Final Activity Report Summary - BIOCELLCHARABRX (Biochemical and cell biological characterization of BRX, a novel regulator of root architecture in Arabidopsis, and its role in hormone homeostasis) The major global challenges in the 21st century, climate change and population growth, are above all challenges to the agricultural sector, as abundant food and natural materials are at the very heart of civilization. They remind us of the need for a sustainable development in agriculture, as it is clear that current productivity levels of crops will not be sufficient if they are to serve more and more people as primary food source, while at the same time they are expected to satisfy emerging accessory needs, such as providing energy in the form of biofuels. Thus, increasing plant productivity is a topic of utmost importance, which can only be addressed by increased research efforts in the plant sciences. In this project, we investigated the molecular mechanisms by which a regulatory gene of the model plant Arabidopsis influences plant growth and development. We generated evidence that this gene is a limiting factor for plant growth through regulating, and responding to plant hormone pathways in an unprecedented molecular manner. It turned out that the protein encoded by this gene initially localizes to the plasma membrane of cells, and only transfers to the nucleus to regulate gene expression if instructed by a certain plant hormone. Although primarily important for root system development, this gene also has a rate-limiting function in shoot growth. As this gene is highly conserved among all higher plant species for which sequencing information is available, our data suggest that our results are applicable to other plants, including all major crops. Thus, eventually, manipulation of the activity level of the corresponding protein in crops might lead to increased yield, a potential application that should follow our research.