Evidence for a role for L‐proline as a salinity protectant in the cyanobacterium Nostoc muscorum

Nostoc muscorum required an active proline oxidase in order to assimilate exogenous proline as a source of fixed nitrogen. A mutant strain (Acr) resistant to growth inhibition by L‐azetidine‐2‐carboxylate (AC) was found to be deficient in proline oxidase activity, and to be a proline overaccumulator. Proline overaccumulation, resulting either from mutational acquisition of the Acr phenotype or from salinity‐inducible uptake of exogenous proline, conferred enhanced salinity tolerance in this cyanobacterium.