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The shade avoidance syndrome ( SAS) refers to a set of plant responses initiated after perception by the phytochromes of light with a reduced red to far-red ratio, indicative of vegetation proximity or shade. These responses, including elongation growth, anticipate eventual shading from potential competitor vegetation by overgrowing neighboring plants or flowering to ensure production of viable seeds for the next generation. In Arabidopsis thaliana seedlings, the SAS includes dramatic changes in gene expression, such as induction of PHYTOCHROME RAPIDLY REGULATED 1 ( PAR1), encoding an atypical basic helix-loop-helix ( bHLH) protein that acts as a transcriptional co-factor to repress hypocotyl elongation. Indeed, PAR1 has been proposed to act fundamentally as a dominant negative antagonist of conventional bHLH transcription factors by forming heterodimers with them to prevent their binding to DNA or other transcription factors. Here we report the identification of PAR1-interacting factors, including the brassinosteroid signaling components BR-ENHANCED EXPRESSION (BEE) and BES1-INTERACTING MYC-LIKE (BIM), and characterize their role as networked positive regulators of SAS hypocotyl responses. We provide genetic evidence that these bHLH transcriptional regulators not only control plant growth and development under shade and non-shade conditions, but are also redundant in the control of plant viability. Our results suggest that SAS responses are initiated as a consequence of a new balance of transcriptional regulators within the pre-existing bHLH network triggered by plant proximity, eventually causing hypocotyls to elongate. [ABSTRACT FROM AUTHOR]

PAR1 is an atypical basic-helix-loop-helix (bHLH) protein that negatively regulates the shade avoidance syndrome in Arabidopsis thaliana acting as a transcriptional cofactor. Consistently with this function, PAR1 has to be in the nucleus to display biological activity. Previous structure-function analyses revealed that the N-terminal region of PAR1 drives the protein to the nucleus. However, truncated forms of PAR1 lacking this region still display biological activity, implying that PAR1 has additional mechanisms to localize into the nucleus. In this work, we compared the primary structure of PAR1 and various related and unrelated plant bHLH proteins, which led us to suggest that PAR1 contains a non-canonical nuclear localization signal (NLS) in the N-terminal region. By overexpressing truncated and mutated derivatives of PAR1, we have also investigated the importance of other regions of PAR1, such as the acidic and the extended HLH dimerization domains, for its nuclear localization. We found that, in the absence of the N-terminal region, a functional HLH domain is required for nuclear localization. Our results suggest the existence of a dual mechanism for PAR1 nuclear localization: (1) one mediated by the N-terminal non-consensus NLS and (2) a second one that involves interaction with other proteins via the dimerization domain. [ABSTRACT FROM PUBLISHER]