Growth and development: Eukaryotes

If there were one word to define microorganisms, it is flexibility and eukaryotic microorganisms are not strangers to it. We used to associate this flexibility to their life styles as well as with their ability to metabolize an extremely wide source of food; however, less obvious but in a similar range of importance is the fact that eukaryotic microorganisms show highly flexible developmental processes which allow them to adapt to a broad range of conditions. Cell differentiation in simple eukaryotes is no less sophisticated or complex than is found in animals and plants, but it is very different. Simple eukaryotes can vary the timing, extent, and mode of differentiation in response to external signals. It all contributes to making them supremely able to adapt to challenging conditions. This results in a flexibility, which surpasses that of other organisms. The molecular basis for such flexibility is starting to be known. The section on eukaryotes in this issue of Current Opinion in Microbiology centers on how developmental pathways in simple eukaryotes result from the application of rules that allow great variability in the adaptability to environmental cues. A good example of this is provided by the new roles of NDR kinases. First described in Saccharomyces cerevisiae, this enzyme group gained interest as recent research in filamentous fungi, timely covered by Sabine Maerz and Stephan Seiler. NDR kinases are critical for growth and differentiation in all analyzed fungal species. These kinases are part of a broad conserved pathway, the MOR (morphogenesis-related NDR kinase network), and besides the elucidation of the central pathway components and the regulation of NDR kinase activity by phosphorylation and interacting proteins, one of the achievements covered by this review includes the connections with effector elements as diverse as small G-protein or transcription factors. MOR pathway is remarkable since, in spite of the conservation of all MOR pathway elements, impairing its function can result in highly divergent cellular responses, indicating a species-specific integration of the MOR in a cellular signaling context