The TaBBM-TaWRKY41-TacAGPS1a module affects starch synthesis and grain quality by regulating embryo and endosperm development in wheat

The BABY BOOM (BBM) AP2/ERF transcription factor is a key regulator of plant cell totipotency and embryogenesis, yet its function during zygotic development in monocots remains poorly defined. Here, using single-cell and spatial transcriptomic analyses in wheat, we show that BBM is specifically expressed in the embryo and in adjacent endosperm domains, where it coordinately regulates embryo and endosperm development. Upstream, the AT-hook transcription factor TaAHL10-B1 controls TaBBM-B1 expression, whereas downstream TaBBM-B1 directly associates with TaWRKY41-B1 to regulate TacAGPS1a-A1, which encodes a core enzyme in starch biosynthesis, thereby influencing grain nutrient accumulation and size. Natural polymorphisms in both promoter and coding regions fine-tune these regulatory interactions. Introgression of the large-grain TaBBM-B1 haplotype into small-grain wheat varieties significantly increased grain size. Together, our findings define a TaBBM-B1–TaWRKY41-B1–TacAGPS1a-A1 regulatory module that integrates embryonic growth with endosperm nutrient allocation to determine grain size, revealing an unexpected role for BBM in monocots and providing a molecular framework for improving wheat yield.