A Novel Function of the 26S Proteasome in Repressing Class-1 KNOX Genes During Leaf Development.

Leaf organogenesis occurs within the peripheral zone of the shoot apical meristem (SAM). It has been known that several members of the class-1 KNOTTED1-like homeobox (KNOX) genes are expressed in the SAM, and their expression must be prevented during leaf primordium initiation and subsequent leaf development. A number of regulators that repress class-1 KNOX genes have been identified, and characterizations of these regulators greatly improved our knowledge of the genetic basis of leaf organogenesis. We have recently reported that the proteolytic function of the Arabidopsis 26S proteasome is involved in specifying leaf adaxial identity during leaf development, by characterizations of mutants defective in genes encoding several 26S proteasome subunits. Here we demonstrate that in addition to the role in leaf polarity establishment, the 26S proteasome also participates in repression of class-1 KNOX genes during leaf development. We show that loss of functions in RPN8a and RPT2a, two 26S proteasome subunit genes, resulted in leaves that produce ectopic outgrowths on the abaxial side of blades. These outgrowths were accompanied by the ectopic expression of several class-1 KNOX genes. These results indicate that the 26S proteasome is important in repressing class-1 KNOX genes and its function may be required until later leaf developmental stages.