Enhanced Levels of Chalcone Synthase in Alfalfa Nodules Induced by a Fix¯ Mutant of Rhizobium meliloti. Erich Grosskopf. Institute of Genetics, BRC, Hungarian Academy of Sciences, Szeged, Hungary, Institut für Biochemische Pflanzen-pathologie, GSF München, Germany. Dang Thi Cam Ha(1), Ruth Wingender(3), Horst Röhrig(3), Judit Szecsi(4), Eva Kondorosi(4), Jeff Schell(3), and Adam Kondorosi(1,4). (1)Institute of Genetics, BRC, Hungarian Academy of Sciences, Szeged, Hungary; (3)Max-Planck-Institut für Züchtungsforschung, Cologne, Germany; (4)Institut des Sciences Végétales, C.N.R.S., Gif-sur-Yvette Cedex, France.. MPMI 6:173-181. Accepted 28 December 1992. 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, 1993.

Experiments were performed to analyze the level of chalcone synthase (CHS) mRNA expression after inoculation of alfalfa roots with different Fix¯ mutants of Rhizobium meliloti. Similar, low levels of CHS mRNA were detected in roots infected by the wild-type AK631, by the Fix¯ mutant TF178, and in uninfected roots. In contrast, roots infected with another Fix¯ mutant, AK1540, showed increased amounts of CHS mRNA at 16 and 18 days after inoculation. In nodules formed by wild-type AK631, CHS transcripts and proteins were present at low levels, and CHS mRNA was localized in situ in nodule cells occupied by bacteroids. Twenty-five percent of the nodules elicited by AK1540 were empty. In these empty nodules, CHS mRNA was strongly expressed 16-20 days after inoculation, with a maximum level at day 18. The CHS mRNA was localized mainly in the outermost cell layers of the empty nodules. The amount of CHS mRNA in invaded nodules elicited by AK1540 was only slightly increased or was comparable to the level in wild-type nodules. These data support the view that mutant bacteria which are unable to infect nodules can activate the plant defense response.