Question

Is cell differentiation is a feature unique to multicellular eukaryotes?  Discuss the molecular mechanisms behind how nitrogen fixation in the filamentous cyanobacteria Anabeana and the parallels to eukaryotic developmental processes.

Answer 1

Cellular differentiation is the process in which a cell changes from one cell type to another. Usually, the cell changes to a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a Symbioses with N2-fixing cyanobacteria

Some nitrogen-fixing cyanobacteria (Nostoc, Anabaena) form with various plants the symbioses that differ in many respects from the legume nodules. Many of these differences are caused by the ability of cyanobacteria to fix N2 in free-living state and to support this fixation with products of their own photosynthesis. The mechanisms of free-living N2 fixation are used in symbioses, but here it is controlled by the hosts due to the localization of cyanobacteria in special symbiotic organs. In the water fern Azolla., cyanobacteria are located within the leaf cavities, in the gymnosperm Cycas – within the intercellular spaces of coralloid roots, in the gymnosperm Gunnera – within the cells in the glands at the bases of leaf petioles.complex system of tissues and cell types. When transferred into N-free medium, 5–10% of cells in the Nostoc or Anabaena filaments are differentiated into heterocysts that, like the rhizobial bacteroids, are devoid of reproductive ability. Heterocysts are enlarged and have thick walls blocking the oxygen diffusion inside the cells. Due to the arrest of photosynthesis, microaerobic conditions are reached inside the heterocysts permitting the nitrogenase synthesis. Another pathway of cellular differentiation in Nostoc is represented by motile hormogonia – filaments consisting of small cells with gas vacuoles.

These variants of cellular differentiation are also used by cyanobacteria during symbioses with plants, but herein the microbial ontogeny is combined with the development of specialized plant structures and with very high N2-fixing activity required to supply plants with nitrogen. During the pre-infection contacts, massive formation of motile hormogonia is induced in Nostoc that colonize actively the interiors of Gunnera glands. This process may be elicited by host: active differentiation of hormogonia was observed in Nostoc treated with matrix extracted from the Gunnera glands and containing polysaccharides and proteins. Within these glands cyanobacteria propagate actively and infect the plant cells. In the points of tight cellular contact, the plant cell walls are lysed and after the entry of cyanobacteria their integrity is restored. Within plant cytoplasm the cyanobacteria are encapsulated into the symbiosome membranes and later up to 80% of cells are converted into N2-fixing heterocycts.

Calcium is one of the most important signalling molecules in eukaryotes and calcineurin is one of the most important target proteins for calcium ions. Therefore calcineurin is involved in a plethora of cellular and developmental processes in higher and lower eukaryotes. Beside species-specific functions of this phos-phatase a couple of parallels exist between higher and lower eukaryotes. The most striking parallel might be that calcineurin is involved in the development of “filamentous structures” like dendrites, blood vessels, or hyphae by regulating tip dominance and branching. Impairment of calcineurin function leads to de-velopmental defects, which are accompanied in many cases by death of animals during embryogenesis or cell death. However, recent studies show that some mammalian isoforms of calci-neurin have very specific duties in an organ-dependent manner. Furthermore, studies in higher and lower eukaryotes show that the phosphatase acts also spatiotemporal. The function of the different isoforms and the spatiotemporal action need more at-tendance in future studies. A couple of target proteins of calci-neurin have been identified (Li et al., 2011) but current research still focuses mainly on NFAT/Crz1-orthologues. More research is needed to elucidate the role of other calcineurin targets during developmental processes. The parallels between higher and lower eukaryotes concerning the function of calcineurin during developmental processes show that lower eukaryotes with simple developmental pro-grams can be very helpful models to study the involvement of calcineurin in detail.