Plants modulate their growth and development to adjust to the continuous variations in their light environment. During early development, seedlings emerging from buried seed grow vigorously upward in the subterranean darkness toward the soil surface (etiolated growth), a developmental strategy promoted by several members of the phytochrome (phy)-interacting factor (PIF) subfamily of bHLH transcription factors (PIF1,3,4 and 5) in Arabidopsis. Upon reaching the soil surface, light signals perceived by the phy photoreceptor family reverse etiolation (deetiolation) by inducing rapid degradation of the PIF proteins. Thus, an antagonistic functional interaction between active phy and PIFs determines seedling growth during deetiolation. A similar mechanism regulates rhythmic growth in seedlings growing under diurnal conditions (in dark and light cycles), where growth is restricted to the end of the night through phy-regulation of PIFs abundance. Altogether, PIFs are central regulators of plant growth in the dark both during seedling deetiolation and under diurnal conditions. Unraveling how the PIFs function in the dark during these two dark-light transitions is of great interest to understand regulation of plant growth. While recent data shows that PIFs regulate the expression of a plethora of genes, little is known on which are direct PIF targets, and which might in turn be key-regulators of growth. The research proposal aims to identify and characterize novel regulators of plant growth in the dark acting downstream of the PIFs. First, a novel genetic screen will be used to identify suppressors of the pif1pif3pif4pif5 quadruple mutant phenotype in etiolated seedlings. Second, a biochemical and genomic approach will be used to identify PIF3-regulated genes at the end of the night. The funding of this proposal will allow the applicant to initiate his own research project in the host laboratory and facilitate his transition to an independent research position in Europe.