Homeodomain transcription factor regulation of leaf serrations in Arabidopsis thaliana

Arabidopsis thaliana has simple spatulate to obovate shaped leaves, with subtle serrations along the margin. The BEL1-like homeodomain transcription factors (BLH) SAWTOOTH1 and SAWTOOTH2, and the class II KNOTTED1-like homeobox (KNOXII) KNAT3 are suppressors of serration outgrowths. Double saw1 saw2 mutants have enhanced serrations and knat3 single mutants have a subtle increase in serrations relative to the wild type. However, the relationship between these homeodomain proteins is unclear. Here we show that SAW1, SAW2, and KNAT3 have overlapping expression patterns on the adaxial side of the leaf and saw1 saw2 knat3 triple mutants show greatly enhanced leaf serration relative to the saw1 saw2 double mutant and knat3 single mutant. KNAT3 expressed in the adaxial leaf from the endogenous promoter suppresses the saw1 saw2 knat3 phenotype, whereas ectopic expression throughout the leaf or in the abaxial domain enhances the triple mutant serration phenotype. This ectopic KNAT3 activity can be modified by ectopic expression of SAW1. These results demonstrate that the adaxial-abaxial domain specificity of BLH and KNOXII homeodomain proteins has consequences for serration geometry.