Plant architecture is determined by tightly regulated developmental processes that largely depend on the action of the plant hormone auxin. A major determinant in auxin action, besides its signaling pathway, is its polar cell-to-cell transport (PAT) throughout the plant. The direction on this transport depends on the localization of the auxin efflux carriers, the PIN proteins. The PINOID (PID) serine/threonine protein kinase is a key regulator of the subcellular localization of the PINs, which are direct phosphorylation targets of the kinase. This thesis describes the functional analysis of three direct interacting partners of PID, two calcium-binding proteins, TOUCH3 (TCH3) and PID BINDING PROTEIN1 (PBP1), and a BTB and TAZ domain (BT) protein. Several studies have already indicated that calcium signaling is induced by auxin application and is necessary for auxin transport. With the isolation of the two calcium-binding proteins TCH3 and PBP1 as interactors of PID, a molecular link between auxin transport and calcium signaling was identified. In this thesis, we show that calcium is involved in the regulation of both the kinase activity and the subcellular localization of PID. In complement to calcium, BTB scaffold proteins are part of the PID protein complex. A detailed analysis of BT protein family in Arabidopsis indicate a functional redundancy among the five members of this family and their requirement for the female gametophyte development. Moreover the BT proteins are required scaffold components in the PID signaling pathway. The functional analyses of the PBPs described in this thesis uncover a new mechanism of protein kinase activity regulation via calcium signaling, and present novel roles for the BT proteins, not only in PID signaling, but also more in general in plant development.LEI Universiteit LeidenPlant science
