Armadillo Homologues in Dictyostelium discoideum

Armadillo family proteins are present throughout the animal kingdom. In fruit flies, frogs, fish and mammals these proteins are essential for correct embryonic patterning via the Wnt/Wingless signal transduction pathway, acting downstream of glycogen synthase kinase-3 (gsk3). In addition, the same proteins are essential for regulated cell to cell adhesion. These proteins must be tightly controlled to prevent the growth and metastasis of tumours in the adult organism. However, the relationship between their developmental and adhesive role is not well understood at present. The cellular slime mould Dictyostelium discoideum is a model system in which to study multicellular development and cell adhesion. The Dictyostelium homologue of glycogen synthase kinase-3, gskA, is essential for the correct proportioning of cell types during development. The downstream targets of gskA remained to be discovered at the start of this work. I have isolated an Armadillo family gene, aardvark (aar), in Dictyostelium and created a knock-out mutant. The mutant phenotype demonstrates that aar has an important role in the later stages of development. Aardvark is crucial for defining the final structure of the fruiting body, particularly the assembly of the stalk tube and its associated cell-cell adhesion junctions. Perturbation of this structure in the aar mutant leads to a duplication of the developmental axis. Aar also has a cell-autonomous role, promoting the terminal differentiation of stalk and spore cells. Furthermore, I present evidence that two other Armadillo homologues are also present in Dictyostelium. These proteins are developmentally regulated and their levels and distribution are altered in a gskA- mutant. This thesis suggests that the molecular control of pattern formation and cell to cell adhesion may be very highly conserved throughout evolution and that in Dictyostelium, as in animals, Armadillo homologues act to coordinate morphogenesis and cell identity

Understanding “green” multicellularity: do seaweeds hold the key?

International audienceLiving organisms are unicellular, composed of a single cell, or multicellular, where a group of up to ~1012 cells functions co-operatively (Kaiser, 2001). All multicellular organisms evolved from single-celled ancestors; every individual organism arises from a unicell and reproduces by forming unicells. Multicellularity enables competitive advantages, and may have shaped our oxygen-rich atmosphere (Grosberg and Strathmann, 1998; Kaiser, 2001; Schirrmeister et al., 2013). Multicellularity has evolved multiple times: animals, plants, algae, amoebae, fungi, and bacteria are or can all be multicellular (King, 2004; Grosberg and Strathmann, 2007; Rokas, 2008; Claessen et al., 2014). Multicellularity can be clonal (arising from division of a single cell) or aggregative (aggregation of genetically diverse cells), with clonal multicellularity considered evolutionarily more stable (Grosberg and Strathmann, 1998). The molecular mechanisms by which organisms become multicellular are not well understood. In this article, we outline eukaryotic multicellular evolution, and discuss how to increase our future understanding

Effects of green seaweed extract on Arabidopsis early development suggest roles for hormone signalling in plant responses to algal fertilisers

The growing population requires sustainable, environmentally-friendly crops. The plant growth enhancing properties of algal extracts have suggested their use as biofertilisers. The mechanism(s) by which algal extracts affect plant growth are unknown. We examined the effects of extracts from the common green seaweed Ulva intestinalis on germination and root development in the model land plant Arabidopsis thaliana. Ulva extract concentrations above 0.1% inhibited Arabidopsis germination and root growth. Ulva extract less than 0.1% stimulated root growth. All concentrations of Ulva extract inhibited lateral root formation.An abscisic-acid-insensitive mutant, abi1, showed altered sensitivity to germination- and root growth-inhibition. Ethylene- and cytokinin-insensitive mutants were partly insensitive to germination-inhibition. This suggests that different mechanisms mediate each effect of Ulva extract on early Arabidopsis development and that multiple hormones contribute to germination inhibition. Elemental analysis showed that Ulva contains high levels of Aluminium ions (Al3+). Ethylene and cytokinin have been suggested to function in Al3+-mediated root growth inhibition: our data suggest that if Ulva Al3+ levels inhibit root growth, this is via a novel mechanism. We suggest algal extracts should be used cautiously as fertilisers, as the inhibitory effects on early development may outweigh any benefits if the concentration of extract is too high