The receptor-like kinase ALE2 promotes giant cell formation in the sepal epidermis

During sepal development in Arabidopsis thaliana, epidermal pavement cells differentiate into small cells and large, highly endoreduplicated giant cells. While the placement of giant cells differs between sepals, the number of giant cells is fairly consistent. The HD-ZIP class IV transcription factor ATML1 has been found to promote giant cell formation. ATML1 protein fluctuates within epidermal nuclei of developing sepals and high ATML1 concentrations reached in the G2 phase of the cell cycle strongly correlates with giant cell fate specification. A genetic screen for reduced giant cell number identified the receptor-like kinase ABNORMAL LEAF SHAPE2 (ALE2) as being important for giant cell formation. We find that ALE2 functions genetically upstream of ATML1 in promoting the formation of giant cells. We observe that nuclear-localized mCitrine-ATML1 fluctuates in ale2 mutants much as it does in wild type and, importantly, nuclear mCitrine-ATML1 reaches similarly high peak concentrations in ale2-1 mutants as in wild type. This indicates that ALE2 functions upstream of ATML1 by affecting protein activity rather than gene expression or protein degradation. One function of ATML1 is to promote transcription of the CDK inhibitor LGO. We find that LGO transcription is delayed and decreased in ale2-1 mutants as compared to wild-type plants, consistent with ATML1 having impaired protein function in ale2 mutants. Overall, we find that the receptor-like kinase ALE2 is necessary to sensitize cells of the developing sepal epidermis to fluctuations of the transcription factor ATML1.