Comparative transcriptome analysis at seven time points during Haematococcus pluvialis motile cell growth and astaxanthin accumulation

Although the molecular basis of stress-induced astaxanthin accumulation in Haematococcus pluvialis has been closely studied, the reasons for astaxanthin accumulation, rather than other carotenoids, have not been thoroughly investigated to date. With the aim of improving our understanding of the complex metabolic changes, especially astaxanthin biosynthesis, occurring during fast-growing vegetative cell proliferation, transformation of intermediate cells and formation of resting non-motile cells, seven specified developmental points with perceptible color difference were sampled and sequenced using an Illumina NextSeq500 Sequencer. Comparative transcriptome profiling was employed to identify the differentially expressed genes. RT-qPCR experiments were employed to validate some important DEGs identified by the profiling. After pairwise comparisons, 2674 DEGs were identified. Seventeen unigenes related to carotenoid biosynthesis were actively transcribed according to KEGG pathway analysis. Compared with these thirteen DEGs involved in carotenoid synthesis, four unigenes including lut1, vde, nced and crtR-3 had low expression and were not found to be differentially expressed during the whole process. This is probably one of the reasons for astaxanthin accumulation in H. pluvialis. In phase 4, the up-regulation of IcyB was accompanied by the down-regulation of IcyE, which appears to be a good strategy for promoting the accumulation of beta-carotene, rather than delta-carotene. Bkt came into focus for its most extensive change, followed by crtR-1 and crtR-2. BKT and CrtR were further confirmed to play an essential role in the stress-dependent initiation of astaxanthin synthesis. RT-qPCR results were consistent with the transcriptomic data.