Dopamine triggers TOR signaling cascade to drive biomass and astaxanthin hyperaccumulation in Haematococcus lacustris under coordinated salinity and gradient light stress.

Exogenous dopamine (DA) can enhance astaxanthin biosynthesis in stress-induced Haematococcus, although the mechanisms behind this effect are not well understood. This study examined how DA influences biomass and astaxanthin production in H. lacustris when exposed to coordinated salinity and gradient light stress. The application of 25 μM DA resulted in a significant increase in biomass (1.86 g L-1), astaxanthin content (32.69 mg g-1), astaxanthin productivity (4.66 mg L-1 d-1), and cell diameter (30.49 μm), with enhancements of 26%, 44%, 81%, and 29%, respectively, compared to the control group. RNA-seq analysis showed that DA treatment upregulated genes associated with the Calvin cycle, carotenogenesis, Tricarboxylic acid cycle, nitrogen assimilation, and secondary cell wall biosynthesis. Notably, DA treatment activated the TOR pathway through the upregulation of TORs. The addition of AZD8055, a TOR inhibitor, hindered astaxanthin accumulation and secondary cell wall biosynthesis, even in cultures treated with DA. The AZD8055 group exhibited a 42% reduction in astaxanthin content and a 56% decrease in productivity compared to those treated with DA. Furthermore, DA treatment significantly lowered oxidative stress markers, reducing cellular damage, while AZD8055 promoted oxidative stress. DA upregulated the astaxanthin biosynthesis genes but downregulated the autophagy-related gene ATG8, whereas AZD8055 treatment produced the opposite effects on these genes. These findings indicate a promising and effective biotechnological strategy that combines DA, gradient light, and salinity stress to boost biomass and astaxanthin production, and further suggest that the promotive effects of DA on astaxanthin accumulation and stress resistance are associated with TOR-related signaling.