Intercellular communication in filamentous cyanobacteria

Abstract

Cyanobacteria represent one of the morphologically most diverse groups in the bacterial world, and one of the few where true multicellularity evolved. Their diversity ranges from single cells to branched filamentous forms. Some filamentous cyanobacteria are additionally able to undergo cell differentiation forming a two cell type system that represents the simplest model of multicellularity. Anabaena sp. PCC 7120 has been studied extensively in recent years as a model organism. Under nitrogen deprivation it differentiates photosynthetically-active vegetative cells into heterocysts, specialised cells for nitrogen fixation. Accordingly, true multicellularity of nitrogen-fixing cyanobacteria requires different forms of intercellular communication and mutual exchange of nutritional and regulatory compounds. Although several studies could show that molecules are exchanged between cells in Anabaena sp. PCC 7120, little is known about the properties and routes that allow molecules to diffuse between adjacent cells. In this work, the transfer of several physiologically-important molecules was investigated, including the fluorescent sucrose analogue esculin, the fluorescent glucose derivatives 2-NBDG (2-(N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose) and 6-NBDG (6-(N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-6-deoxyglucose), and the artificial fluorescent tracer BCECF (2′,7′-Bis-(2-carboxyethyl)-5-(and-6-)carboxyfluorescein). Furthermore, the role of the septal proteins FraC, FraD and SepJ for the exchange of these molecules was examined in Anabaena sp. PCC 7120. Confocal microscopy and fluorescence recovery after photobleaching (FRAP) experiments reveal that cytoplasmic diffusion through channels between neighbouring cells is the primary route for molecular exchange. Loss of FraC, FraD and SepJ strongly reduced the movement of molecules between cells. Overexpression of sepJ altered the heterocyst spacing pattern in Anabaena sp. PCC 7120, suggesting a role of SepJ for the diffusion of regulators. Further, multicellularity in terms of intercellular communication was explored in Mastigocladus laminosus, a true branching cyanobacterium that is one of the morphologically most complex prokaryotes. FRAP analyses indicate the cytoplasmic continuity of the filament network and the dependency of molecular exchange on the morphology of the trichomes.