A model for the 26S proteasome and ribosome actions in leaf polarity formation.

Leaf morphogenesis requires the establishment of adaxial-abaxial polarity in emerging leaf primordia, and a number of genes participating in this process have been identified in recent years. We previously reported that the 26S proteasome is important in specifying the leaf adaxial fate. More recently, two papers from separate researches showed that several genes encoding ribosomal large subunit proteins also play an important role in leaf adaxial-abaxial patterning. Here we show that plants with a single mutation in the genes encoding either 26S proteasome subunits or ribosomal proteins shared similar abnormalities in some leaves, with an outgrowth formed on the distal part of the leaf abaxial side. Plants harboring these 26S proteasome or ribosome mutations in combination with an additional mutation asymmetric leaves1 or 2 (as1 or as2) demonstrated severely defective leaves, and the phenotypes of these double mutants were very similar. Because activities of the 26S proteasome and ribosome both affect the level of functional proteins, the recent findings suggest that a previously unrecognized regulation, the protein level regulation, is critical in normal leaf patterning. A regulatory model for the 26S proteasome and ribosome actions in leaf patterning is discussed.