The nucleolar complex FAN-FIP1 mediates ribosome biogenesis in Arabidopsis and is critical for BR signaling and heat tolerance

Ribosome biogenesis is critical for plant development and environmental responses. A large number of ribosomal proteins (RPs) and ribosomal biogenesis factors (RBFs) are required for ribosome biogenesis, many of which remain uncharacterized in plants. We report here the identification of Arabidopsis RBF FAN and its interacting partner FAN-INTERACTING PROTEIN 1 (FIP1). As their human and yeast orthologues, FAN-FIP1 interact. Both FAN and FIP1 participate in the processing of pre-rRNAs. Functional loss of FAN or FIP1 knock-down results in developmental retardation and hypersensitivity to heat stresses. We demonstrate that FAN-FIP1 positively mediates brassinosteroid (BR) signaling by ensuring the translation efficiency of the BR receptor-coding gene BRASSINOSTEROID INSENSITIVE 1 (BRI1) through the presence of its upstream open reading frame (uORF). Importantly, BR signaling positively mediates the processing of pre-rRNAs, which may be critical not only for development but also for heat tolerance.