Summary• The ability of Burkholderia phymatum STM815 to effectively nodulate Mimosa spp., and to fix nitrogen ex planta , was compared with that of the known Mimosa symbiont Cupriavidus taiwanensis LMG19424.• Both strains were equally effective symbionts of M. pudica , but nodules formed by STM815 had greater nitrogenase activity. STM815 was shown to have a broader host range across the genus Mimosa than LMG19424, nodulating 30 out of 31 species, 21 of these effectively. LMG19424 effectively nodulated only nine species. GFP-marked variants were used to visualise symbiont presence within nodules.• STM815 gave significant acetylene reduction assay (ARA) activity in semisolid JMV medium ex planta , but no ARA activity was detected with LMG19424. 16S rDNA sequences of two isolates originally from Mimosa nodules in Papua New Guinea (NGR114 and NGR195A) identified them as Burkholderia phymatum also, with nodA , nodC and nifH genes of NGR195A identical to those of STM815.• B. phymatum is therefore an effective Mimosa symbiont with a broad host range, and is the first reported beta-rhizobial strain to fix nitrogen in free-living culture.

Repoils of i-iodiihition in tbt-LcKLiminosae an-c\aniiiK-il in tbe li.ulit ol curri-nt views on tlu-taxonomy of tbe family. In tile subfamilv-C'ac-salpinioideac, nodulation is largely restricted to the tribe Caesalpmieae and the Ljeiuis ('/tctiiittccristit lrom tlu-C'assieae. .All nodules studied have rhizobiii retained within infection threads diirinu the nitrogen lixin^ period. In tbe .Mimosoideae, nodulation is general, except lor 4 groups w ithin the tribe Mimoseae. and a verv-few species of Acacia. The onlv' tribe from tbe Papilionoideae which appears not to ncHlulate is the Diptery^eae, altbouyh the monogeneric luichresteae has not been examined. .\ tiumbei-of genera in the Swartzieae ilo not nodulate. Takint; the family as a whole, nodulation appears to be very iinitorm certain sections niHhilate, otbers do not.

Synchronized akinete differentiation occurred following the transition from exponential to non-exponential (linear) growth, the major trigger being energy limitation. Young akinetes first accumulated cyanophycin, then developed a multilayered extracellular envelope and a thickened wall. The dry weight, chlorophyll a, glycogen and carbon contents of mature akinetes were greater than those of vegetative cells, while their contents of DNA, RNA, protein, phycocyanin and nitrogen were similar to those of vegetative cells. Akinetes were resistant to desiccation and low temperatures, but not to temperatures above the maximum for vegetative cell growth.In N,-grown cultures heterocyst differentiation ceased at the end of exponential growth, while cell division continued, and akinetes first appeared in a regular pattern at a fixed distance (9 cells) from the nearest heterocyst. Exogenous NH,+ inhibited the differentiation of heterocysts and, in their absence, akinetes developed in irregular positions. The regular spatial pattern imposed on akinete differentiation by heterocysts was, like the heterocyst spatial pattern itself, independent of N, fixation. Similar changes in both patterns induced by 7-azatryptophan suggested that they share a common mechanism of control.

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