Investigating Suberin Biosynthesis in Poplar: Candidate Genes and Chemical Composition
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For plants, lipid barriers represent a critical adaptation to terrestrial life, providing a first line of defense against biotic and abiotic stressors. Among these lipid barriers, suberized cell walls protect both internal and external tissues of the plant, where they play vital roles including regulation of water and solute exchange, and defense against pathogens. Suberin is a heteropolymer deposited under normal development or as a stress response that is composed of two covalently-linked domains; the polyphenolic domain and the polyaliphatic domain. Because of this complexity, our understanding of the structure, biosynthesis and deposition of suberin has been limited. Overall, the goal of this thesis was to use combined chemical and molecular approaches to identify candidate genes for suberin biosynthesis as well as to investigate the structure of the polyester using the model tree, Populus trichocarpa. RNA-Sequencing of cork tissue generated 455 candidates for suberin biosynthesis, and chemical characterization of the suberin polymer across 4 developmental stages allowed connections between the chemical products and differentially expressed genes to generate hypothesis and candidate genes for future analysis. These chemical methods included the quantification of glycerol using a recently published methodology for the related polymer, cutin, to propose structural motifs. The ratio of acyl monomers to glycerol was determined to be 5:1, leading to the hypothesis that suberin forms an extended polymer in poplar cork. In addition to glycerol, a modified green solvent protocol for thioacidolysis was introduced and shown to perform as well as conventional dioxane methods. By studying the time course of wound-healing suberization, the chemistry and gene expression of selected genes were compared with those of the suberin deposition during normal development. The results indicate that wound healing suberin is chemically distinct from native suberin deposition and therefore may be under different genetic control. The work contained in this thesis furthers our understanding of suberin structure in tree bark, establishes improved methodologies, and generates hypothesis for future targeted studies.