View the Current Issue

Article

  • The EMBO Journal advance online publication 22 October 2009; doi:10.1038/emboj.2009.306

Inhibition of the shade avoidance response by formation of non-DNA binding bHLH heterodimers

Patricia Hornitschek1, Séverine Lorrain1, Vincent Zoete2, Olivier Michielin2 and Christian Fankhauser1

  1. Center for Integrative Genomics, University of Lausanne, Lausanne, Switzerland
  2. Swiss Institute of Bioinformatics, Molecular Modeling Group, Lausanne, Switzerland

Correspondence to:

Christian Fankhauser, Center for Integrative Genomics, University of Lausanne, Genopode Building, Lausanne CH-1015, Switzerland. Tel.: +41 21 692 3941; Fax: +41 21 692 3925; E-mail: christian.fankhauser@unil.ch

Received 27 April 2009; Accepted 23 September 2009

In shade-intolerant plants such as Arabidopsis, a reduction in the red/far-red (R/FR) ratio, indicative of competition from other plants, triggers a suite of responses known as the shade avoidance syndrome (SAS). The phytochrome photoreceptors measure the R/FR ratio and control the SAS. The phytochrome-interacting factors 4 and 5 (PIF4 and PIF5) are stabilized in the shade and are required for a full SAS, whereas the related bHLH factor HFR1 (long hypocotyl in FR light) is transcriptionally induced by shade and inhibits this response. Here we show that HFR1 interacts with PIF4 and PIF5 and limits their capacity to induce the expression of shade marker genes and to promote elongation growth. HFR1 directly inhibits these PIFs by forming non-DNA-binding heterodimers with PIF4 and PIF5. Our data indicate that PIF4 and PIF5 promote SAS by directly binding to G-boxes present in the promoter of shade marker genes, but their action is limited later in the shade when HFR1 accumulates and forms non-DNA-binding heterodimers. This negative feedback loop is important to limit the response of plants to shade.

  • Keywords:

    • HFR1,
    • phytochrome,
    • PIF/bHLH factor,
    • shade avoidance