Evidence for plant-mediated regulation of nitrogenase expression in theAnthoceros-Nostoc symbiotic association

Summary: A pleiotropic dinitrogen and nitrate assimilation mutant was obtained by mutagenesis of Nostoc sp. strain ATCC 29133 with N-methyl-N-nitro-N-nitrosoguanidine followed by penicillin counterselection in the presence of NO- 3 and N2. Mutant strain UCD 223 was capable of reducing acetylene in the free-living growth state only under anaerobic conditions, or under atmospheric conditions when in symbiotic association with Anthoceros punctatus. Heterocysts of strain UCD 223 were noticeably lacking the cohesive outer polysaccharide layer of wild-type heterocysts. Oxygen microelectrode profiles of symbiotic Anthoceros-Nostoc tissue revealed an anaerobic environment in the symbiotic cavities containing Nostoc. The acetylene-reducing activities of strain UCD 223, and of its spontaneously-arising Fix+ revertant strain UCD 236, were not repressed by the presence of 10 mm-NO- 3 when in the free-living growth state, in contrast to wild-type Nostoc ATCC 29133. However, in situ activities of acetylene reduction by symbiotically associated Nostoc ATCC 29133 and strains UCD 223 and UCD 236 were repressed by the presence of 10 mm-NO- 3. It appears that the symbiotic cavities of Anthoceros punctatus can physiologically replace the function of the heterocyst outer wall and that the repression of nitrogenase activity in symbiotic Nostoc in response to the presence of NO- 3, and probably NH+ 4, is mediated by Anthoceros.