The Roles of WOL and APL in Phloem Development in Arabidopsis thaliana Roots

The plant vascular tissue is necessary for transport of water, nutrients and macromolecules, and functions as a supporting structure. The network of xylem and phloem is expanded throughout plant life, both in the radial direction, due to cambial activity and in the apical-basal directions due to meristem activity. Although plant hormones, such as auxin and cytokinins, have been shown to be important for the general establishment of the vascular tissue, the genetic regulation of the patterning process is yet poorly understood. In this study, the structural analysis of the provascular meristem of the Arabidopsis root and the identification of two genes that are involved in the regulation of this patterning process are presented. The Arabidopsis phloem poles are established after a series of asymmetric cell divisions. The first divisions, of the provascular cells adjoining the quiescent center (QC), appear random. The phloem specific cell divisions that form the companion cells (CC) and sieve element (SE) cell files are highly reproducible, indicative of a tightly regulated process. The wooden leg (wol) mutation results in a vascular defect, in which cell proliferation of the vascular initials is restrained, resulting in fewer vascular cells. The remaining vascular cells all differentiate as protoxylem and therefore the primary root lacks phloem. Molecular cloning of the mutation identified the gene as a twocomponent histidine kinase receptor, which is specifically expressed within the vascular bundle, indicating that a phosphorelay signaling pathway regulates cell proliferation within the vascular tissue. A second mutant, named altered phloem development (apl), has a more specific defect. Whereas the outer cell layers of the apl roots have a normal radial pattern, the vascular system is abnormal. The phloem specific tangential and periclinal cell divisions occur less frequent and subsequently the prospective SEs and CCs start to differentiate as xylem elements. The APL gene encodes a MYB (myeloblastosis, DNA binding domain) coiled-coil-type transcription factor, and from late embryogenesis onwards it is specifically expressed in developing phloem, consistent with a key role in phloem development. Ectopic APL expression in the vascular bundle inhibits xylem development. The presented results suggest that APL has a dual role both in promoting phloem differentiation and in repressing xylem differentiation during vascular development. A model of vascular development is proposed in which both WOL and APL are critical steps for vascular patterning. Thesis_start.pmd 5.11.2004, 12:37 10