Because plants have a sessile lifestyle, they must adjust to various external cues and coordinate their developmental program accordingly. To facilitate survival in often hostile environments, plants have evolved a notable developmental plasticity that allows them to shape their body and optimise their metabolism according to specific environmental demands. As with other multicellular organisms, the coordination of plant development relies on the communication across cellular boarders. In this context, plants employ a chemical-based network to control their developmental and physiological processes. Central to this network are a limited number of small molecules, called phytohormones. They are responsible for the adequate transcriptional reprogramming of tissues answering to external cues or for the maintenance of coordinated development. Although several molecules are known to function as plant hormones, plant shape is largely controlled by auxin. Over the past decades the knowledge of auxin action and signalling has greatly improved. By contrast, the synthesis of auxin is still not fully defined with respect to catalysed reactions and enzymes involved. Currently, it is assumed that a small number of alternative biosynthetic pathways contribute to auxin biosynthesis in plants. Due to prevailing knowledge gaps, extensive functional redundancy, and tissue and plant specific variations in expression patterns of the identified components, the relevance of each of these pathways is difficult to assess. The main focus of this project is to close the existing knowledge gaps and to decipher the pathways of auxin formation in plants. In addition, the cross-talk between the individual pathways and their regulation will be studied. To achieve these goals, a truly Systems Biology approach will be taken, encompassing genetics, transcriptomics, metabolomics, and bioinformatics workflows.