ZmXTH1, a new xyloglucan endotransglucosylase/hydrolase in maize, affects cell wall structure and composition in Arabidopsis thaliana*

Xyloglucan endotransglucosylase/hydrolases (XTHs; EC 2.4.1.207 and/or EC 3.2.1.151) are enzymes involved in the modification of cell wall structure by cleaving and, often, also re-joining xyloglucan molecules in primary plant cell walls. Using a pool of antibodies raised against an enriched cell wall protein fraction, a new XTH cDNA in maize, ZmXTH1, has been isolated from a cDNA expression library obtained from the elongation zone of the maize root. The predicted protein has a putative N-terminal signal peptide and possesses the typical domains of this enzyme family, such as a catalytic domain that is homologous to that of Bacillus macerans beta-glucanase, a putative N-glycosylation motif, and four cysteine residues in the central and C terminal regions of the ZmXTH1 protein. Phylogenetic analysis of ZmXTH1 reveals that it belongs to subgroup 4, so far only reported from Poaceae monocot species. ZmXTH1 has been expressed in Pichia pastoris (a methylotrophic yeast) and the recombinant enzyme showed xyloglucan endotransglucosylase but not xyloglucan endohydrolase activity, representing the first enzyme belonging to subgroup 4 characterized in maize so far. Expression data indicate that ZmXTH1 is expressed in elongating tissues, modulated by culture conditions, and induced by gibberellins. Transient expression assays in onion cells reveal that ZmXTH1 is directed to the cell wall, although weakly bound. Finally, Arabidopsis thaliana plants expressing ZmXTH1 show slightly increased xyloglucan endohydrolase activity and alterations in the cell wall structure and composition.