Arabidopsis epigenetic factor AS2 mitigates nucleolar stress and promotes flat leaf development by repressing a CDK inhibitor

The Arabidopsis thaliana leaf, which exhibits a symmetrically extended flat morphology, consists of two distinct cellular domains: adaxial and abaxial layers. The ASYMMETRIC LEAVES2 (AS2) gene is essential for specifying the adaxial domain, and its protein forms nucleolar structures, termed AS2 bodies, at ribosomal-DNA loci. Numerous nucleolus-related genes have been reported to be cooperatively involved in leaf adaxialization together with AS2. However, the molecular relationships between AS2 and these genes remain unclear. To identify chemical modulators of AS2 function, we screened a chemical library containing natural products and identified eight molecules, including camptothecin, that induced filamentous leaves in the as2 mutant. Camptothecin is an inhibitor of topoisomerase I, which is required for transcription of ribosomal RNA genes, and induces nucleolar stress. Treatment with 10-hydroxy-camptothecin increased transcript levels of the CDK inhibitor KIP-RELATED-PROTEIN5/INHIBITOR-OF-CYCLIN-DEPENDENT-KINASE3 (KRP5/ICK3), and caused notable changes in the number and morphology of AS2 bodies. Given its role in leaf morphogenesis, our findings suggest that AS2 is a key factor in leaf adaxialization, which leads to the formation of flat leaves, and in protection against nucleolar stress through coordination with nucleolar proteins.