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 mutant Hairy Sheath Frayed1 (Hsf1). Hsf1 plants produce leaves with outgrowths consisting of proximal segments - sheath, auricle and ligule - emanating from the distal blade margin. Analysis of three independent Hsf1 alleles revealed gain-of-function missense mutations in the ligand binding domain of the maize cytokinin (CK) receptor Zea 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 the Hsf1 leaf phenotypes. Results from expression and epistatic analyses indicated the Hsf1 mutant receptor appears to be hypersignaling. Our results demonstrate that hypersignaling of CK in incipient leaf primordia can reprogram developmental patterns in maize.

叶片形态发生需要沿三个轴（近端-远端，内侧-外侧和背面-轴向）极化的生长。草叶显示出明显的近端-远端（PD）极性，该极性由近端鞘管组成，该近端鞘管通过耳廓和舌叶与远端刀片隔开。尽管正确指定这四个部分对于正常形态至关重要，但我们对影响像玉米（Zea mays）这样的单子叶植物PD规格的机理的知识尚不完善。在这里，我们报告的基因鉴定的基础是半显性，叶片模式，玉米突变体毛鞘Frayed1（Hsf1）。Hsf1植物产生的叶片向外生长，叶片的近端部分（鞘，耳廓和舌状叶）从远端叶片边缘散发出。对三个独立的Hsf1等位基因的分析揭示了玉米细胞分裂素（CK）受体玉米组氨酸激酶1（ZmHK1）基因的配体结合域中的功能增强错义突变。生化分析和结构建模表明，靠近CK结合袋的突变残基会影响CK结合亲和力。外源CK表型处理Hsf1叶表型的野生型幼苗。表达和上位性分析的结果表明Hsf1突变受体似乎是高信号的。我们的结果表明，初生叶原基中CK的超信号化可以重编程玉米的发育模式。生化分析和结构建模表明，靠近CK结合袋的突变残基会影响CK结合亲和力。外源CK表型处理Hsf1叶表型的野生型幼苗。表达和上位性分析的结果表明Hsf1突变受体似乎是高信号的。我们的结果表明，初生叶原基中CK的超信号化可以重编程玉米的发育模式。生化分析和结构建模表明，靠近CK结合袋的突变残基会影响CK结合亲和力。外源CK表型处理Hsf1叶表型的野生型幼苗。表达和上位性分析的结果表明Hsf1突变受体似乎是高信号的。我们的结果表明，初生叶原基中CK的超信号化可以重编程玉米的发育模式。