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LAF3, a Novel Factor Required for Normal Phytochrome A Signaling^1,[w]

Laboratory of Plant Molecular Biology, The Rockefeller University, 1230 York Avenue, New York 10021 (P.D.H., S.G.M., L.-F.H., N.-H.C.); and Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom (S.G.M.)

Phytochrome A (phyA) is the photolabile plant light receptor that mediates broad spectrum very low-fluence responses and high irradiance responses to continuous far-red light (FR_c). An Arabidopsis mutant laf3-1 (long after far-red 3) was recovered from a screen for transposon-tagged mutants that exhibit reduced inhibition of hypocotyl elongation in FR_c. The laf phenotype correlated well with a strongly attenuated disappearance of XTR7 transcript in FR_c. The effects of laf3-1 on phyA-controlled CAB, CHS, and PET H expression were more subtle, and the mutation had no clear effects on PET E and ASN1 transcript levels in FR_c. The use of two alternative transcription initiation sites in the LAF3 gene generates two isoforms that differ only at their N termini. Transcripts encoding both isoforms were induced during germination and were present at slightly higher levels in de-etiolated seedlings than in those grown in darkness. No significant differential regulation of the two isoforms was observed upon exposure to either FR_c or continuous red light. Transcripts encoding the shorter isoform (LAF3_ISF2) always appear to be more abundant than those encoding the longer isoform (LAF3_ISF1). However, both isoforms were capable of full complementation of the laf3-1 hypocotyl phenotype in FR_c. When fused to a yellow fluorescent protein, both isoforms localize to the perinuclear region, suggesting that LAF3 encodes a product that might regulate nucleo-cytoplasmic trafficking of an intermediate(s) involved in phyA signal transduction.

¹ This work was supported by NIH grant GM 44640, a JST (CREST) grant, a grant from the Biotechnology and Biological Science Research Council (S.G.M.), and by the South African National Research Foundation (postdoctoral fellowship to P.D.H.).

^[w] The online version of this article contains Web-only data.

^* Corresponding author; e-mail chua{at}rockefeller.edu; fax 212-327-8327.

Received June 12, 2003; returned for revision July 2, 2003; accepted August 28, 2003.

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