LOB DOMAIN-CONTAINING PROTEIN 15 Positively Regulates Expression of VND7, a Master Regulator of Tracheary Elements

Xylem includes xylem parenchyma cells, fibers and tracheary elements. Differentiation of tracheary elements is an irreversible process that is controlled by the master regulator VASCULAR-RELATED NAC-DOMAIN 7 (VND7). Molecular events occurring downstream of VND7 are well understood, but little is known regarding upstream regulation of VND7. In this study, we identified LOB DOMAIN-CONTAINING PROTEIN 15 (LBD15)/ASYMMETRIC LEAVES2-LIKE (ASL11) as a regulator of VND7. LBD15 was expressed in immature vascular cells and positively regulated both VND7 expression and differentiation of tracheary elements. LBD15 directly associated with the upstream sequence of VND7 and positively regulated VND7 expression. A 25 bp upstream sequence was essential for VND7 expression in the elongation zone of Arabidopsis roots. Taken together with previous studies identifying LBD15 as a target of VND7, we propose that LBD15 acts in a positive feedback regulation system that promotes and accelerates VND7 expression during the initiation phase of tracheary element differentiation in roots.