Title : Recombinant glycinebetaine improves metabolic activities, ionic balance and salt tolerance in diazotrophic freshwater cyanobacteria
Abstract:
To find out the physiologic mechanisms contributing to increased tolerance to salinity by glycinebetaine in diazoptrophic cyanobacteria, methyl transferase gene ApGSMT-DMT catalyzing glycinebetaine synthesis in Aphanothece halophytica was transferred into N2-fixing filamentous cyanobacteria Anabaena PCC7120 and Anabaena doliolum. The transformants synthesized/accumulated significant amount of glycinebetaine. Under salt-stress (0.1 M NaCl), the transformants were more efficient in reducing nitrate and its incorporation into amino compounds compared to wild-types indicating that glycinebetaine not only shielded the enzymes from deleterious effect of salt but enhanced their activities. High photosynthetic rate of ApGSMT-DMT transformants under salt-stress exhibited haloprotection of the photosynthetic machinery, while reduced respiration revealed efficient use of photosynthates. Salinity reduced the electron flow activity of PS II causing damage to cytochrome b6-f complex, which was overcome to an extent by glycinebetaine. The observed limited electrons transport from PSII to PSI under salinity favored additional excitation of PSI resulting in increased electron transport activity of PSI. Salinity reduced the intracellular K+/Na+ and Ca2+/Na+ ratios, while transformants showed the highest K+/Na+ ratio. This attempt to develop salt tolerant N2-fixers can increase the nitrogen availability under saline conditions, and opens the way to transform symbiotic and free-living N2-fixers to increase the salt tolerance and productivity.
Key words: N2-fixing cyanobacteria, Glycinebetaine, Intracellular cation, Nitrate assimilation, Photosynthesis, Respiration, Salinity
In natural ecosystems, organisms often grow under suboptimal growth conditions and face biotic and abiotic stresses. Salinity is one of the most deleterious factors that limit the growth and yield of the organisms. Salinity increases the intracellular levels of Na+ which competes with the transport of K+ and Ca2+ resulting in an ion imbalance, ion toxicity and nutrient deficiencies. Cyanobacteria, owing to their great evolutionary age and great adaptability are usefulmodels for eukaryotic organisms. In a typical prokaryotic cell organization, they possess water-oxidizing photosynthesis of higher plants as well as nitrogen-fixing ability. The latter property makes cyanobacteria of agronomic importance. Salinization of cultivable land is a global concern. The deleterious effect of salinity on soil fertility and plant productivity has become a serious threat to the global food security culminating into various socio-economic problems. To address the problem, we reported the first successful transformation of nitrogen-fixing, filamentous and heterocyst forming cyanobacteria with ApGSMT-DMT genes. The transformants synthesized and accumulated GB. The
present study analyzes the effect of salinity on physiologic processes such as nitrate assimilation, photosynthesis and photosynthetic electron transport chain activities, respiration and intracellular cation concentrations (Na+, K+ and Ca2+) in transformed (with ApGSMT-DMT) cyanobacterial cells.
