Light boosts cell proliferation and astaxanthin accumulation in nitrogen-starved Chromochloris zofingiensis via TOR...

Light is a crucial regulatory factor for astaxanthin biosynthesis in microalgae under non-stress and abiotic stresses. However, its physiological impacts, molecular mechanisms and signaling pathway remain unclear. The present study showed that light could significantly promote the cell proliferation, nitrogen redistribution and astaxanthin accumulation via Target of Rapamycin (TOR) signaling pathway under nitrogen starvation condition. Compared with the dark condition, the cell density, protein content, astaxanthin content and TOR activity increased by 22 %, 100 %, 136 % and 335 % under 200 μmol m² s^-1 light intensity. But the above induction effects were significantly impaired by the inhibition of the TOR signaling pathway. Interestingly, the level of reactive oxygen species (ROS) was not positive regulator in light-induced astaxanthin accumulation, as it was decreased by light under nitrogen starvation condition. Comparative transcriptome analysis revealed that TOR-mediated light exposure upregulated the expression of key genes involved in energy production pathways, as well as carotenoid biosynthesis. Weighted gene co-expression network analysis identified genes such as MYB3R and bZIP as potential key regulatory genes downstream of TOR, contributing to high light-induced cell proliferation and carotenoid production. The whole-genome DNA methylation analysis suggested that TOR was involved in the suppression of global DNA methylation under high light, potentially facilitating gene expression. This study emphasized the regulatory mechanisms of TOR mediated light-induced astaxanthin accumulation, providing theoretical basis and induction strategy for astaxanthin production.