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Understanding the patterned deposition of lignin in secondary cell walls Chou, Eva Yi


Lignin, one of the three main components of the secondary cell wall, is an important phenolic biopolymer that provides strength and rigidity to the cell walls of tracheary elements and fibers in vascular plants. Lignin is composed of phenolic alcohol monomers called monolignols, which are synthesized in the cytoplasm. These monolignols are exported to the apoplast where they polymerize by random radical coupling following oxidation by laccases and peroxidases. Two laccases found in Arabidopsis thaliana, LAC4 and LAC17, were localized to secondary cell wall, and required for lignification of protoxylem tracheary elements. The localization of LAC4 and LAC17 to spiral secondary cell walls could be due to either: 1) the diffuse secretion of laccases followed by remobilization to the secondary wall, or 2) a reorientation of post-Golgi vesicle trafficking to secondary cell wall specific plasma membrane domains. Localization studies with LAC4-RFP driven by a constitutive promoter found laccases localized to all regions of the primary cell wall prior to differentiation, then the localization shifted into the helical secondary cell wall bands during protoxylem tracheary elements differentiation. This change in localization suggests there is a change in vesicle traffic during secretion of secondary cell wall components (such as laccases). Furthermore, Fluorescence Recovery After Photobleaching (FRAP) was used to determine if laccase localization in secondary cell walls was due to constraint by the secondary cell walls or exclusion from the primary cell wall. Laccases were also found to be immobile in secondary cell wall domains, but mobile when expressed ectopically in primary cell wall domains. Further drug and mutant FRAP studies found laccases remain immobile in the absence of secondary cell wall: cellulose, xylan, lignin and xylan/lignin. These results suggest laccases are not only anchored to secondary cell wall specific components but may be anchored to multiple components of the secondary cell wall or an unknown component of the secondary cell wall.

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