A suppressor screen of an Arabidopsis thaliana REDUCED COMPLEXITY (RCO)-expressing strain provides insight into the genetics of leaf margin complexity.

The varied leaf morphologies of seed plants provide an attractive system for studying the development and evolution of biological forms. Here, we consider the genetic mechanisms underlying variation in leaf margin geometry, as leaves can bear protrusions ranging from shallow serrations to lobes to fully separated leaflets. Leaflet formation in the complex-leaved species Cardamine hirsuta requires the REDUCED COMPLEXITY (RCO) homeobox gene. RCO was lost in the lineage of its simple-leaved relative Arabidopsis thaliana, and re-introduction of ChRCO into A. thaliana as a transgene increases leaf complexity by triggering the generation of deep lobes in the leaf margin. As the genetic mechanisms for RCO-mediated outgrowth formation are only partially understood, we performed a mutagenesis screen for suppressors of lobe formation in A. thaliana plants harboring a ChRCO transgene. From this screen, we identified CUP-SHAPED COTYLEDON 2 (CUC2), PIN-FORMED 1 (PIN1), CYCLOPHILIN 71 (CYP71), NUCLEOLAR PROTEIN 2A (NOP2A), RIBOSOMAL PROTEIN L34 (RPL34), and RIBOSOMAL PROTEIN L10aB/PIGGYBACK1 (PGY1). We also showed that the C. hirsuta CYP71 gene is required for leaflet development, as the cyp71 mutant has simplified leaves. Our results suggest that CUC2-auxin-PIN1-mediated marginal patterning, the CYP71 gene, and ribosome biogenesis are required for RCO to drive increased leaf complexity.