title: Shade Promotes Phototropism through Phytochrome B-Controlled Auxin Production.
abstract: Phototropism is an asymmetric growth response enabling plants to optimally position their organs. In flowering plants, the phototropin (phot) blue light receptors are essential to detect light gradients. In etiolated seedlings, the phototropic response is enhanced by the red/far-red (R/FR)-sensing phytochromes (phy) with a predominant function of phyA. In this study, we analyzed the influence of the phytochromes on phototropism in green (de-etiolated) Arabidopsis seedlings. Our experiments in the laboratory and outdoors revealed that, in open environments (high R/FR ratio), phyB inhibits phototropism. In contrast, under foliar shade, where access to direct sunlight becomes important, the phototropic response was strong. phyB modulates phototropism, depending on the R/FR ratio, by controlling the activity of three basic-helix-loop-helix (bHLH) transcription factors of the PHYTOCHROME INTERACTING FACTORs (PIFs) family. Promotion of phototropism depends on PIF-mediated induction of several members of the YUCCA gene family, leading to auxin production in the cotyledons. Our study identifies PIFs and YUCCAs as novel molecular players promoting phototropism in photoautotrophic, but not etiolated, seedlings. Moreover, our findings reveal fundamental differences in the phytochrome-phototropism crosstalk in etiolated versus green seedlings. We propose that in natural conditions where the light environment is not homogeneous, the uncovered phytochrome-phototropin co-action is important for plants to adapt their growth strategy to optimize photosynthetic light capture.
authors.affiliation: Center for Integrative Genomics, Faculty of Biology and Medicine, University of Lausanne, 1015 Lausanne, Switzerland.
authors.forename: Anupama
authors.initials: A
authors.lastname: Goyal
authors.affiliation: IFEVA, Facultad de Agronomia, Universidad de Buenos Aires and CONICET, Av. San Martin 4453, 1417 Buenos Aires, Argentina.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Karayekov
authors.affiliation: Center for Integrative Genomics, Faculty of Biology and Medicine, University of Lausanne, 1015 Lausanne, Switzerland.
authors.forename: Vinicius Costa
authors.initials: VC
authors.lastname: Galvão
authors.affiliation: Plant Biology Laboratory, Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, CA 92037, USA.
authors.forename: Hong
authors.initials: H
authors.lastname: Ren
authors.affiliation: IFEVA, Facultad de Agronomia, Universidad de Buenos Aires and CONICET, Av. San Martin 4453, 1417 Buenos Aires, Argentina; Fundación Instituto Leloir, Instituto de Investigaciones Bioquímicas de Buenos Aires-CONICET, 1405 Buenos Aires, Argentina.
authors.forename: Jorge J
authors.initials: JJ
authors.lastname: Casal
authors.affiliation: Center for Integrative Genomics, Faculty of Biology and Medicine, University of Lausanne, 1015 Lausanne, Switzerland. Electronic address: christian.fankhauser@unil.ch.
authors.forename: Christian
authors.initials: C
authors.lastname: Fankhauser
mesh_terms: D017360:Arabidopsis / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D018506:Gene Expression Regulation, Plant / Q000502:physiology; D007210:Indoleacetic Acids / Q000378:metabolism*; D008027:Light; D018524:Phototropism / Q000502:physiology*; D051939:Phytochrome B / Q000502:physiology*; D036226:Seedlings / Q000502:physiology
chemical_list: D029681:Arabidopsis Proteins; D007210:Indoleacetic Acids; D051939:Phytochrome B
keywords: Arabidopsis thaliana; PHYTOCHROME-INTERACTING FACTORs; YUCCAs; photoreceptor crosstalk; phototropin 1; phototropism; phytochrome B; shade avoidance