabaena strains – A. laxa RPAN8, A. iyengarii RPAN9, A. variabilis RPAN59 and A. oscillarioides RPAN69 (with A. variabilis RPAN16 serving as negative control) were evaluated in time course studies involving incubation under three levels of phosphorus and pH conditions. Total chlorophyll, proteins, chitosanase, endoglucanase and CMCase activity were measured and inhibition assayed against phytopathogenic fungi. The four weeks old RPAN69 culture showed significantly higher chlorophyll which was 41% higher than control. This was also linked with an enhancement of 18.26% and 9.18% in chitosanase and CMCase activity respectively over control in the treatment involving half dose of phosphorus. Chlorophyll and CMCase activity showed a high degree of correlation with highest values at pH 9.5. A pH of 5.5 was the most suitable condition for the maximum activity of chitosanase for all the strains except RPAN16. The strains RPAN8 and RPAN9 showed the highest activity of endoglucanase at pH 5.5 while the other strains exhibited maximum activity at pH 7.5. This study provides insight into the role of P and pH in modulating fungicidal activity in different Anabaena strains, which can be valuable for enhancing their efficiency as a biocontrol agent.

Analyses of diversity among fungicidal Anabaena strains

This study characterized 28 Anabaena strains exhibiting fungicidal activity using different molecular approaches. Based on 16S rDNA and internal transcribed spacer region sequences, all the Anabaena strains isolated from India were found genetically diverse from the other reference strains in the National Center for Biotechnology Information database. Out of these, six belonging to five different species were found to produce microcystin toxins as confirmed by polymerase chain reaction amplification of microcystin synthase gene and an enzyme-linked immunosorbent assay-based method. All the sequenced Anabaena strains with the fungicidal trait formed a separate group in both the trees, when compared to other reference Anabaena strains. Thus, it can be hypothesized that the antifungal trait has led to evolutionary divergence among these strains. This study illustrates the diversity among the Anabaena strains with fungicidal activity and their uniqueness vis a vis Anabaena strains available globally.

Development of microtitre plate-based assay for evaluation of fungicidal potential of cyanobacterial metabolites

An investigation was undertaken to optimise the microtitre plate-based assay for undertaking in-depth analyses of the potency and mode of action of cyanobacterial metabolites exhibiting fungicidal activity. The 96-well titre plate, using potato dextrose agar medium was standardised for evaluating minimum inhibitory concentration (MIC) of cyanobacterial metabolites against several phytopathogenic fungi, in terms of volume of media, concentration/volume of metabolite, inoculum and wavelength to be used for scanning. The optimised protocol was employed for recording growth inhibition in terms of MIC and facilitating microscopic analyses of morphological abnormalities induced by cyanobacterial metabolites in the fungal hyphae. This study not only illustrated the utility of the newly developed titre plate assay for analyses of large number of samples simultaneously and but also represented a first time report on microscopic observations related to various facets of fungicidal activity exhibited by cyanobacterial metabolites. Future research is directed towards scale up of this method for studies on tripartite interactions of cyanobacterial metabolites, target fungi with selected host plants, as a prelude to their use as biocontrol agents.