TaSG-D1-TaGAMyb signaling module regulates seed weight in wheat (Triticum aestivum L.).

Grain weight is one of the critical determinants of wheat (Triticum aestivum L.) yield, and understanding its genetic and molecular mechanisms is essential for improving crop productivity. Here, we find that TaSG-D1, which encodes an STKc-GSK3 kinase, negatively regulates grain weight. TaSG-D1 interacts with and phosphorylates TaGAMyb to reduce its degradation. Overexpression of TaGAMyb results in decreased grain length and weight, whereas its knockout increases both agronomic traits in wheat. Further investigation reveals that TaGAMyb directly activates the expression of TaCKX2.2.1, a negative regulator of grain development. Transcriptome profiling shows differential expression of TaCKX2.2.1 in 15-DAP grain of WT and TaGAMyb knockout lines, ultimately leading to a decreased concentration of active cytokinin in grains. Taken together, our findings demonstrate that TaSG-D1 negatively regulates grain development by increasing the protein abundance of transcription factor TaGAMyb, which in turn promotes the expression of downstream gene TaCKX2.2.1, a key regulator of cytokinin signaling.