Identification of a morphogene required for tapered filament termini in filamentous cyanobacteria

Although the photosynthetic cyanobacteria are monophyletic, they exhibit substantial morphological diversity across species, and even within an individual species due to phenotypic plasticity in response to life cycles and environmental signals. This is particularly prominent among the multicellular filamentous cyanobacteria. One example of this is the appearance of tapering at the filament termini. However, the morphogenes controlling this phenotype and the adaptive function of this morphology are not well defined. Here, using the model filamentous cyanobacterium Nostoc punctiforme ATCC29133 (PCC73102), we identify tftA, a morphogene required for the development of tapered filament termini. The tftA gene is specifically expressed in developing hormogonia, motile trichomes where the tapered filament morphology is observed, and encodes a protein containing putative amidase_3 and glucosaminidase domains, implying a function in peptidoglycan hydrolysis. Deletion of tftA abolished filament tapering inidcating that TftA plays a role in remodelling the cell wall to produce tapered filaments. Genomic conservation of tftA specifically in filamentous cyanobacteria indicates this is likely to be a conserved mechanism among these organisms. Finally, motility assays indicate that filaments with tapered termini migrate more efficiently through dense substratum, providing a plausible biological role for this morphology.