Analysis of physiological function(s) of a laccske-like multicopper oxidase in Arabidopsis thaliana
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Laccase and related laccase-like multicopper oxidases (LMCOs) have been studied in plants for more than a century, yet our understanding of their physiological function(s) remains limited. The work described in this dissertation was directed toward understanding of the physiological function of one particular Arabidopsis LMCO gene, At2g30210, by measuring its enzyme activities, patterns of tissue-specific expression, subcellular localization, effects of loss-of-function mutations, and regulation by external factors. Heterologously expressed At2g30210 LMCO showed phenoloxidase active, but no ferroxidase activity. RT-PCR and transcriptional profiling showed that the At2g30210 LMCO gene is expressed primarily in root tissues. Histochemical analyses of transgenic, GUS-expressing plants revealed that At2g30210 transcripts were preferentially expressed in developing endodermis of the root elongation zone. Analyses of subcellular localization indicated that the At2g30210-YFP fusion product was localized to the cell periphery. A loss-of-function At2g30210 mutant displayed phenotypic responses to sugar availability and salt levels consistent with alterations in endodermal function. The At2g30210 LMCO gene was not regulated by changes in iron levels. However, the gene was up-regulated in seedlings grown on MS medium lacking sucrose, a response that was consistent with a phenotype observed in knockout mutants of this gene. These results altogether suggest a possible physiological function for the At2g30210 LMCO gene product in endodermal cell development, possibly in formation of the lignin-like phenolic polymers observed in the specialized cell walls that constitute the Casparian strip in endodermal cells.