Molecular dissection of Arabidopsis thaliana endodermis development, structure and function.

The endodermis is a fundamental cell layer common to all higher plants, separating outer from inner cell layers in the root. It does so by virtue of its Casparian Strips, which are precisely aligned bands of specialized cell wall material. The endodermis resembles an animal epithelium both in organization and function, but lacks the molecular modules that establish polar epithelia in animals. Despite its fundamental importance, we know nothing about the mechanisms that establish endodermal structures. I now define the developmental sequence of events that leads to endodermal differentiation. I show that the plasma membrane at the Casparian Strip acts as a diffusional barrier between the outer and inner domains of the endodermal plasma membrane and that this region mediates tight attachment to the cell wall matrix. In addition, I have identified molecular markers for both the inner and the outer domains of the plasma membrane and demonstrate that already meristematic cells display polarized distributions, but that this polarity becomes refined and the domains separated upon cellular differentiation and formation of the Casparian Strip. I demonstrate that endodermal central-peripheral polarity co-exists with the apical-basal polarity defined by PIN proteins within the same cells, but utilizes different polarity determinants. My work establishes the Arabidopsis endodermis as a new polarity model that displays a strictly defined separation of domains. Polarization and barrier establishment within the endodermis should be relevant for a plethora of nutrient uptake processes in roots of higher plants and its mechanistic dissection should further our understanding of plant nutrition in general. I also report on progress of a forward genetic approach to identify actors of Casparian Strip establishment.