Groups of multi-cellular passage cells in the root exodermis of Echinochloa crus-galli varieties lack not only suberin lamellae but also lignin deposits

Abstract

Passage cells are frequently found in the exodermis and the endodermis of the roots. Because passage cells lack an apoplastic diffusion barrier, they are thought to provide pathways for the transport of nutrients and the entrance of endomycorrhizal fungi. Exodermal passage cells possess Casparian strips but not suberin lamellae. So far, exodermal passage cells have not been associated with a particular internal structure. In some wetland plants, the outer part of the root (i.e., epidermis, exodermis, and sclerenchyma) of emerging lateral root primordia has an oxygen leaky zone called a window. The exodermis at the window site also lacks suberin lamellae, but it remains unclear whether the exodermis at the window site also lacks Casparian strips. Here, we report that several of the exodermal cells in the window of Echinochloa crus-galli grown under aerated or deoxygenated stagnant agar nutrient solution also lack lignin, which is a major constituent of Casparian strips. The sclerenchyma cells that form part of the window also lacked lignin deposits. Sites at which lateral root primordia developed were highly permeable to an apoplastic tracer (periodic acid). These observations indicate that windows consist of a novel type of passage cell at the exodermis that lacks lignin as well as suberin lamellae.