GENOMES UNCOUPLED1-independent retrograde signaling targets the ethylene pathway to repress photomorphogenesis

When germinating in the light, Arabidopsis (Arabidopsis thaliana) seedlings undergo photomorphogenic development, characterized by short hypocotyls, greening, and expanded cotyledons. Stressed chloroplasts emit retrograde signals to the nucleus that induce developmental responses and repress photomorphogenesis. The nuclear targets of these retrograde signals are not yet fully known. Here, we show that lincomycin-treated seedlings (which lack developed chloroplasts) show strong phenotypic similarities to seedlings treated with ethylene (ET) precursor 1-aminocyclopropane-1-carboxylic acid, as both signals inhibit cotyledon separation in the light. We show that the lincomycin-induced phenotype partly requires a functioning ET signaling pathway, but could not detect increased ET emissions in response to the lincomycin treatment. The two treatments show overlap in upregulated gene transcripts, downstream of transcription factors ETHYLENE INSENSITIVE3 and EIN3-LIKE1. The induction of the ET signaling pathway is triggered by an unknown retrograde signal acting independently of GENOMES UNCOUPLED1. Our data show how two apparently different stress responses converge to optimize photomorphogenesis.This work was supported by grants from FEDER/Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación (Project References BIO2015-68460-P and PGC2018-099987-B-I00), from the Spanish Ministry of Economy and Competitiveness (FJCI-2016-30876 to C.G.) and from the CERCA Programme/Generalitat de Catalunya (Project Reference 2017SGR-718) to E.M. We acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa Programme for Centres of Excellence in R&D” 2016-2019 (SEV‐2015‐0533).Peer reviewe