Phenylpropanoids are Vital for Endodermal Diffusion Barrier Formation and Root Transport Properties

Critical for the root’s function in water and selective nutrient transport are two apoplastic diffusion barriers in the endodermis, Casparian strips and suberin lamellae. Whereas the Casparian strip is dominated by the aromatic polymer lignin, suberin consists predominantly of lipids and only minor amounts of aromatics. In the past decade multiple genes in the biosynthesis of aliphatic suberin monomers have been characterized which demonstrated that severely lowering aliphatic suberin results in increased permeability across suberized tissue. However, little is know about the physiological importance of the aromatic phenylpropanoids in Casparian strip lignin and suberin lamellae. They have mostly been disregarded due to their low abundance in suberin polyester or the difficulty to isolate and analyse lignin from specific tissues such as the endodermis.

We determined that modifications in the phenylpropanoid pathway, providing the aromatic Casparian strip and suberin monomers, affect both Casparian strip and suberin deposition and barrier properties. Consequently, water transport, nutritional ion homeostasis, salt and drought tolerance was altered in transgenic plants modified in aromatics biosynthesis. Furthermore, mutations in downstream genes required for aromatic crosslinking led to delayed deposition of suberin lamellae. Moreover, in secondary cell walls of the endodermis we observed a disturbance of the highly organized ultrastructure, which has been hypothesized to also contribute to endodermal barrier properties.