Quantitative control of ASYMMETRIC LEAVES2 expression is critical for leaf axial patterning in Arabidopsis

ASYMMETRIC LEAVES2 (AS2) is one of the key genes required for specifying leaf adaxial identity during leaf adaxial-abaxial polarity establishment. Previous data have shown that, in leaf development, AS2 is directly repressed by an abaxially located transcription factor KANADI1 (KAN1), so that the AS2 transcripts are restricted only in the adaxial leaf domain. It is shown here that, different from the spatial repression by KAN1, the quantitative repression of AS2 in the adaxial domain is also critical for ensuring normal leaf pattern formation. By analysing two gain-of-function as2 mutants, as2-5D and isoginchaku-2D (iso-2D), it is shown that the similar AS2-over-expressed phenotypes of these mutants reflect two different kinds of AS2 misexpression patterns. While as2-5D causes disruption of a KAN1-binding site at the AS2 promoter leading to derepression of AS2 in the abaxial side but without changing its expression level of a leaf, iso-2D results in over-expression of AS2 but without altering its adaxial expression pattern. In addition, it was found that, in iso-2D, levels of histone H3 lysine 27 trimethylation (H3K27me3) and H3K4me3 at the AS2 locus are significantly reduced and increased, respectively, compared with those in the wild type and as2-5D. These results suggest that during leaf patterning, quantitative control of the AS2 expression level might involve epigenetic regulations.