Professor Farhah Assaad
Technische Universität München
Adaptive responses can be broken down into four steps: sensing, signal integration, decision making processes and the execution of these decisions. While progress has been made in the field of Arabidopsis to understand signal perception and integration, little is known about decision making processes (here referred to as allocation decisions) and their execution. To address this, we have set up a forward genetic screen to identify major players in allocation decisions. We focused on the seedling, which reaches binary shoot versus root growth decisions in terms of allocating the limited energy resources contained in the germinating seed. Seed were germinated under growth conditions designed to place contradictory demands on shoot versus root growth. Our screen identified a number of genes in the brassinosteroid signaling pathway, notably the shaggy-like kinase BIN2. To elucidate the role of BIN2 in allocation decisions, we carried out a large scale yeast-two hybrid screen. Together with in vitro kinase assays, this identified the multisubunit tethering complex TRAPPII as a novel target of BIN2. TRAPPII resides at the trans-Golgi Network, where it regulates sorting decisions and mediates both exocytosis and endocytosis. We have genetic evidence that the TRAPPII complex acts as a guanine-exchange factor (GEF) for RabA GTPases. GEFs catalyze exchange of GDP for GTP; the GTP-bound, activated Rab subsequently recruits a diverse local network of Rab effectors to the membrane to give it identity in vesicle fusion events. Understanding the control of GEF-Rab interactions is crucial to unraveling the operation of membrane traffic and the mechanisms that alter trafficking pathways in response to developmental or environmental signals. We are currently exploring the role of Arabidopsis BIN2/TRAPPII/Rab-GTPase modules in allocation decisions and adaptive responses.
This seminar will take place in the Large Lecture Theatre in the Department of Plant Sciences.