Barriers Formed in Different Ways – The Exodermis and the Endodermis in Tomato

The exodermis cell type is present within most angiosperms but is understudied due to its absence in Arabidopsis thaliana. Exodermis cells are able to form a barrier which is thought to be analogous to the Casparian strip in the root endodermis. We describe the morphology and the timing of the formation of this barrier in the exodermis relative to the endodermis, the ability of the exodermis to act as a functional barrier, and the composition of this barrier. Cell type-specific molecular profiling and gene reporters are used to assess the degree to which the endodermis development and Casparian strip regulatory module has been co-opted for acquisition of exodermis identity.