Reduced Phosphorus Requirement of a Mutant Azolla-Anabaena Symbiotic N2-fixing Complex

The use of the photo-autotrophic nitrogen-fixing water fernAzolla as an effective source of organic nitrogen in tropicalpaddy fields has been limited by a high phosphorus requirement.Azolla species with a minimum of 1.5 to 2.0 mM phosphate (P)requirement, under controlled conditions, are known. A local Azolla species requiring at least 1.5 mM sodium phosphatefor a normal rate of multiplication and N₂ fixation was exposedto N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The resultingmutant population had a significantly lower P requirement, butwas auxotrophic for glutamine with an extremely reduced glutaminesynthetase (GS) activity. An L-methionine-DL-sulphoximine (MSX)-resistant(MSX^r) Azolla population, having an approximately 1.5 timeshigher GS activity than that of the wild type (WT) parent organism,was cultured and subjected to MNNG-induced mutation for lowP requirement while putting MSX as a control in the mutant selectionmedium. The resulting population of mutant Azolla was a normalprototroph with a P requirement as low as 0.75 mM for its ‘WTparent-like’ usual growth and N₂ fixation. Key words: Azolla, phosphorus requirement, mutation