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Subcellular Partitioning-Dependent Functional Switching of Arabidopsis Photoreceptor Phytochrome B in Response to Brassinosteroids
Jong Sang Ryu1,4, Hyunmo Choi1, Sung Hyun Hong1, Tomonao Matsushita2, Akira Nagatani2 and Hong Gil Nam1,3,*
1Division of Molecular Life Sciences and National Core Research Center for Systems Bio-Dynamics, POSTECH, Pohang, Kyungbuk 790-784, Republic of Korea
2Laboratory of Plant Physiology, Graduate School of Science, Kyoto University, Sakyo-Ku, Kyoto 606-8502, Japan
3The I-BIO graduate program, POSTECH, Pohang, Kyungbuk 790-784, Republic of Korea
4Current address: Samsung Advanced Institute of Technology, Yongin, Gyeonggi, Korea
*Corresponding author
  Received : March 06, 2009
  Accepted : March 30, 2009
  Published : March 30, 2009
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Synopsis

Many organisms control their physiology and behavior in response to the local light environment, which is first perceived by photoreceptors that undergo light-dependent conformational changes. Phytochromes are one of the major photoreceptors in plants, controlling wide aspects of plant physiology by recognizing the light in red (R) and far-red (FR) spectra. Higher plants have two types of phytochromes; the photo-labile type I (phyA in Arabidopsis) and photo-stable type II (phyB-E in Arabidopsis). Phytochrome B (phyB), a member of the type II phytochromes in Arabidopsis, shows classical R and FR reversibility between the inter-convertible photoisomers, Pr and Pfr. Interestingly, the Pr and Pfr isomers show partitioning in the cytosol and nucleus, respectively. In the over 50 years since its discovery, it has been thought that the type II phytochromes only function to mediate R light. As described in the text, we have now discovered phyB has an active function in FR light. Even striking is that the R and FR light exert an opposite effect. Thus, FR light is not simply nullifying the R effect but has an opposing effect to R light. What is more interesting is that the phyB-mediated actions of FR and R light occur at different cellular compartment of the plant cell, cytosol and nucleus, respectively, which was proven through utilization of the cytosolic and nuclear-localized mutant versions of phyB. Our observations thus shoot down a major dogma in plant physiology and will be considered highly provocative in phytochrome function. We argue that it would make much more sense that plants utilize the two isoforms rather than only one form, to effectively monitor the changing environmental light information and to incorporate the information into their developmental programs.

Keyword: brassinosteroid, seedling growth, red light, far red light, nucleo-cytoplasmic partitioning
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