Early Communication in the Gunnera-Nostoc Symbiosis: Plant-Induced Cell differention and Protein Systhesis in the Cyanobacterium. Ulla Rasmussen. Department of Botany, Stockholm University, S-10691 Stockholm, Sweden. Christina Johansson, and Birgitta Bergman, Department of Botany, Stockholm University, S-10691 Stockholm, Sweden. MPMI 7:696-702. Accepted 26 July 1994. Copyright 1994 The American Phytopathological Society.

Establishment of the symbiosis between the angiosperm Gennera L. and the N₂-fixing cyanobacterium Nostoc is achieved by infection of specialized plant stem glands by the cyanobacterium. Here we report the finding that the acidic mucilage secreted by the Gunnera glands carries signal molecules that specifically 1) induced the differentiation of Nostoc filaments into motile so-called hormogonia, essential for infection. This effect was seen in compatible as well as incompatible strains. Proteinase K treatment of the mucilage abolished hormogonium induction, indicating that the inducing compound was a protein. Neither extracts from different Gunnera plant parts, not seed rinse induced hormogonium formation. In addition, red light as well as darkness induced, while low pH per se inhibited hormogonium differentiation. The mucilage also 2) stimulated growth, and 3) rapidly induced a new polypeptide of approximately 40 kDa and an increased synthesis of a 65-kDa polypeptide exclusively in compatible Nostoc strains. Hence, we conclude that mucilage secreted by the Gunnera glands is a major component in a signaling system between the plant and the cyanobacterum, and that establishment depends on both symbiosis specific and unspecific events.