The endodermis: The building of a plant polarized epithelium
The endodermis is the innermost cortical cell layer of plant roots and surrounds their central vascular strand. It features Casparian strips, ring-like hydrophobic cell wall impregnations that surround each endodermal cells as a median belt and which are fused into a supracellular, network-like structure. This Casparian strip network represents the major extracellular (apoplastic) diffusion barrier in young roots. It serves to separate and protect the inner, extracellular space of the vascular cylinder from that of the cortex, which is continuous with the soil. Thereby, the endodermis is functionally equivalent to an animal polarized epithelium, such as the gut epithelium, for example. Very little was known in molecular terms about the building of this intricately structured cell layer that has evolved independently from animal epithelia. In a series of publication in recent years, we were able to describe the progression of endodermal differentiation and obtain markers that reveal the presence of a median, ring-like, lateral diffusion barrier in the plasma membrane, now termed the “Casparian strip domain” (CSD), as well as a strict polarity within the endodermis (see Alassimone et al., PNAS, 2010). We were also able to identify a previously uncharacterized family of proteins, now named “CAsparian Strip domain Proteins” (CASPs) as well as “CASP-likes” (CASPLs) for the extended family of CASP-related proteins. We could demonstrate that the CASPs are functionally equivalent to animal tight junction proteins in that they set up a median domain that acts as a lateral diffusion barrier as well as the determining the position of the Casparian strip cell wall impregnation itself (see Roppolo et al., Nature, 2011). We were also able to clarify a long-standing debate about the molecular nature of this cell wall impregnation, demonstrating that the Casparian strips are made of lignin and not suberin, as often presumed (see Naseer et al., PNAS, 2012). Based on this finding, we could recently provide mechanistic insights into how the CASPs provide a transmembrane protein scaffold that assembles what we call a “lignin polymerizing complex”, consisting of a specific NADPH oxidase, as well as peroxidase (in addition to other as yet unidentified proteins). This model can explain the subcellular precise polymerization of lignin in a centrally located belt (see Lee et al., Cell, 2012). We have also written a number of reviews that further discuss diverse aspects of our work and of endodermal development in general (see our reviews).
The functional relevance of the endodermis
Our molecular and cell biological investigations of endodermal differentiation has now provided us with a number of specific mutants in endodermal barrier formation that we think will be invaluable novel tools to validate and better understand the many supposed roles of the endodermis in root function, specifically its role in the selective uptake and retention of nutrients, but also its proposed function as a protective barrier to pathogens, for example. We have now started using our new molecular insights in order to address the role of the endodermis in nutrient uptake. For this we collaborate with a number of experts on nutrient physiology. Currently, we have very active collaborations with the laboratory of David Salt in Aberdeen, as well as with the laboratory of Junpei Takano at Hokkaido University, with which we have an HFSP Young Investigator grant.