Die Rolle von Phosphoinositiden bei der Auxinsignalleitung von Arabidopsis thaliana

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The influence of the phosphoinositide network on auxin-signaling in Arabidopsis thaliana The phytohormone auxin (IAA) is a key regulator of growth and organ formation as well as a signal for tropical responses in plants, including the model plant Arabidopsis thaliana. An IAA-gradient is established towards the basal sides of plant tissues that serves as a developmental cue or guides the direction of gravitropic bending responses. The polar distribution of IAA is mediated by auxin transport systems, especially PIN auxin efflux carriers, which localize intracellularly in a strictly polar pattern at one side of the plasma membrane of plant cells. It was a working hypothesis of this thesis that phosphoinositides (PIs), regulatory membrane phospholipids, contribute to the polar distribution of PIN-proteins and, thus, partake in auxin signaling. In previous experiments it has been established that PIs are required for plant gravitropic bending, but so far the mechanisms by which PIs or PI-derived signals interact with auxin signaling are not clear. The PI, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) has an important function for vesicle trafficking and in recruiting proteins which are crucial for cytosis processes at the plasma membrane. A. thaliana pip5k1 pip5k2 double mutants showed reduced gravitropic bending and disturbed PIN localization, accompanied by reduced auxin transport. These results show that PtdInsP(4,5)P2 has an important role for the polar distribution of PINs and auxin. In a second part of this thesis, it was hypothesized that not only the intact lipid, PtdInsP(4,5)P2 is important for auxin signaling in plants but also PtdInsP(4,5)P2-derived inositolpolyphosphates (IPPs). In previous studies the IPP inositolhexakisphosphate (InsP6) was found as a cofactor in the structure of the auxin receptor TIR1. To address whether InsP6 had functional relevance for auxin perception, auxin-dependent processes were studied in Arabidopsis wild type and in ipk1-1 mutants and transgenic InsP 5-Ptase plants with reduced levels of InsP6. As an example for auxin-dependent tropical growth the gravitropic response of hypocotyls and roots was monitored and both ipk1-1 mutants and InsP 5-Ptase plants exhibited substantially reduced gravitropic curvature for both organs. TIR1 is a part of the SCFTIR1-complex. Binding of auxin to TIR1 mediates proteasomal degradation of AUX/IAA transcriptional repressors, enabling auxin-induced gene expression. Because changes in auxin-induced gene expression represent a read-out for TIR1 function, the induction of gravity-induced genes was monitored in root tips using transcript arrays. The array data indicate that transcript levels of auxin-dependent genes induced by gravistimulation in wild type controls were mostly not changed or differently regulated in the ipk1-1 mutants and InsP 5-Ptase plants. The findings present evidence that InsP6 is a cofactor required for full TIR1 functionality in Arabidopsis. Overall, the data presented in this thesis establish PIs and PI-derived IPPs as integral aspects of auxin signaling in A. thaliana that control the distribution of auxin as well as its perception.