CRABS CLAW is a small protein belonging to the YABBY family, a plant specific protein family. In Arabidopsis thaliana it is expressed in the developing carpels and regulates the apical fusion of the two carpels, transmitting tract development, lateral growth, and nectary formation. The expression of CRC is rather complex with multiple expression domains throughout the young gynoecium and as for other YABBY proteins a non-cell-autonomous action has been described. However, only few regulators of CRC expression and target genes are described and the mode of non-cell-autonomous action is still unknown. This dissertation aims to identify transcriptional regulators, responsible for the proper temporal and spatial expression of CRC, the specification of CRC s place in the adaxial-abaxial regulatory network and to clarify the means of its non-cell-autonomous action. The regulation of CRC expression has been analyzed via a large scale Yeast-1-Hybrid screen and identified over 100 potential regulators of CRC expression, integrating CRC tightly into the carpel developmental regulatory protein network.Further analysis of CRC function through expression analysis led to the identification of target genes of CRC like mir165/166, members of the KANADI gene family, and the HD ZIP III gene family. Both gene families are major players in the adaxial-abaxial regulatory network, involved in the development of all lateral plant organs such as leaves and floral organs. CRC supports KANADI action and activates the expression of other involved factors. In addition, CRC directly targets members of the HD ZIP III family. However, CRC s position in the adaxial- abaxial regulatory network seems to be not conserved in other eudicots. CRC exhibits a non- cell-autonomous action which is conferred by at least two signaling pathways. Abaxial polarity is regulated by the activation of the mobile miRNA165/166. At the same time, localizations of GFP tagged CRC revealed the CRC protein to be mobile as it migrates into the adaxial domain in young gynoecia. In older gynoecia it was excluded from the adaxial domain.This study identified multiple unique features of CRC compared to its relatives. Its thightly controlled expression by over 100 putative regulators, integration in complex co-expression networks, adaxial and abaxial target genes, and its two mode non-cell-autonomous action indicate the important role in the complicated carpel development.
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