Realizing pipe dreams – a detailed picture of vascular development

Realizing pipe dreams – a detailed picture of vascular development This special issue of Journal of Experimental Botany focuses on the developmental mechanisms required to generate plant vascular tissue. The focus is Arabidopsis, including the three models for initial patterning involving the interaction of auxin and cytokinin, and going on to look at expansion and differentiation into xylem and phloem. Bryophyte and tree models are also considered, as well as new techniques for analyzing the vasculature of mature plants. There are many advantages to using plant vascular tissue as a model for studying developmental biology. These include a very high level of tissue organization that is apparent both early in development, where highly coordinated cell divisions in the embryo are required to specify the vascular tissue (Berleth and Jurgens, 1993), and during secondary growth, where highly oriented cell divisions in the cambium, a bifa-cial meristem, drive radial growth and expansion (Chaffey et al., 2002) (Box 1). Differentiated tissues, the xylem and phloem, are derived from divisions in vascular meristems. Consequently studying vascular tissue formation takes in classic developmental biology themes of cell division, tissue organization and differentiation. Ohtani et al. (2017) review the evolution of water and nutrient transport systems in bryophytes through to the vascular system widely recognizable in higher plants. Terrestrial plants require water and nutrients from the soil and sunlight captured in aboveground organs for growth and development. There would have been considerable selective pressure on early land plants for a transport mechanism that allowed all cells in the plant to access these resources. Vascular systems have enabled plants to increase in size through evolutionary time by facilitating the transport of these resources around the plant and are consequently pivotal to higher plant evolution. Furthermore, innovations such as the rigid secondary cell wall, present in the water-conducting cells of the xylem, enable these vessels to withstand the negative pressures of water transport, but also provide the support allowing increases in plant height to improve light capture. Ohtani et al. (2017) also review our knowledge of how transcription factors regulate vas-cular differentiation, and explore the role these transcription factors may have in driving the evolution of vascular systems in lower plants. The vascular tissue is specified early in development, and a number of recent papers have focused on how the vascu-lar systems that have evolved in higher plants initiate and expand. In both the embryo and the root …