Ribosomal proteins promote leaf adaxial identity

Establishing abaxial-adaxial polarity is central to leaf morphogenesis and function. Groups of genes that encode different components for leaf patterning have been identified in recent years. These include transcriptional factors, small RNAs, 26S proteasome and components required for post-transcriptional gene silencing and chromatin remodeling, showing a complex regulatory network and indicating that the regulation occurs at different levels. In this work, we report the identification and characterization of asymmetric leaves1/2 enhancer5 (ae5) and ae6 mutants. These two mutants had a phenotype of abnormal leaf patterning, with the abaxial mesophyll features appearing in the adaxial mesophyll domain, and double mutants ae5 as1/2 and ae6 as1/2 producing severely abaxialized leaves. AE5 and AE6 encode the ribosomal large subunit proteins RPL28A and RPL5A, respectively, and mutations in two other ribosomal protein genes, RPL5B and RPL24B, resulted in plant phenotypes similar to those of ae5 and ae6. Because these four ribosomal proteins are located in distinct sites in the ribosomal large subunit, we propose that the conserved translational function of the ribosome may be required for regulating key components during leaf patterning. Collectively, our data indicate that specific ribosome subunit-mediated translational control is essential in leaf polarity establishment.