Complex Symbiotic Phenotypes Result from Gluconeogenic Mutations in Rhizobium meliloti. Turlough M. Finan. Department of Biology, McMaster University, Hamilton, Ontario, Canada, L8S 4K1. Elizabeth McWhinnie(1), Brian Driscoll(1), and Robert J. Watson(2). (1)Department of Biology, McMaster University, Hamilton, Ontario, Canada, L8S 4K1; (2)Plant Research Centre, Agriculture Canada, Ottawa, Ontario, Canada K 1 A 0C6.. MPMI 4:386-392. Accepted 17 April 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991.

Mutants of Rhizobium meliloti defective in one of the following enzyme activities have previously been reported: phosphoenol-pyruvate carboxykinase (Pck‾), enolase (Eno‾), glyceraldehyde-3-phosphate dehydrogenase (Gap‾), and 3-phosphoglycerate kinase (Pgk‾) (Finan, Oresnik, and Bottacin. J. Bacteriol. 170:3396-3403). Here we report the symbiotic N₂-fixing activities (acetylene reduction and plant dry weight) and structure of alfalfa nodules induced by these gluconeogenic mutants. In addition, we have determined the activities of the above enzymes in extracts from wild-type bacteroids isolated from alfalfa nodules. The Gap‾, Eno‾, and Pgk‾ mutants formed small white Fix‾ nodules with acetylene-reducing activities of less than 5% of wild type. Each of these mutants induced nodules with a similar structure; the nodules lacked a distinct symbiotic zone and had relatively few infected cells. Plants inoculated with the Pck‾ mutants had acetylene-reducing activities of approximately 60% of plants inoculated with the wild-type bacteria. No significant abnormalities in the ultrastructure of nodules induced by the Pck‾ mutants were found. Paradoxically, no phosphoenolpyruvate carboxykinase activity was detectable in wild-type bacteroid extracts, indicating that gluconeogenesis does not occur in bacteroids. A possible role for phosphoenolpyruvate carboxykinase in controlling bacteroid proliferation is suggested.