Dirigent proteins and dirigent sites in lignifying tissues

Tissue-specific dirigent protein gene expression and associated dirigent (s ite) localization were examined in various organs of Forsythia intermedia u sing tissue printing, in situ mRNA hybridization and immunolabeling techniq ues. respectively. Dirigent protein gene expression was primarily noted in the undifferentiated cambial regions of stem sections, whereas dirigent pro tein sites were detected mainly in the vascular cambium and ray parenchyma cell initials. Immunolocalization also revealed cross-reactivity with parti cular regions of the lignified cell walls, these being coincident with the known sites of initiation of lignin deposition. These latter regions are co nsidered to harbor contiguous arrays of dirigent (monomer binding) sites fo r initiation of lignin biopolymer assembly. Dirigent protein mRNA expressio n was also localized in the vascular regions of roots and petioles, whereas in leaves the dirigent sites were primarily associated with the palisade l ayers and the vascular bundle. That is, dirigent protein mediated lignan bi osynthesis was initiated primarily in the cambium and ray cell initial regi ons of stems as well as in the leaf palisade layers, this being in accordan ce with the occurrence of the lignans for defense purposes. Within lignifie d secondary xylem cell walls, however, dirigent sites were primarily locali zed in the SI sublayer and-compound middle lamella, these being coincident with previously established sites for initiation of macromolecular lignin b iosynthesis. Once initiation occurs, lignification is proposed to continue through template polymerization. (C) 2001 Elsevier Science Ltd. All rights reserved.