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

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.