| The concept of cell surface perception of external stimuli is well established in living organisms, in particular animal systems. Recent advances in plant biology have shown that plants utilize receptor protein kinases (RPK) to perceive and respond to a variety of external stimuli. Plant RPKs are involved in an array of growth and developmental processes, steroid and peptide hormone perception, recognition and defense responses towards pathogens, and recognition and rejection of self-pollen. Though the function of a number of plant RPKs are known, the signaling pathways that they initiate are not well understood.; The recognition of “self” is of particular importance not only to plants but also to other species such as fungi (mating systems) and eukaryotes (major histocompatibility complex—MHC). In plant reproduction, the mechanism of self-incompatibility, utilized by over half of all flowering species, is essential for preventing inbreeding and promoting out-crossing. In Brassica species self-incompatibility involves a cell-cell signaling event between the pollen and the stigma. Within the stigma, signaling via the receptor kinase, SRK (S Locus Receptor Kinase) governs self-incompatibility. Recently, a number of advances have been made in the understanding of the molecular basis of self-pollen recognition. However, the events following the activation of SRK and the mechanism by which the signaling pathway facilitates the rejection of self-pollen is not known.; The novel Brassica napus protein ARC1 (Arm Repeat Containing 1) was identified as a potential downstream component of SRK signaling. The work presented in this thesis provides evidence for a biological role for ARC1 in self-incompatibility. The loss of ARC1 expression in self-incompatible Brassica napus W1 transgenic plants correlated with a partial breakdown in self-incompatibility and the production of a significant number of seeds after self-pollination. ARC1 was further shown to be an E3 ubiquitin ligase capable of mediating protein ubiquitination. The data presented indicate that during self-pollination, ARC1 is recruited by SRK to mediate protein ubiquitination and proteasomal degradation so as to promote the inhibition of self-pollen. |
