The maizeHairy Sheath Frayed1(Hsf1) mutant alters leaf patterning through increased cytokinin signaling

Leaf morphogenesis requires growth polarized along three axes - proximal-distal, medial-lateral and abaxial-adaxial. Grass leaves display a prominent proximal-distal (P-D) polarity consisting of a proximal sheath separated from the distal blade by the auricle and ligule. Although proper specification of the four segments is essential for normal morphology, our knowledge is incomplete regarding the mechanisms which influence P-D specification in monocots like maize (Zea mays). Here we report the identification of the gene underlying the semi-dominant, leaf patterning, maize mutantHairy Sheath Frayed1(Hsf1).Hsf1plants produce leaves with outgrowths consisting of proximal segments – sheath, auricle and ligule – emanating from the distal blade margin. Analysis of three independentHsf1alleles revealed gain-of-function missense mutations in the ligand binding domain of the maize cytokinin (CK) receptorZea mays Histidine Kinase1(ZmHK1)gene. Biochemical analysis and structural modeling suggest the mutated residues near the CK binding pocket affect CK binding affinity. Treatment of wild type seedlings with exogenous CK phenocopied theHsf1leaf phenotypes. Results from expression and epistatic analyses indicated theHsf1mutant receptor appears to be hypersignaling. Our results demonstrate that hypersignaling of CK in incipient leaf primordia can reprogram developmental patterns in maize.SummaryIncreased cytokinin signaling in the maizeHairy Sheath Frayed1mutant modifies leaf development leading to changes in pattering, growth and cell identity.