Xylem tissue specification, patterning, and differentiation mechanisms

Vascular plants (Tracheophytes) have adapted to a variety of environments ranging from arid deserts to tropical rainforests, and now comprise >250,000 species. While they differ widely in appearance and growth habit, all of them share a similar specialized tissue system (vascular tissue) for transporting water and nutrients throughout the organism. Plant vascular systems connect all plant organs from the shoot to the root, and are comprised of two main tissue types, xylem and phloem. In this review we examine the current state of knowledge concerning the process of vascular tissue formation, and highlight important mechanisms underlying key steps in vascular cell type specification, xylem and phloem tissue patterning, and, finally, the differentiation and maturation of specific xylem cell types.

Quantifying leaf venation patterns: two-dimensional maps

The leaf vasculature plays crucial roles in transport and mechanical support. Understanding how vein patterns develop and what underlies pattern variation between species has many implications from both physiological and evolutionary perspectives. We developed a method for extracting spatial vein pattern data from leaf images, such as vein densities and also the sizes and shapes of the vein reticulations. We used this method to quantify leaf venation patterns of the first rosette leaf of Arabidopsis thaliana throughout a series of developmental stages. In particular, we characterized the size and shape of vein network areoles (loops), which enlarge and are split by new veins as a leaf develops. Pattern parameters varied in time and space. In particular, we observed a distal to proximal gradient in loop shape (length/width ratio) which varied over time, and a margin-to-center gradient in loop sizes. Quantitative analyses of vein patterns at the tissue level provide a two-way link between theoretical models of patterning and molecular experimental work to further explore patterning mechanisms during development. Such analyses could also be used to investigate the effect of environmental factors on vein patterns, or to compare venation patterns from different species for evolutionary studies. The method also provides a framework for gathering and overlaying two-dimensional maps of point, line and surface morphological data.

Trees of the people: the growing science of poplars in Canada and worldwideThis commentary is one of a selection of papers published in the Special Issue on Poplar Research in Canada

Poplars are trees of the angiosperm genus Populus , a group of generally diploid, dioecious, and deciduous pioneers that are widespread in forests around the Northern Hemisphere. Canada is a centre of poplar abundance and diversity, with 7 of the 29 global Populus species occurring naturally, and other species being introduced. Numerous interspecific hybrids also occur, including native hybrids that create centres of biodiversity, and artificial hybrid poplars that are planted for wood and fiber production, as well as for environmental and landscape applications. This paper introduces poplars and their distribution across Canada and explores Canadian and international contributions to poplar research. It also discusses papers of the special issue Poplar Research in Canada in the November 2007 and December 2007 issues of the Canadian Journal of Botany, which contains a collection of 19 contributions from Canadian institutions that report on a diverse range of topics including genomics and molecular biology, physiology, ecology, population genetics, and systematics. Many of these contributions address issues pertinent to silviculture and other applications, and one article chronicles poplar research and poplar utilization in Canada. These studies that appear in this special issue reflect the growing international emphasis on poplar, not only as an important and widespread group of trees, but also as a model that provides broader insights relevant to fundamental and applied sciences involving trees and forest systems.