Tanaka Kiwamu, Asami Tadao, Yoshida Shigeo, Nakamura Yasushi, Matsuo Tomoaki, Okamoto Shigehisa
Department of Agricultural Sciences and Natural Resources , Kagoshima University, Kagoshima 890-0065, Japan.
Plant Physiol. 2005 Jun;138(2):1117-25. doi: 10.1104/pp.104.058040. Epub 2005 May 20.
Homeostasis of brassinosteroids (BRs) is essential for normal growth and development in higher plants. We examined responsiveness of 11 BR metabolic gene expressions to the decrease or increase of endogenous BR contents in Arabidopsis (Arabidopsis thaliana) to expand our knowledge of molecular mechanisms underlying BR homeostasis. Five BR-specific biosynthesis genes (DET2, DWF4, CPD, BR6ox1, and ROT3) and two sterol biosynthesis genes (FK and DWF5) were up-regulated in BR-depleted wild-type plants grown under brassinazole, a BR biosynthesis inhibitor. On the other hand, in BR-excessive wild-type plants that were fed with brassinolide, four BR-specific synthesis genes (DWF4, CPD, BR6ox1, and ROT3) and a sterol synthesis gene (DWF7) were down-regulated and a BR inactivation gene (BAS1) was up-regulated. However, their response to fluctuation of BR levels was highly reduced (DWF4) or nullified (the other eight genes) in a bri1 mutant. Taken together, our results imply that BR homeostasis is maintained through feedback expressions of multiple genes, each of which is involved not only in BR-specific biosynthesis and inactivation, but also in sterol biosynthesis. Our results also indicate that their feedback expressions are under the control of a BRI1-mediated signaling pathway. Moreover, a weak response in the mutant suggests that DWF4 alone is likely to be regulated in other way(s) in addition to BRI1 mediation.
油菜素甾醇（BRs）的稳态对于高等植物的正常生长发育至关重要。我们检测了11个BR代谢基因表达对内源BR含量在拟南芥中降低或升高的响应，以拓展我们对BR稳态潜在分子机制的认识。五个BR特异性生物合成基因（DET2、DWF4、CPD、BR6ox1和ROT3）以及两个甾醇生物合成基因（FK和DWF5）在生长于BR生物合成抑制剂油菜唑下的BR缺失野生型植物中上调。另一方面，在施用油菜素内酯的BR过量野生型植物中，四个BR特异性合成基因（DWF4、CPD、BR6ox1和ROT3）和一个甾醇合成基因（DWF7）下调，一个BR失活基因（BAS1）上调。然而，在bri1突变体中，它们对BR水平波动的响应大幅降低（DWF4）或完全消失（其他八个基因）。综上所述，我们的结果表明，BR稳态是通过多个基因的反馈表达来维持的，每个基因不仅参与BR特异性生物合成和失活，还参与甾醇生物合成。我们的结果还表明，它们的反馈表达受BRI1介导的信号通路控制。此外，突变体中的微弱响应表明，除了BRI1介导外，DWF4可能还以其他方式受到调控。
