The role of endocytosis in the self-incompatibility response of Arabidopsis lyrata

The self-incompatibility (SI) response in crucifer plants is due to the allele-specific interaction of the S -locus receptor kinase (SRK) at the surface of papillar cells with its ligand in the pollen coat. SRK and its ligand are encoded by one unique, multiallelic S -locus. SRK is part of a large family of plant receptor kinases (PRK) that show similarities to mammalian receptor kinases regarding regulation of their activity. One mechanism regulating receptor kinase activity is endocytosis. The SRK kinase domain interacts with the endosomal protein SNX1, which is involved in endocytic processes of mammalian receptor kinases. Todate, it is not known whether SRK internalizes and whether this process might regulate the SI response. The aim of my Master’s project was to find out whether SNX1 mediated endocytosis is required for the SI response and to establish a model system for studies of SRK endocytic trafficking. The first approach consisted in the observation of SI during overexpression, downregulation and mislocalization of SNX1 in papillar cells of the self-incompatible plant Arabidopsis lyrata. Only two putative SNX1 overexpressing plants were obtained. In both, no phenotype concerning self-incompatibility was observed, which is likely to be due to insufficient SNX1 overexpression. However, as SNX1 was fused to the Green Fluorescent Protein (GFP), it was possible to visualize in papillar cells compartments that are supposed to be endosomes. In a second part of my project, the self-compatible ”model-plant” Arabidopsis thaliana, a close relative of A. lyrata, was used to establish a model system for an easier study of SRK trafficking. Due to transactivated expression of Red Fluorescent Protein (RFP) fused markers of the plasma membrane, endosomes and the trans golgi network, these important compartments of the endocytic pathway were visualized by laser scanning confocal microscopy in A. thaliana papillar cells. In order to co-express these markers with a Green Fluorescent Protein GFP tagged SRK, we isolated a new, entire SRK allele from A. lyrata. Transient overexpression of the GFP fused AlSRK in planta was successful. Thus, my Master’s work provided the foundations for analyzing SRK endocytic trafficking in the model plant A. thaliana.