Abstract

My lab investigates the molecular and developmental basis of endodermal structure and polarity. The endodermis is an invariable barrier within the root of all vascular plants. Its barrier function is mediated by the Casparian Strips, ring-like hydrophobic cell wall thickenings that are coordinated between endodermal cells and form a supracellular network. Casparian Strips effectively blocks passage of nutrients and pathogens through the extracellular space, while still allowing for signal perception and nutrient uptake. This very much resembles the dual, protective/uptake function of polarised gut epithelia in animals, for example. The molecular players and mechanisms that underlie this intricately structured cell layer have remained obscure. Our group has recently described the developmental events leading to a differentiated endodermis. Its initial differentiation, less than 1 mm away from the root tip, occurs in a switch-like “burst” of events rather than as a gradual maturation. We have visualised a strict polarity within the endodermis, with proteins being distributed exclusively to the outer domain of the plasma membrane (facing the root cortex) or the inner domain (facing the cells of the stele). The two polar domains are separated by a central membrane diffusion-barrier. We identified an unknown protein family that forms this central membrane barrier and predicts the site of Casparian Strip formation. Mutants in these CASP genes display disorganized deposition of Casparian Strips. In addition, we have obtained a number of mutants in Casparian Strip development, both by forward and reverse genetic approaches, which display interrupted, or strongly delayed, deposition of Casparian Strips, some by interfering with CASP localization. These mutants now provide the unprecedented opportunity to directly test the many supposed roles of the Casparian Strips and I will report on our latest results from the analysis of these mutants.