| dc.description.abstracteng | The complex sphingolipid classes glycosylceramides (GlcCers) and glycosyl inositolphosphorylceramides (GIPCs) are essential membrane components in plant cells. However, the regulation of their synthesis and their distinct physiological roles in plants is still poorly understood. GlcCers and GIPCs both contain a ceramide backbone consisting of a long-chain base (LCB) that is connected to a fatty acid. The syntheses of these two complex sphingolipids are alternative pathways in plant metabolism. It is assumed that distinct structural modifications in the LCB moiety determine the metabolic fate and physiological function of sphingolipids. In the bryophyte Physcomitrella patens, channelling of sphingolipid metabolites into complex sphingolipid formation appears to be stricter than in vascular plants. The physiological relevance of GlcCers, GIPCs, and their specific LCB moieties was therefore investigated in P. patens. P. patens GlcCers are enriched in ceramides with a dihydroxy, Δ4,8-diunsaturated LCB moiety while P. patens GIPCs mostly contain ceramides with a trihydroxy LCB moiety. The establishment of a sophisticated cultivation system and of various mutant characterisation assays is a prerequisite for in-depth examinations of P. patens mutants. P. patens knockout mutants were generated by homologous recombination that targeted key steps of sphingolipid biosynthesis. Disruption of the LCB C-4 hydroxylase, PpS4H, which is involved in GIPC formation, resulted in plants that were severely impaired in growth and development. These growth impairments might have derived from cell plate formation defects during cytokinesis. Loss of the trihydroxy LCB moiety also caused global changes in all sphingolipid classes. Disruption of the LCB Δ4-desaturase, PpSD4D, did not substantially affect plant viability. The mutant only showed mild cell elongation defects. However, sd4d-1 mutants had substantially reduced GlcCer levels, which confirms that LCB Δ4-desaturation is important for channeling sphingolipids into GlcCer formation in P. patens. In contrast, P. patens plants that had a disturbed glycosylceramide synthase, PpGCS, activity, were affected in plant growth and cell differentiation and showed cell death-like lesions. gcs-1 plants lacked all GlcCers and accumulated precursor hydroxyceramides. Cumulative findings from this work show that disruption of individual steps in P. patens sphingolipid biosynthesis differently affect plant physiology. The results give first insights into sphingolipid biosynthesis in P. patens. While some aspects of plant sphingolipid metabolism known from studies in Arabidopsis thaliana have been confirmed in the bryophyte, novel features of the sphingolipid pathway could also be identified in P. patens. These new findings contribute to our knowledge on how sphingolipid synthesis and function have diversified during land plant evolution. | de |