Summary

• Asymmetrically transported auxin-efflux carriers cause a polarized flow of auxin, which leads to the formation of continuous columns of procambial cells. In addition, one or more auxin-flow-independent mechanisms might also be involved in the formation of the continuous columns.
• Adaxial- and abaxial-identity genes might interact to specify xylem and phloem formation, respectively, to produce the intravascular radial pattern of vascular bundles of leaves.
• Cytokinin has a crucial role in the formation and/or maintenance of procambial cells, probably together with auxin.
• Brassinosteroids might initiate differentiation of procambial cells to xylem cells. This process might be associated with the positive and negative regulation of the expression of specific members of the HD-ZIP-III gene family by brassinosteroids and by the RNAi machinery, respectively.
• Molecular and cellular studies with Zinnia elegans xylogenic cultures have uncovered details of a vascular-cell specification process — tracheary-element differentiation — during which coordinated gene expression is induced in association with patterned secondary-cell-wall formation and vacuole-executed programmed cell death.