The development of the characteristic planar shape of leaves depends on the establishment of an abaxial-adaxial axis in the leaf primordia. Many different genes are involved in this crucial developmental process. REVOLUTA[REV, which encodes a class III homeodomain-leucine zipper(HD-ZIPIII) transcription factor], ASYMMETRIC LEAVES1 (AS1,a Myb-domain transcription factor gene) and ASYMMETRIC LEAVES2(AS2, which encodes a leucine-zipper transcription factor) are among the genes that specify the adaxial fate. KANADI family transcription factors,by contrast, help specify the abaxial identity by antagonizing HD-ZIPIII genes. In this issue, two papers unexpectedly reveal that ribosomal proteins are also involved in leaf polarity establishment in Arabidopsis. On , Pinon et al. describe three genes - PIGGYBACK1 (PGY1), PGY2 and PGY3 - that, when mutated, enhance the mild leaf polarity defects of Arabidopsis as1 mutants. All three pgy mutants develop ectopic outgrowths on the adaxial (upper) side of leaves when on an as1 mutant background. This phenotype is enhanced by REVmutations and is suppressed by KANADI gene mutations, highlighting the possibility that PGY genes might influence leaf development by interacting with the HD-ZIPIII-KANADI pathway. Surprisingly, the researchers show that PGY1, PGY2 and PGY3 encode the cytoplasmic large subunit ribosomal proteins RPL10a, RPL9 and RPL5, respectively. On , Yao et al. report that several other Arabidopsis ribosomal protein genes are also involved in leaf patterning. The ae5 and ae6 mutants,they report, develop abaxial-like photosynthetic tissue (mesophyll) in the adaxial mesophyll domain and produce severely abaxialised leaves when crossed to as1/2 mutants. Again, surprisingly, AE5 and AE6were found to encode the ribosomal large subunit proteins - in this case,RPL28A and RPL5A, respectively. Yao et al. also report that plants with mutations in two other ribosomal protein genes (RPL5B and RPL24B) have similar phenotypes to the ae5 and ae6mutants. Like Pinon et al., they also provide evidence that AE5 and AE6 interact with the HD-ZIPIII-KANADI pathway. Taken together, the results of these two papers strongly suggest that ribosome-mediated translational control is an important, and unexpected, regulator of leaf patterning.
