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 (0.1M 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.