title: Growth and Thallus Morphogenesis of Ulva mutabilis (Chlorophyta) Depends on A Combination of Two Bacterial Species Excreting Regulatory Factors.
abstract: Axenic Ulva mutabilis gametes develop parthenogenetically into callus-like colonies consisting of undifferentiated cells without normal cell walls. From the accompanying microbial flora of established laboratory strains of U. mutabilis with normal morphology, a Roseobacter, a Sulfitobacter, and a Halomonas species were isolated. Each of these microbe species alone induced the development of the Ulva gametes into thalli composed of differentiated cells with characteristic deficiencies. Typical traits of these thalli were: an enhanced rate of cell division not followed by cell expansion, the presence of unusual cell wall protrusions, and the absence of differentiated rhizoid cells. The addition of a Cytophaga species, also derived from the same microbial flora, to either one of the three other strains resulted in the development of normal fast growing thalli with the typical morphology of the algal strain used. These effects are mediated by specific regulatory factors that are excreted into the environment by the bacteria and could be also isolated from the bacterial cell extracts. In contrast with the Cytophaga-factor, the regulatory factor of the three other bacterial species was also found intracellularly in other bacterial strains not associated with Ulva, but in this case it was not excreted. Functionally, the Roseobacter-, Sulfitobacter-, and Halomonas-factors resemble a cytokinin, while the Cytophaga-factor acts similar to auxin. Neither factor could be replaced by known phytohormones. The Roseobacter species exhibits a specific chemotactic affinity to the rhizoid cells of U. mutabilis and seems to cooperate with the Cytophaga strain and the alga by chemical communication forming a symbiotic tripartite community.
authors.affiliation: Institute of Biochemistry, Genetics and Microbiology, University of Regensburg, Universitätsstraße 31, 93053, Regensburg, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Spoerner
authors.affiliation: Institute of Biochemistry, Genetics and Microbiology, University of Regensburg, Universitätsstraße 31, 93053, Regensburg, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Wichard
authors.affiliation: Institute of Biochemistry, Genetics and Microbiology, University of Regensburg, Universitätsstraße 31, 93053, Regensburg, Germany.
authors.forename: Tanja
authors.initials: T
authors.lastname: Bachhuber
authors.affiliation: Institute of Biochemistry, Genetics and Microbiology, University of Regensburg, Universitätsstraße 31, 93053, Regensburg, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Stratmann
authors.affiliation: Institute of Biochemistry, Genetics and Microbiology, University of Regensburg, Universitätsstraße 31, 93053, Regensburg, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Oertel
keywords: Cytophaga sp; Roseobacter sp; Ulva mutabilis; axenic alga; developmental mutant; differentiation; symbiosis; thallus morphology