Spatial Specificity of H2O2-generating Oxalate Oxidase Gene Expression During Wheat Embryo Germination

Germin, a molecular marker of wheat embryo germination, is a protease-resistant, apoplastic, homopentameric glycoprotein with peroxide-generating oxalate oxidase activity. The spatial specificity of germin-like oxalate oxidase (gl-OXO) gene expression has been determined in tissues of germinating wheat embryos by a combination of histochemical, immunocytochemical and in situ hybridization techniques. The synthesis and accumulation of gl-OXO mRNA and protein is localised within the enveloping tissues of the embryonic axis (particularly the coleorhiza) during the first 24 h of imbibition. By 48 h germination, gl-OXO accumulation is detected throughout the root, with the exception of the postmitotic zone of cell elongation, where accumulation of its transcript is restricted to outer cell layers. At this time in the elongating shoot, gl-OXO is restricted to the coleoptile where it is detected only in the epidermal cell layer, the vascular bundles and bundle sheath cells. In older seedlings (approximately 9 days post-imbibition) gl-OXO activity is detected in leaves, but only within the vascular bundles. These patterns of expression are consistent with the hypothesis that the biological function of gl-OXO is to restrict cell growth by participating in cell-wall restructuring through the local provision of hydrogen peroxide for cross-linking of wall components.