Genotypic and phenotypic diversity of Anabaena isolates from diverse rice agro-ecologies of India

A critical review of soil algae as a crucial soil biological component of high ecological and economic significance

Aero-terrestrial algae are ecologically and economically valuable bioresources contributing to carbon sequestration, sustenance of soil health, and fertility. Compared to aquatic algae, the literature on subaerial algae is minimal, including studies of distinctive habitats such as forest soils, agricultural fields, deserts, polar regions, specific subaerial zones, artificial structures, and tropical soils. The primary goal here was to identify the gaps and scope of research on such algae. Accordingly, the literature was analyzed per sub-themes, such as the "nature of current research data on terrestrial algae," "methodological approaches," "diversity," "environmental relationships," "ecological roles," and "economic significance." The review showed there is a high diversity of algae in soils, especially members belonging to the Cyanophyta (Cyanobacteria) and Chlorophyta. Algal distributions in terrestrial environments depend on the microhabitat conditions, and many species of soil algae are sensitive to specific soil conditions. The ecological significance of soil algae includes primary production, the release of biochemical stimulants and plant growth promoters into soils, nitrogen fixation, solubilization of minerals, and the enhancement and maintenance of soil fertility. Since aero-terrestrial habitats are generally stressed environments, algae of such environments can be rich in rare metabolites and natural products. For example, epilithic soil algae use wet adhesive molecules to fix them firmly on the substratum. Exploring the ecological roles and economic utility of soil and other subaerial algae could be helpful for the development of algae-based industries and for achieving sustainable soil management.

Influence of cyanobacterial inoculation on the culturable microbiome and growth of rice

Rice plants are selective with their associations with bacteria that are beneficial for growth, nutrient uptake, exhibit induced resistance or antagonism towards pathogens. Cyanobacteria as bioinoculants are known to promote the growth and health of rice plants. The present investigation was aimed at understanding whether and how cyanobacterial (Calothrix elenkinii) inoculation influenced the rice plant growth and the culturable bacterial populations and identifying the dominant culturable "microbiome" members. The plant tissue extracts were used to enumerate populations of the culturable microbiome members using selected enrichment media with different nutrient levels. About 10-fold increases in population densities of culturable microbiome members in different media were recorded, with some isolates having metabolic potential for nitrogen fixation and phosphorus solubilization. Fatty acid methyl ester (FAME) analysis and 16S rRNA sequencing of selected microbial morphotypes suggested the predominance of the members of Bacillaceae. Significant increases in plant growth attributes, nitrogenase activity and indole acetic acid production, and activities of hydrolytic and defense enzymes were recorded in the Calothrix inoculated plants. The PCR-based analysis and scanning electron microscopic (SEM) observations confirmed the presence of inoculated cyanobacterium inside the plant tissues. This investigation illustrated that cyanobacterial inoculation can play significant roles in improving growth and metabolism of rice directly and interact with the beneficial members from the endophytic microbiome of rice seedlings synergistically.

Deciphering the factors associated with the colonization of rice plants by cyanobacteria

Cyanobacteria-rice plant interactions were analyzed using a hydroponics experiment. The activity of plant defense and pathogenesis-related enzymes, scanning electron microscopy, growth, nitrogen fixation (measured as ARA), and DNA fingerprinting assays proved useful in illustrating the nature of associations of cyanobacteria with rice plants. Microscopic analyses revealed the presence of short filaments and coiled masses of filaments of cyanobacteria near the epidermis and cortex of roots and shoot tissues. Among the six cyanobacterial strains employed, Calothrix sp. (RPC1), Anabaena laxa (RPAN8), and Anabaena azollae (C16) were the best performing strains, in terms of colonization in roots and stem. These strains also enhanced nitrogen fixation and stimulated the activity of plant defense/cell wall-degrading enzymes. A significantly high correlation was also recorded between the elicited plant enzymes, growth, and ARA. DNA fingerprinting using highly iterated palindromic sequences (HIP-TG) further helped in proving the establishment of inoculated organisms in the roots/shoots of rice plants. This study illustrated that the colonization of cyanobacteria in the plant tissues is facilitated by increased elicitation of plant enzymes, leading to improved plant growth, nutrient mobilization, and enhanced plant fitness. Such strains can be promising candidates for developing "cyanobacteria colonized-nitrogen-fixing rice plants" in the future.

Developing biochemical and molecular markers for cyanobacterial inoculants

Markers for evaluating the establishment of cyanobacteria based on their sensitivity or resistance to antibiotics, saccharide utilization patterns and PCR generated fingerprints were developed. Four selected strains (isolates from rhizosphere soils of diverse agro-ecosystems) have shown potential as diazotrophs and exhibited plant growth promoting abilities. Different responses were obtained on screening against 40 antibiotics, which aided in developing selectable antibiotic markers for each strain. Biochemical profiles generated using standardized chromogenic identification system (including saccharide utilization tests) revealed that 53 % of the saccharides tested were not utilized by any strain, while some strains exhibited unique ability for utilization of saccharides such as melibiose, cellobiose, maltose and glucosamine. PCR based amplification profiles developed using a number of primers based on repeat sequences revealed the utility of 3 primers in providing unique fingerprints for the strains.

Identification, characterization, and regulation of a novel antifungal chitosanase gene (cho) in Anabaena spp

Two contrasting cyanobacterial species (Anabaena fertilissima and Anabaena sphaerica) were selected based on differences in antifungal behavior in order to study the mechanism for production of an antifungal enzyme and the genes responsible for this production. In A. fertilissima, chitosanase and antifungal activities were increased significantly under of growth-limiting conditions (8 of light and 16 h of darkness). The lack of such activities in A. sphaerica was associated with high levels of protein that accumulated during the stationary phase (at 28 days) under the same light conditions. The gene putatively responsible for chitosanase and antifungal activities was amplified using specific primers, and sequence analysis of the amplified products (1.086 and 1.101 kb in A. sphaerica and A. fertilissima, respectively) showed that they belong to the glycoside hydrolase 3 (GH3)-like family of Anabaena variabilis ATCC 29413. Pairwise alignment of the corresponding protein sequences identified a putative signal peptide (amino acids 1 to 23) and some amino acid changes in the sequence of A. fertilissima which may be responsible for functioning of the chitosanase and the observed antifungal activity. Hydrolysis of the chitosan oligosaccharide (GlcN)(5) to (GlcN)(2) and (GlcN)(3) confirmed the presence of chitosanase activity in A. fertilissima. Site-directed mutagenesis of the A. fertilissima chitosanase-encoding gene (cho) led to identification of catalytic residues (Glu-121 and Glu-141) important for the antifungal effect of the cho product. The level of expression of cho was monitored by quantitative real-time reverse transcription-PCR, which indicated that transcription of this gene is significantly enhanced under conditions that retard growth, such as a long dark period.