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Plant and Cell Physiology Advance Access originally published online on February 23, 2008
Plant and Cell Physiology 2008 49(4):501-511; doi:10.1093/pcp/pcn024
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Phytochrome-Regulated PIL1 Derepression is Developmentally Modulated

Yong-sic Hwang1,2,3 and Peter H. Quail1,2,*

1Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA
2USDA/ARS-Plant Gene Expression Center, 800 Buchanan Street, Albany, CA 94710, USA
3Department of Bioscience and Biotechnology, Konkuk university, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Korea

*Corresponding author: E-mail, quail{at}nature.berkeley.edu; Fax, +1-510-559-5678.


  Abstract

We define the photoresponsiveness, during seedling de-etiolation, of PHYTOCHROME-INTERACTING FACTOR 3-LIKE 1 (PIL1), initially identified by microarray analysis as an early-response gene that is robustly repressed by first exposure to light. We show that PIL1 mRNA abundance declines rapidly, with a half-time of 15 min, to a new steady-state level, 10-fold below the initial dark level, within 45 min of first exposure to red light. Analysis of phy-null mutants indicates that multiple phytochromes, including phyA and phyB, impose this repression. Conversely, PIL1 expression is rapidly derepressed by subsequent far-red irradiation of previously red light-exposed seedlings. However, the magnitude of this derepression is modulated over time, in a biphasic manner, in response to increasing duration of pre-exposure to continuous red light: (i) an early phase (up to about 6 h) of relatively rapidly increasing effectiveness of far-red reversal of repression, as declining phyA levels relieve initial very low fluence suppression of this response; and (ii) a second phase (beyond 6 h) of gradually declining effectiveness of far-red reversal, to only 20% of maximal derepression, within 36 h of continuous red light exposure, with no evidence of circadian modulation of this responsiveness, an observation in striking contrast to a previous report for entrained, green seedlings exposed to vegetative shade. These data, together with analysis of phytochrome signaling mutants and overexpressors with aberrant de-etiolation phenotypes, suggest that the second-phase decline in robustness of PIL1 derepression is an indirect consequence of the global developmental transition from the etiolated to the de-etiolated state, and that circadian coupling of derepression requires entrainment.

Keywords: De-etiolation - Derepression - Early response gene - Phytochrome - PHYTOCHROME-INTERACTING FACTOR 3-LIKE 1 (PIL1)

Abbreviations: bHLH, basic helix–loop–helix; FRc, continuous far-red light; FRp, far-red light pulse; phy, phytochrome; PIF, PHYTOCHROME-INTERACTING FACTOR; PIL1, PHYTOCHROME-INTERACTING FACTOR 3-LIKE 1; Rc, continuous red light; Rp, red light pulse; VLFR, very low fluence response.

(Received November 23, 2007; Accepted February 11, 2008)
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