Zinnia elegans: the missing link from in vitro tracheary elements to xylem

Abstract

For the last 20 years, in vitro xylogenic cultures of Zinnia elegans have been routinely used to study tracheary element (TE) formation. That said, the precise anatomical relationship between in vitro and in planta xylogenesis in Zinnia has been completely ignored. In order to make this comparison, we provide herein a much needed description of xylem tissue of the Zinnia plant. Based on the proportions of secondary wall thickenings, the in vitro TE system most closely resembles hypocotyl vasculature. Moreover, we have shown by confocal microscopy that vessel-like structures of up to five individual TEs are produced in vitro, suggesting that the formation of multicellular structures and cell-cell communication during in vitro TE formation are far more extensive than previously suspected. Finally, as more and more genes become available through genomic approaches of Zinnia TEs, it will be necessary to precisely localize them in planta as a first step in elucidating gene function. Our results provide the histological groundwork for this very purpose.

Non-Cell-Autonomous Postmortem Lignification of Tracheary Elements inZinnia elegans

“…16-µm-thick transversal cryosections were taken in the four first epicotyl internodes of 5-week-old Z. elegans plants (Figure 2A), and three to four consecutive proto-and metaxylem vessels were analyzed in the primary xylem of the vascular bundle ( Figures 2C to 2F). Primary xylem vessels are easily distinguishable, exhibit secondary cell wall patterns similar to TEs in vitro, and are completely isolated from neighboring lignified cells (Pesquet et al, 2003(Pesquet et al, , 2006. These cells also mature very rapidly after their formation, as revealed by the phloroglucinol-HCl staining of transverse sections from the different epicotyl internodes (Figures 2C to 2F).…”

Novel Markers of Xylogenesis in Zinnia Are Differentially Regulated by Auxin and Cytokinin

“…Based on comparative localization of gast1 and exp5 gene expression in planta and in vitro, we provide an additional argument in favor of the presence of xylem parenchyma equivalents in vitro by associating gene expression of gast1/exp5-expressing TEs in vitro with cambium cells in planta, and a subgroup of gast1/exp5-expressing non-TEs in vitro with xylem parenchyma cells in planta. In addition, the localization of gast1 and exp5 in planta was restricted to cambial cells immediately above vessel strands, implying that the cambium is a heterogeneous cell layer composed of cells characterized by different gene expression patterns, despite the lack of any major morphological differences among the cells (Pesquet et al, 2003). In conclusion, we show that the judicious combination of macroarrays and multiplex IS-RT-PCR on plant cells and organs opens new prospects in the quest to associate gene expression and cell fate at the single cell level in the context of xylem differentiation.…”

Galactoglucomannans Increase Cell Population Density and Alter the Protoxylem/Metaxylem Tracheary Element Ratio in Xylogenic Cultures of Zinnia

“…Protoxylem (PX) TEs with annular or helical secondary cell wall thickening differentiate while an organ is still expanding, whereas metaxylem (MX) TEs with reticulate or pitted secondary cell wall thickening differentiate after organ expansion has ceased (Esau, 1977). The relative proportions of PX and MX TEs that make up a given vascular bundle vary among plant organs and within a given organ throughout plant growth (Fahn, 1990;Pesquet et al, 2003). Until now, the underlying molecular mechanisms, signaling events, and positional information dictating the formation of either PX or MX were largely unknown.…”

“…Indeed, zinnia cultures mimic many aspects of xylogenesis in planta, including the formation of PX-and MX-like TEs (Falconer and Seagull, 1985;Pesquet et al, 2003). Therefore, the zinnia system provides a unique opportunity to search for actors involved in upstream events that determine PX or MX formation.…”

“…Theoretically, the 800-gene macroarray contains PX and MX genes because the library was made from cultures containing both types of TEs (Pesquet et al, 2003). Two comparisons were made: the first with a pool of RNA from cells cultured in the presence versus absence of GGMOs for 12 h, and the second from cells cultured for 60 h, again, in the presence or absence of GGMOs.…”