The Elongator complex regulates hypocotyl growth in darkness and during photomorphogenesis

The Elongator complex (hereafter Elongator) promotes RNA polymerase II-mediated transcript elongation through epigenetic activities such as histone acetylation. Elongator regulates growth, development, immune response and sensitivity to drought and abscisic acid. We demonstrate that elo mutants exhibit defective hypocotyl elongation but have a normal apical hook in darkness and are hyposensitive to light during photomorphogenesis. These elo phenotypes are supported by transcriptome changes, including downregulation of circadian clock components, positive regulators of skoto- or photomorphogenesis, hormonal pathways and cell wall biogenesis-related factors. The downregulated genes LHY, HFR1 and HYH are selectively targeted by Elongator for histone H3K14 acetylation in darkness. The role of Elongator in early seedling development in darkness and light is supported by hypocotyl phenotypes of mutants defective in components of the gene network regulated by Elongator, and by double mutants between elo and mutants in light or darkness signaling components. A model is proposed in which Elongator represses the plant immune response and promotes hypocotyl elongation and photomorphogenesis via transcriptional control of positive photomorphogenesis regulators and a growth-regulatory network that converges on genes involved in cell wall biogenesis and hormone signaling.