Poster: Hormones

Abs # 565: The regulation of biosynthesis and secretion of brassinosteroids during xylem differentiation

Presenter:	Yamamoto, Ryo , yamamoto@biol.s.u-tokyo.ac.jp
Authors	Yamamoto, Ryo  (A)   Fujioka, Shozo  (B)   Yoshida, Shigeo  (B)   Fukuda, Hiroo  (A)
Affiliations:	(A): Dept. of Biol. Sci., Univ. of Tokyo (B): Plant Functions Lab., RIKEN

In Zinnia elegans cell cultures, mechanically-isolated single mesophyll cells transdifferentiate directly into tracheary elements in response to exogenous auxin and cytokinin. Using the zinnia system, we have found that brassinosteroids act as a key factor for the initiation of final stage of tracheary element differentiation. Quantification of the levels of endogenous brassinosteroids revealed that some brassinosteroid species increased dramatically before the entry into the final stage. Moreover we found that some brassinosteroids in medium also increased, but the increased kinds of brassinosteroids in medium differed from those in cells. In the present study, we investigated molecular mechanism regulating the accumulation and/or the secretion of specific endogenous brassinosteroids prior to final differentiation of tracheary elements. A combination of auxin and cytokinin, which induces tracheary element differentiation, affected accumulation patterns of brassinosteroids. Auxin and cytokinin promoted conspicuously the accumulation of typhasterol and 6-deoxotyphasterol. However secretion of castasterone, which is a precursor of an active brassinosteroid, brassinolide, was not affected by auxin and cytokinin. Furthermore we isolated several genes involved in brassinosteroid biosynthesis from zinnia cells. Of them, only ZeDWF4 and ZeCPD1 are transcribed actively when cultured with a combination of auxin and cytokinin. Taken together with these results, we will discuss the regulation mechanism of brassinosteroid biosynthesis and secretion in relation to tracheary element differentiation in zinnia.