Brassinosteroid levels increase drastically prior to morphogenesis of tracheary elements

As the first step toward understanding the involvement of endogenous brassi nosteroids (BRs) in cytodifferentiation, we analyzed biosynthetic activitie s of BRs in zinnia (Zinnia elegans L. cv Canary Bird) cells differentiating into tracheary elements. The results of feeding experiments suggested that both the early and late C6-oxidation pathways occur during tracheary eleme nt differentiation. Gas chromatography-mass spectrometry analysis revealed that five BRs, castasterone, typhasterol, 6-deoxocastasterone, 6-deoxotypha sterol, and 6-deoxoteasterone, actually existed in cultured zinnia cells an d culture medium. Quantification of endogenous BRs in each stage of trachea ry element differentiation by gas chromatography-mass spectrometry exhibite d that they increased dramatically prior to the morphogenesis, which was co nsistent with the idea that BRs are necessary for the initiation of the fin al stage of tracheary element differentiation. Moreover, the proportion of each BR in culture medium was quite different from that in cells, suggestin g that specific BRs are selectively secreted into medium and may function o utside the cells.