The Plasma membrane Intrinsic Proteins (PIPs) in grey poplar Populus . canescens identification of the PIP gene family and characterization of its function on multi scale Zusammenfassung: This PhD thesis provides a molecular and physiological evaluation of the functionality of the PIP subfamily of aquaporins in leaf water relations and CO2 flux of grey poplar (Populus x canescens) as a tree model system.Plasma membrane intrinsic proteins (PIPs) are widely studied in herbaceous plants; however little is known regarding their role in water and CO2 transport in woody species. In the perennial woody plant Populus × canescens 15 PIP genes were cloned and according to the phylogenic analysis all these genes had high homology with their Populus trichocarpa orthologs. The transcript abundance of the 15 PcPIP genes was examined in six different tissues of Populus × canescens including young mature old leaves roots phloem and xylem. All PcPIP genes except PIP2;8 and PIP2;10 were expressed in all above mentioned tissues. However significant difference was present at the level of expression. To reveal the function of PIPs in poplar transgenic poplar plants were generated in which the translation of PIP genes was down regulated by RNA interference (RNAi). To target the two main groups of PIPs two specific RNAi cassettes for PIP1 and PIP2 genes were constructed. The on off target analysis revealed no additional targets of the two RNAi sequences besides PIP genes. With the PcPIP RNAi transgenic plants molecular screening was done on the transcript level by semi quantitative RT PCR and on the protein level by ELISA respectively. Based on these results three PcPIP RNAi transgenic lines were selected in which PIP genes were the most down regulated.With the selected PcPIP RNAi transgenic lines a series of physiological and morphological studies were performed to characterize the plant’s response under optimal and multi scale stress (elevated CO2 drought and heat stress) conditions. Under optimal conditions the leaves of the PcPIP RNAi transgenic lines had wider opened stomata and therefore had a higher net CO2 assimilation and transpiration rate compared with the wild type (WT) and empty vector control (EV) plants. As a consequence of the re organization of the cellular structures the leaves from PcPIP RNAi transgenic plants had greater mesophyll conductance for CO2 (gm) higher leaf hydraulic conductance (Kleaf) and higher apoplastic water flux and overall the leaves were smaller. Based on these results it is concluded that PIP proteins not only play essential roles in whole leaf water and CO2 flux but also have impo ...