UC Berkeley

Abstract - The nascent plant cell wall begins with the deposition of pectin into the apoplastic space between two developing daughter cells. This polysaccharide has three distinct domains of which homogalacturonan (HG) is the largest. Consisting of a repeating α- 1-4 linked D-galacturonic acid (GalA) homopolymer, HG can be further modified with methyl (C6) and/or O-acetyl (C2 or C3) esters. GalA and associated divalent cations play a critical role in the formation of a complex called the "egg box" which can be modified through the esterification of HG. The construction and strength of this egg-box structure is believed to play an important role in plant cell wall plasticity and integrity throughout development. Another cell wall component, cellulose, is similarly critical in its role as the main load-bearing polysaccharide in the primary cell wall. Co-expression analysis of primary cellulose synthase genes has led to the identification of several critical genes necessary for cell wall development. One such gene is TBR1 (TRICHOME BIREFRINGENCE1). Subsequent research confirmed that mutation of TBR1 causes aberrant phenotypes with reduction of cellulose content and altered pectin composition.

In this thesis, the function of TBR1 in plant cell wall development has been studied through the detailed characterization of cell wall composition, which has demonstrated that loss of TBR1 results with depreciated HG O-acetyl transferase activity. Pectinolytic enzymes were developed, characterized and used as novel tools to probe changes in native pectin esterification structure. Research on the tbr1 mutant demonstrated that the resulting decrease in cellulose deposition is accompanied by a decrease in HG and HG O-acetylation due to the absence of a functional TBR1. This suggests that the changes in HG O-acetyl esterification observed in mutants of TBR1 impacts the divalent cation interactions in such a way that the deposition and potentially the arrangement of the cellulose fibers are affected. This ultimately results in a decrease in cellulose deposition and cell wall elasticity in specific cell types, in this case trichomes and hypocotyls. These results contribute to a novel understanding of how pectin influences cell wall dynamics.