Suberin is a cell wall-associated biopolymer found in specific cell types, such as root epidermis, root endodermis (including the Casparian band), bundle sheath cells and the periderm (cork) of woody species and underground organs (e.g., tubers). Suberin is unique and distinct from other non-carbohydrate cell wall bio polymers such as lignin and cutin for several reasons: (1) it is an assembly of two distinct polymeric domains, one poly(phenolic) and the other poly(aliphatic), (2) each domain has a unique chemical composition, and (3) both domains are present in the same cells.
Suberin functions as a physical barrier, preventing water loss from the tissues it surrounds as well as providing protection against pathogens. It also forms a significant component of the wound healing process in plants; indeed the main model system to study suberin and the suberization process is the potato tuber. Upon wounding, tubers undergo a massive re-arrangement of metabolism, channeling significant metabolic energy into the formation of the phenolic and aliphatic monomeric precursors of suberin via two disparate metabolic pathways.
Our research efforts have focused on identifying unique metabolic steps associated with the biosynthesis of aliphatic suberin monomers, as these are excellent markers for the suberization process. Specifically, we have cloned the potato cytochrome P-450 (CYP86A33) thought to be responsible for the formation of ω-hydroxy fatty acids (StFAωH-1) characteristic of aliphatic suberin, and are currently characterizing its promoter. Similarly, in soybean, the major aliphatic components are ω-hydroxy fatty acids, and these have been shown to correlate with root pathogen disease resistance. Soybean contains six FAωH genes (GmFAωH-1 - GmFAωH-6), two of which (GmFAωH-5 and GmFAωH-6) are strongly expressed in roots. Using an Agrobacterium rhizogenase-mediated gene transfer system, we are manipulating suberin levels in soybean "hairy" roots to test the hypothesis that suberin functions in the resistance of soybean to root pathogens. Importantly, soybean hairy roots have the same anatomy and suberin deposition patterns as normal roots.