On December 8, research teams led by Wei Wei and Jinsong Zhang from the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, published a study online in Journal of Integrative Plant Biology titled:
“A single-MYB transcription factor GmMYB331 regulates seed oil accumulation and seed size/weight in soybean.”
This research identifies a key single-MYB domain transcription factor, GmMYB331, which positively regulates both seed oil accumulation and seed size/weight in soybean in a unique dosage-dependent manner. The discovery provides a highly promising genetic target for developing high-yield, high-oil soybean varieties.
During the experiments, soybean plants were cultivated under illumination provided by the JIUPO LED Plant Supplemental Light Panel, supplied by Fujian Jiupo Biotechnology Co., Ltd. Growth conditions were maintained at 26 °C during the day and 22 °C at night, with a 16-hour light / 8-hour dark photoperiod.
Soybean is one of the world’s most important oilseed crops, and seed oil content directly determines its quality and economic value. Understanding the molecular mechanisms underlying oil accumulation and identifying key regulatory genes are critical for molecular breeding.
Seed oil biosynthesis is controlled by complex multi-gene networks, in which transcription factors play central roles. While R2R3-MYB transcription factors have been extensively studied, single-MYB domain transcription factors remain poorly characterized, particularly in oil metabolism regulation.
Through transcriptome sequencing and co-expression network analysis of developing high-oil and low-oil soybean seeds, followed by functional validation in Arabidopsis, the researchers identified a 331-amino-acid single-MYB domain gene, named GmMYB331.
Subcellular localization analysis showed that GmMYB331 localizes to both the nucleus and the endoplasmic reticulum. Transactivation assays confirmed that it functions as an active transcription factor.
Overexpression of GmMYB331 in Arabidopsis significantly increased seed fatty acid content and seed weight, demonstrating its positive regulatory role in oil accumulation and seed development.
Further validation in soybean revealed a clear dosage-dependent effect:
- Low-expression lines (OE-9, OE-6):
Significantly increased fatty acid content with normal plant growth. - High-expression lines:
Markedly enlarged seeds. In line OE-1, 100-seed weight increased by 50.9%, while other lines showed increases ranging from 5.3% to 14.2%. However, these lines also displayed phenotypic abnormalities, including enlarged leaf angles and thinner stems. - Knockout lines:
Showed significant reductions in both fatty acid content and 100-seed weight.
The data demonstrate a dosage effect:
Moderate expression primarily enhances fatty acid accumulation, whereas high expression more strongly promotes seed enlargement.
Notably, high-expression lines showed reduced stem cellulose content, suggesting that GmMYB331 may influence plant architecture by repressing cellulose biosynthesis.
RNA-seq analysis showed that overexpression of GmMYB331 significantly upregulated genes involved in seed oil biosynthesis and sugar metabolism.
Both in vitro and in vivo assays confirmed that GmMYB331 directly binds to promoters of key lipid biosynthesis genes:
- EMSA assays demonstrated binding to promoters of GmOLEO1, GmFatB, and GmACP4.
- ChIP-qPCR validated specific binding to the GmOLEO1-1 promoter.
In addition, GmMYB331 indirectly activates known lipid regulatory genes such as GmZF392, GmZF351, and GmNFYA, forming a multi-layered regulatory network controlling oil biosynthesis.
This study proposes a new molecular framework explaining how GmMYB331 mediates trait-specific regulation in a dosage-dependent manner:
- At moderate expression levels, GmMYB331 preferentially activates lipid biosynthesis genes, significantly increasing seed oil content.
- At high expression levels, it strongly activates cell cycle regulatory networks, promoting seed enlargement.
This differential regulatory strategy provides a new conceptual model for understanding how transcription factors dynamically balance multiple agronomic traits during crop development.