Pesquet E, Jauneau A, Goffner D
Zinnia elegans is an excellent model for xylogenesis: from in vitro and in planta
In Floriculture, Ornamental and Plant Biotechnology: Advances and Topical Issues, Jaime A Teixeira da Silva editor, Global Science Books, Ltd. (London, UK), 171-178

Abstract
Vascular tissues play an essential role in plant growth, allowing for transport of raw sap via the xylem from roots to leaves and elaborated sap by the phloem from source to sink. The formation of xylem, or xylogenesis, is an integral part of the growth of each organ and allows vascular continuity in the organism, interconnecting organs and establishing irrigation of the entire plant. Xylem is a complex tissue comprised of several cell types (parenchyma, conducting cells and fibers), each formed by specific developmental processes and each with distinct morphological characteristics. One interesting cell type is the conducting cell, called the tracheary element (TE), which, in order to be functional, must undergo secondary cell wall formation and programmed cell death (PCD) processes. This results in the formation of an interconnected series of strengthened hollow tubes suitable for raw sap conduction. One of the major difficulties in studying TE formation in planta is that very few cells actually differentiate at a given moment and a given time. In order to study TE formation, an in vitro TE differentiation system has been established from isolated mesophyll cells of an ornamental asteraceae, Zinnia elegans. This systemis characterized byahigh rate of differentiation and the semi-synchronous nature of the morphological changes that occur. In order to compare TE formation in vitro and in planta, we provide herein i) a detailed anatomical description of the vascular system of Zinnia elegans, ii) a morphological comparison of TE differentiation in vitro vs. in planta and iii) gene expression localization both in planta and in vitro. The morphological and gene expression data provided in this chapter allow us to integrate in vitro-derived information in the context of the whole plant.