The HMG-box protein PtHMG1 modulates lipid accumulation and fatty acid composition in the diatom Phaeodactylum tricornutum

Introduction The diatom Phaeodactylum tricornutum is a premier biological platform for producing high-value polyunsaturated fatty acids (PUFAs). However, the transcriptional mechanisms coordinating lipid accumulation and fatty acid composition remain poorly understood. Methods In this study, we characterized the role of the HMG-box transcriptional regulator PtHMG1 in P. tricornutum through overexpression (OE) and CRISPR/Cas9-mediated knockout (KO). Results Our results identify PtHMG1 as a global regulator of lipid metabolism. PtHMG1-OE strains exhibited a 90% increase in total fatty acids and a rise in total lipids from 0.3 g g -1 to 0.5 g g -1 without significant growth inhibition. Although OE upregulated key front-end desaturases (PTD5a, PTD5b, and PTD6) and increased absolute eicosapentaenoic acid (EPA) content by 18%, the relative proportion of PUFAs declined due to the disproportionate accumulation of saturated and monounsaturated fatty acids. Conversely, PtHMG1-KO led to an 11% decrease in total lipids and significant growth defects. While KO strains showed downregulation of EPA-specific desaturases, a compensatory upregulation of alternative desaturases maintained a stable relative fatty acid profile. Conclusion These findings demonstrate that PtHMG1 plays a crucial role in regulating total lipid accumulation and modifying fatty acid composition in P. tricornutum . The decoupling of lipid enhancement from growth inhibition underscores the significant potential of utilizing PtHMG1 for targeted genetic modification, facilitating the sustainable development of high-yield microalgal oils.