Expression Patterns and Functional Characterisation of Arabidopsis GALACTOSE OXIDASE-LIKE (GOXL) Genes Suggest Specialised Roles for Galactose Oxidases in Plants.

Galactose oxidases (GalOxs) are well-known enzymes that have been identified in several fungal species and characterised using structural and enzymatic approaches. However, until very recently, almost no information on their biological functions was available. The Arabidopsis (Arabidopsis thaliana) gene RUBY PARTICLES IN MUCILAGE (RUBY) encodes a putative plant GalOx that is required for pectin cross-linking through modification of galactose (Gal) side-chains and promotes cell-cell adhesion between seed coat epidermal cells. RUBY is one member of a family of seven putative GalOxs encoded in the Arabidopsis genome. To examine the function(s) of GalOxs in plants, we studied the remaining six GALACTOSE OXIDASE-LIKE (GOXL) proteins. Like RUBY, four of these proteins (GOXL1, 3, 5 and 6) were found to localize primarily to the apoplast, while GOXL2 and 4 were found primarily in the cytoplasm. Complementation and GalOx assay data suggested that GOXL1, GOXL3, and possibly GOXL6 have similar biochemical activity to RUBY, whereas GOXL5 only weakly complemented, and GOXL2 and GOXL4 showed no activity. Members of this protein family separated into four distinct clades prior to the divergence of the angiosperms. There have been recent duplications in Brassicaceae resulting in two closely related pairs of genes that have either retained similarity in expression (GOXL1 and GOXL6) or show expression divergence (GOXL3 and RUBY). Mutant phenotypes were not detected when these genes were disrupted, but their expression patterns suggest that these proteins may function in tissues that require mechanical reinforcements in the absence of lignification.