EARLY COMMUNICATION IN THE GUNNERA-NOSTOC SYMBIOSIS - PLANT-INDUCED CELL-DIFFERENTIATION AND PROTEIN-SYNTHESIS IN THE CYANOBACTERIUM

Establishment of the symbiosis between the angiosperm Gunnera L. and t he N-2-fixing cyanobacterium Nostoc is achieved by infection of specia lized plant stem glands by the cyanobacterium. Here we report the find ing that the acidic mucilage secreted by the Gunnera glands carries si gnal molecules that specifically 1) induced the differentiation of Nos toc filaments into motile so-called hormogonia, essential for infectio n, This effect was seen in compatible as well as incompatible strains, Proteinase K treatment of the mucilage abolished hormogonium inductio n, indicating that the inducing compound was a protein, Neither extrac ts from different Gunnera plant parts, nor seed rinse induced hormogon ium formation, In addition, red light as well as darkness induced, whi le low pH per se inhibited hormogonium differentiation. The mucilage a lso 2) stimulated growth, and 3) rapidly induced a new polypeptide of approximately 40 kDa and an increased synthesis of a 65-kDa polypeptid e exclusively in compatible Nostoc strains, Hence, we conclude that mu cilage secreted by the Gunnera glands is a major component in a signal ing system between the plant and the cyanobacterium, and that establis hment depends on both symbiosis-specific and unspecific events,