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.