J. Plant Physiol. 161. 265 – 270 (2004) http://www.elsevier-deutschland.de/jplhp Phytochrome regulation of pea phototropin Robert C. Elliott 1 *, J. Damien Platten 1 , John C. Watson 2 , James B. Reid 1 1 School of Plant Science, University of Tasmania, Private Bag 55, Hobart,Tasmania 7001,Australia 2 Department of Biology, 723 West Michigan St, Indiana University-Purdue University Indianapolis, Indianapolis IN, 46202-5132, USA Received June 19, 2003 · Accepted August 21, 2003 Summary Type 1 phototropin, one of the blue light receptors responsible for phototropism, is encoded in peas by at least two genes, PsPHOT1A and PsPHOT1B (formerly PsPK4 and PsPK5), both of which are more similar to Arabidopsis PHOT1 than to Arabidopsis PHOT2. We show here that PsPHOT1B enco- des a full-length phototropin, whose expression pattern suggests that Psphot1b is the predominant phot1-type phototropin in etiolated seedlings. The gene encoding the other type 1 phototropin, PsPHOT1A, is expressed at low levels, with its highest levels in the leaves and stems of more mature, light-grown plants. Studies with phyA, phyB and the phyAphyB double mutants show that phyA and phyB have partially redundant roles in the reduction of PsPHOT1B expression under red light. Key words: phototropin – phototropism – phytochrome Abbreviations: LOV = light, oxygen, voltage domain. – PCR = polymerase chain reaction. – PHOT1 = Arabidopsis phototropin 1. – PHOT2 = Arabidopsis phototropin 2. – phyA = phytochrome A. – phyB = phytochrome B. – RACE PCR = rapid amplification of cDNA ends polymerase chain reaction. – SSPCR = single-sided polymerase chain reaction. – VLF = very-low-fluence Introduction Light plays an important role in regulating the levels and activity of the plant photoreceptors and their downstream tar- gets. Light has several effects on the sensitivity of the plant to unilateral blue light. Dark-grown seedlings in many species require a pulse of low-fluence red light to achieve maximal bending in response to unilateral blue light (Chon and Briggs 1966, Janoudi and Poff 1992, Liu and Iino 1996). This is prob- ably phytochrome acting on a component downstream of the blue light receptor phototropin (Britz and Carrol 1993), per- haps via phosphorylation of an auxin response factor (ARF) * E-mail corresponding author: Robert.Elliott@utas.edu.au (Stowe-Evans et al. 2001). Blue light also affects the sensitivity of the plant. Exposure of the plant to overhead blue light cau- ses a short period of reduced sensitivity to subsequent unila- teral blue light (Janoudi and Poff 1991, 1993). This adaptation to blue light may be due to inactivation of the photoreceptor via autophosphorylation (Christie and Briggs 2001, Liscum 2002). In Arabidopsis longer exposures to blue light cause reductions in the holoenzyme phot1 (Sakamoto and Briggs 2002). This reduction is paralleled by a decrease in PHOT1 transcript (Sakamoto and Briggs 2002). Transcript levels of a second phototropin with a lower sensitivity to blue light, PHOT2, increase in response to white, blue, red, or UV-A (Jarillo et al. 2001, Kagawa et al. 2001, Sakai et al. 2001). Although the short-term adaptation process is mediated by 0176-1617/04/161/03-265 $ 30.00/0