Class III HD-ZIPs govern vascular cell fate: an HD view on patterning and differentiation

Plant vasculature is required for the transport of water and solutes throughout the plant body. It is constituted of xylem, specialized for transport of water, and phloem, that transports photosynthates. These two differentiated tissues are specified early in development and arise from divisions in the procambium, which is the vascular meristem during primary growth. During secondary growth, the xylem and phloem are further expanded via differentiation of cells derived from divisions in the cambium. Almost all of the developmental fate decisions in this process, including vascular specification, patterning, and differentiation, are regulated by transcription factors belonging to the class III homeodomain-leucine zipper (HD-ZIP III) family. This review draws together the literature describing the roles that these genes play in vascular development, looking at how HD-ZIP IIIs are regulated, and how they in turn influence other regulators of vascular development. Themes covered vary, from interactions between HD-ZIP IIIs and auxin, cytokinin, and brassinosteroids, to the requirement for exquisite spatial and temporal regulation of HD-ZIP III expression through miRNA-mediated post-transcriptional regulation, and interactions with other transcription factors. The literature described places the HD-ZIP III family at the centre of a complex network required for initiating and maintaining plant vascular tissues.