Soybean MYB transcription factors GmMYB73 and GmMYB85 regulate lipid biosynthesis through lncRNA-mediated mechanisms.

Two MYB transcription factors, GmMYB73 and GmMYB85, were identified as positive regulators of soybean oil biosynthesis. They enhance seed oil content and fatty acid composition through distinct protein interaction partners. This study establishes a novel 'ncRNA-TF-oil trait' regulatory framework in soybean. MYB transcription factors (MYBs), which function as trans-acting regulatory proteins containing the conserved MYB DNA-binding domain, represent the largest family of transcription factors in plants. In this study, we investigated the regulatory role of soybean MYB transcription factors in lipid biosynthesis. By integrating existing transcriptome data from soybean pods at defined developmental stages (R3-R5 stages, days after flowering, DAF), we systematically identified two MYB transcription factors (GmMYB73 and GmMYB85). To investigate the biological functions of GmMYB73/GmMYB85, we constructed plant overexpression vectors and generated transgenic Arabidopsis thaliana lines. The transgenic seeds showed significantly increased thousand-grain weight (by 10.94 and 10.53% for GmMYB73 and GmMYB85, respectively) and elevated triglyceride content (by 11.34 and 9.07%, respectively). Conversely, we observed substantial reductions in soluble sugars (decreased by 34.44 and 16.67%) and starch content (decreased by 2.08 and 43.75%). Analysis of fatty acid composition revealed significant increases in three major unsaturated fatty acids: oleic acid (up by 127.82 and 205.95%), linoleic acid (increased by 138.92 and 187.98%), and α-linolenic acid (elevated by 125.97 and 172.64%). Through screening in the soybean young pod transcriptome database, upstream genes of GmMYB73 and GmMYB85 were identified as lncRNA44155 and lncRNA22084. Quantitative PCR confirmed their interaction in transgenic hairy roots, with lncRNA44155 and lncRNA22084 activating GmMYB73 and GmMYB85 expression respectively. Protein interaction analysis demonstrated differential regulatory mechanisms: GmMYB73 physically associates with the fatty acid biosynthetic enzyme GmFAB2, whereas GmMYB85 directly binds to GmLEC1b, the master controller of seed lipid accumulation. This study provides the first evidence that MYB transcription factors regulate soybean oil metabolism through a noncoding RNA-mediated multilevel mechanism, establishing a novel 'ncRNA-TF-oil trait' regulatory framework.