Early Determinants of Heterocyst Differentiation in Multicellular Cyanobacteria

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Certain multicellular cyanobacteria develop trichomes in which cells are connected by cell-cell adhesion and communicate through gated septal junctions. These cyanobacteria differentiate up to four different cell types and the trichomes respond to certain stimuli as a whole. Thus, these cyanobacteria fulfill all hallmarks of true multicellular organisms. Our proposal targets the earliest steps in the differentiation process of heterocysts. Heterocysts are specialized for the fixation of atmospheric N2 using the enzyme nitrogenase and are photosynthetically inactive. Heterocysts and the neighboring photosynthetically active vegetative cells crucially depend on each other and exchange metabolites and signals, which makes the heterocyst-specific N2 fixation the emergent function that allows these cyanobacteria to survive in nitrogen-poor environments. In cyanobacteria belonging to the Nostocales, mature heterocysts differentiate from vegetative cells upon removal of combined nitrogen from the environment over a time course of ~24 h, making the process of cell differentiation accessible for detailed research. Our hypothesis 1 is that the vegetative cells in a filament are not uniform but that pre-existing spatial heterogeneities render certain cells more likely to enter the pathway toward heterocyst differentiation than others. Cells that enter the differentiation process may be determined in a process that results from the stochastic nature of transcription (hypothesis 2). Previous approaches often screened mutant libraries for the inability to grow on N2. Thus, essential and redundant genes or factors affecting patterning or time of commitment would have been missed. Therefore, we expect to identify genes and molecular factors that have remained entirely uncharacterized or unrecognized for their role in heterocyst differentiation thus far our (hypothesis 3). The multicellularity of heterocyst-forming cyanobacteria is an important element in natural assemblages of different bacteria, which will, however, only materialize if a very specific set of parameters is set in the environment (hypothesis 4). The project focuses on understanding the earliest steps in the heterocyst differentiation process in the genetically tractable and well-characterized model Nostoc 7120, on the evolution of the involved genes and mechanisms from non-heterocystous cyanobacteria and on the impact heterocyst differentiation has on environmental bacterial communities. We will apply single cell transcriptomics, identify and characterize early regulators of multicellular decision making, identify tipping points for heterocyst differentiation and extend from analyses in the laboratory to the impact on environmental populations.