Spatial and temporal deposition of suberin during maturation of the onion root exodermis

Suberin is a complex biopolymer composed of a poly(aliphatic) domain (SPAD) and a poly(phenolic) domain (SPPD). Suberin is typically confined to specialized cell types including root exodermal cells, but its synthesis in a maturing exodermis is still not well understood. For the current work, Allium cepa roots were used as a model to analyze SPAD and SPPD synthesis in a maturing uniseriate exodermis. Roots were divided into four maturation zones based on their growth rate and the deposition of suberin lamellae in maturing exodermal cells. Exodermal and epidermal cell layers were separated from the underlying layers in each maturation zone, then soluble and insoluble suberin monomers were extracted chemically, and quantified and identified by gas chromatography - mass spectrometry. Temporal patterns for the synthesis of the SPAD, but not for the SPPD, were revealed upon resolution of the metabolite profiles. The composition of the soluble fraction was essentially unchanged as the exodermis matured. In contrast, the SPAD composition differed during maturation, mainly owing to significant increases in the deposition of C18: 1 alpha,omega-dioic acid and omega-OH fatty acids. It is proposed that the exodermal maturation zones with their corresponding metabolite profiles be used as targets for the functional enzymatic characterization of suberin biosynthetic pathways.