A new insight on structural and some functional aspects of peri-endodermal thickenings, a specific layer in Noccaea caerulescens roots

Cell walls of peri-endodermis, a layer adjacent to the endodermis in alpine pennycress (Noccaea caerulescens) roots, form C-shaped peri-endodermal thickenings (PET). Despite its specific position close to the endodermis, assumed similarity of PET to phi thickenings in many other species, and fact that N. caerulescens is a well-studied heavy metals hyperaccumulating plant, PET as a root trait is still not understood. Here, we characterized PET cell walls by histochemical techniques, by Raman spectroscopy, immuno-labelling, and by electron microscopy. Moreover, a role of PET in solute transport was tested and compared with Arabidopsis thaliana plants, which does not form PET in roots. The cell walls with PET have a structured relief mainly composed of cellulose and lignin. Suberin, typical for endodermal cells is missing but pectins are present on inner surface of PET. Penetrating dyes are not able to cross PET neither by apoplasmic nor symplasmic pathway, and significantly higher content of metals is found in root tissues outside of PET than in innermost tissues. Based on development and chemical composition, PET are different from endodermis and highly resemble phi thickenings. Contrary, different structure and dye impermeability of PET, not known in case of phi thickenings, point to an additional barrier function which makes peri-endodermis with PET a unique and rare layer.