Grain number is a flexible trait that strongly contributes to grain yield. In rice (Oryza sativa), the OsMKKK10-OsMKK4-OsMPK6 cascade, which is negatively regulated by the dual-specificity phosphatase GSN1, coordinates the trade-off between grain number and grain size. However, the specific components upstream and downstream of the GSN1-MAPK module that regulate spikelet number per panicle remain obscure. Here, we report that ERECTA1 (OsER1), a negative regulator of spikelet number per panicle, acts upstream of the OsMKKK10-OsMKK4-OsMPK6 cascade and that the OsER1-OsMKKK10-OsMKK4-OsMPK6 pathway is required to maintain cytokinin homeostasis. OsMPK6 directly interacts with and phosphorylates the zinc finger transcription factor DST to enhance its transcriptional activation of CYTOKININ OXIDASE2 (OsCKX2), indicating that the OsER1-OsMKKK10-OsMKK4-OsMPK6 pathway shapes panicle morphology by regulating cytokinin metabolism. Furthermore, overexpression of either DST or OsCKX2 rescued the spikelet number phenotype of the oser1, osmkkk10, osmkk4, and osmpk6 mutants, suggesting that the DST-OsCKX2 module genetically functions downstream of the OsER1-OsMKKK10-OsMKK4-OsMPK6 pathway. These findings reveal specific crosstalk between a MAPK signaling pathway and cytokinin metabolism, shedding light on how developmental signals modulate phytohormone homeostasis to shape the inflorescence.

粒数是一种灵活的性状，对谷物产量有很大贡献。在水稻 ( Oryza sativa ) 中，OsMKKK10-OsMKK4-OsMPK6 级联受到双特异性磷酸酶 GSN1 的负调控，协调籽粒数量和籽粒大小之间的权衡。然而，GSN1-MAPK 模块上游和下游调节每穗小穗数量的具体组件仍然不清楚。在这里，我们报道 ERECTA1 (OsER1) 是每穗小穗数量的负调节因子，作用于 OsMKKK10-OsMKK4-OsMPK6 级联的上游，并且 OsER1-OsMKKK10-OsMKK4-OsMPK6 途径是维持细胞分裂素稳态所必需的。OsMPK6 直接与锌指转录因子 DST 相互作用并磷酸化，增强其对细胞分裂素氧化酶 2的转录激活( OsCKX2 )，表明OsER1-OsMKKK10-OsMKK4-OsMPK6途径通过调节细胞分裂素代谢来塑造穗形态。此外，DST或OsCKX2的过表达挽救了oser1、osmkkk10、osmkk4和osmpk6突变体的小穗数量表型，表明 DST-OsCKX2 模块在 OsER1-OsMKKK10-OsMKK4-OsMPK6 途径下游发挥遗传功能。这些发现揭示了 MAPK 信号通路和细胞分裂素代谢之间的特定串扰，揭示了发育信号如何调节植物激素稳态以塑造花序。