A single-MYB transcription factor GmMYB331 regulates seed oil accumulation and seed size/weight in soybean

Seed oil accumulation is an important process affecting seed quality, and regulatory factors modulating this process remain less understood, especially in soybean. In this study, through RNA-seq and gene co-expression network analysis, we identified a single MYB (Myeloblastosis)-type transcription factor GmMYB331, which promotes seed oil accumulation in soybean seeds and enhances seed size/weight as well. Transgenic soybean plants with mild GmMYB331 overexpression showed higher total fatty acid contents in seeds and higher seed yield per plant compared to the control plants. In contrast, transgenic soybean plants with strong GmMYB331 overexpression showed only increased seed size/weight but much reduced seed yield per plant, along with an altered plant architecture. Knocking out GmMYB331 by CRISPR/Cas9 produced mutants with less total fatty acids, smaller seeds, and less seed weight, indicating that the gene is required for oil accumulation and seed size/weight control. GmMYB331 may achieve these functions by differential binding to the gene promoters and activation of the downstream genes, namely, GmOLEO1/2/4 for oil accumulation in mild overexpressing plants and GmCYCD2;2 for seed size/weight increase in strong overexpressing plants. Our study reveals a possible mechanism involving differential regulation by GmMYB331 toward oil accumulation and seed size/weight increase. Manipulation of the GmMYB331 gene may facilitate breeding for high-oil and/or -yield soybean cultivars.