DELLAgenes restrict inflorescence meristem function independently of plant height

DELLA proteins associate with transcription factors to control plant growth in response to gibberellin1 . Semi-dwarf DELLAmutants with improved harvest index and decreased lodging greatly improved global food security during the ‘green revolution’ in the 1960–1970s2 . However, DELLAmutants are pleiotropic and the developmental basis for their effects on plant architecture remains poorly understood. Here, we show that DELLA proteins have genetically separable roles in controlling stem growth and the size of the inflorescence meristem, where flowers initiate. Quantitative three-dimensional image analysis, combined with a genome-wide screen for DELLA-bound loci in the inflorescence tip, revealed that DELLAs limit meristem size in Arabidopsisby directly upregulating the cell-cycle inhibitor KRP2in the underlying rib meristem, without affecting the canonical WUSCHEL-CLAVATAmeristem size regulators3 . Mutation of KRP2in a DELLAsemi-dwarf background restored meristem size, but not stem growth, and accelerated flower production. In barley, secondary mutations in the DELLAgain-of-function mutant Sln1d4 also uncoupled meristem and inflorescence size from plant height. Our work reveals an unexpected and conserved role for DELLAgenes in controlling shoot meristem function and suggests how dissection of pleiotropic DELLA functions could unlock further yield gains in semi-dwarf mutants. During gibberellic acid signalling, DELLAs restrict the size of the shoot apical meristem by targeting the cell cycle regulator KRP2. The roles of DELLAs in the shoot apical meristem and stem growth can be genetically uncoupled in Arabidopsisand barley.