Elucidation of molecular mechanism by which plant constructs cell-type specific cell-wall architecture

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Xyloglucan endotransglucosylase/hydrolases (XTHs) are a class of enzymes that are implicated in the construction and restructuring of the cellulose/xyloglucan framework. Thirty-three members of the XTH gene family are found in the genome of Arabidopsis thaliana, but their roles remain unclear. In this research we have analyzed the tissue-specific and growth stage-dependent expression profiles of promoter::GUS fusion constructs for four Arabidopsis XTH genes, AtXTH17, -18, -19, and -20, which are phylogenetically closely related to one another. AtXTH17 and AtXTH18 were expressed in all cell types in the elongating and differentiating region of the root, while AtXTH19 was expressed in the apical dividing and elongating regions, as well as in the differentiation zone, and was up-regulated by auxin. In contrast, AtXTH20 was specifically expressed in vascular tissues in the basal mature region of the root. This expression analysis also disclosed cis-regulatory sequences that are conserved a … More mong the four genes, and are responsible for the root-specific expression profile. Next, we examined phenotypes of mutants in which each of the Arabidopsis XTH genes was disrupted, to identify the role of each member of this gene family, This was undertaken using a reverse genetic approach, and disclosed two loss-of-function mutants for the AtXTH27 gene, xth27-1 and xth27-2. These exhibited short-shaped tracheary elements in tertiary veins, and reduced the number of tertiary veins in the first leaf. In mature rosette leaves of the mutant, yellow lesion-mimic spots were also observed. Extensive expression of the pXTH27::GUS fusion gene was observed in immature tracheary elements in the rosette leaves. The highest level of AtXTH27 mRNA expression in the rosette leaves was observed during leaf expansion, when the tracheary elements were elongating. These findings indicate that AtXTH27 plays an essential role during the generation of tracheary elements in the rosette leaves of Arabidopsis. These results indicate that the four XTH genes, which were generated by gene duplication, have diversified their expression profile within the root in such a way to take responsibility for particular physiological roles in the cell wall dynamics. Less