Arabidopsis 26s Proteasome Pathway And Transcription Elongation Factor Elongator Regulation Of Leaf Adaxial - Abaxial Polarity Built

1. The proteolytic function of the Arabidopsis 26S proteasome is required for specifying leaf adaxial identityPolarity formation is central to leaf morphogenesis, and several key genes that function in adaxial-abaxial polarity establishment have been identified and characterized extensively. We previously reported that Arabidopsis ASYMMERTIC LEAVES1 and 2 (AS1 and AS2) are important in promoting leaf adaxial fates. We obtained an as2 enhancer mutant, asymmetric leaves enhancer3 (ae3), which demonstrated pleiotropic plant phenotypes, including a defective adaxial identity in some leaves, while the ae3 as2 double mutant leaves were severely abaxialized. The aberrant ae3 as2 leaves were accompanied by elevated expression levels of leaf abaxial promoting genes FILAMENTOUS FLOWER, YABBY3, KANADI1 and KANADI2, and a reduced expression level of the adaxial promoting gene REVOLUTA. We identified AE3, which encodes a putative 26S proteasome subunit RPN8a. Furthermore, other double mutant combinations of as2 with other 26S subunit mutations, including rpt2a, rpt4a, rpt5a, rpt1a, rpn1a, rpn9a, rpn2a, rpn10, pad1, pbe1 and paa1,all displayed comparable phenotypes with those of ae3 as2, albeit with varying phenotypic severity. Since these mutated genes encode subunits that are located in different parts of the 26S proteasome, it is possible that the proteolytic function of the 26S holoenzyme is involved in leaf polarity formation. Together, our findings reveal that posttranslational regulation is essential in proper leaf patterning. 2. The Arabidopsis Elongator and the 26S proteasome may function together in specifying leaf adaxial identityThe Arabidopsis 26S proteasome was demonstrated to be required for specifying leaf adaxial identity. However, the molecular mechanism for such a specification is largely unknown. Here we show that mutants with the defective Elongator subunit genes, elongata1 (elo1) and elongata2 (elo2), and with an aberrant Elongator-associating protein gene, deformed roots and leaves1/elongata4 (drl1/elo4), phenotypically resembled those of the 26S proteasome subunit mutants. Double mutants by combining as1/as2 with elo mutants and with 26S proteasome subunit mutants were also similar. Furthermore, double mutants with elo2 and one of those that contained a mutated 26S proteasome subunit gene were embryonically lethal. The Elongator is a conserved protein complex, which consists of six subunits with the histone acetyl transferase activities in eukaryotes. The biological function of this protein complex was thought to facilitate transcriptional elongation. Since one of the previously reported 26S proteasome functions is also involved in regulating transcription through degradation of some transcription factors/cofactors, we propose that Elongator and 26S proteasome may act in the same pathway to regulate transcription of genes required for the leaf polarity control, or act in parallel pathways by which common downstream genes are regulated.