N-acetyltransferase HOOKLESS1 (HLS1) in Arabidopsis, a recognized pivotal factor in hook formation, is also implicated in multifaceted developmental processes including plastochron and leaf senescence. However, its molecular function remains to be elucidated. We utilized LC-MS for plant hormone quantification and implemented a series of molecular genetic and physiological approaches to analyze HLS1 functions. Our findings indicate that HLS1 functions in tissues distant from its expression sites in the regulation of apical hook formation, plastochron, and leaf senescence, implying its non-cell-autonomous mode of action. We observed that extranuclear localization of HLS1 can complement the hls1 phenotype, suggesting a revision of our understanding that HLS1 is a nuclear-specific histone acetyltransferase. The double mutant of HLS1 and its paralog shows highly pleiotropic and enhanced phenotypes similar to those of the double mutant of the putative carboxypeptidase gene ALTERED MERISTEM PROGRAM1 (AMP1) and its paralog. Genetic analysis revealed that HLS1 and AMP1 act in the same genetic pathway to regulate the plastochron. No significant differences in the contents of known plant hormones were observed in the hls1 mutant. HLS1 is an N-acetyltransferase that functions in a non-cell-autonomous manner across pleiotropic developmental phenomena in conjunction with AMP1, potentially independent of known plant hormone signaling.