Isolation and expression analyses of methyl-d-erythritol 4-phosphate (MEP) pathway genes from Haematococcus pluvialis

Haematococcus pluvialis is a green alga known to accumulate the keto-carotenoid astaxanthin under stress conditions. In H. pluvialis, carotenoids are derived from isopentenyl diphosphate (IPP), which is synthesized via the non-mevalonate methyl-D-erythritol 4-phosphate (MEP) pathway. The present study revealed that several treatments caused changes in pigment profiles and the expression levels of genes encoding MEP pathway enzymes. Additionally, photosynthesis fluorescence was monitored. Generally, under stress conditions, there was an increase in astaxanthin, along with a decrease in total chlorophyll and photo capacity. Six IPP biosynthetic genes were cloned from H. pluvialis. Expression analysis revealed that these transcripts were upregulated under stress culture conditions. However, the extent of MEP pathway gene expression varied with the stress conditions. 4Diphosphocytidyl- 2-C-methyl-D-erythritol (CDP-ME) synthase (CMS) and CDP-ME kinase (CMK) exhibited significantly higher transcriptional expression under nitrogen starvation treatments. While 1-deoxy-D-xylulose 5-phosphate (DXP) synthase (DXS), CMS, CMK 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate (HMBPP) synthase (HDS), and HMBPP reductoisomerase (HDR) showed significant upregulation on the second day under nitrogen starvation and high light (HL-N). The enhanced expression of these genes was also observed on the third day under high light. The high expression of MEP pathway genes was correlated with the accumulation of astaxanthin under HL-N stress. This is the first report of the isolation of IPP biosynthetic genes and their differential expression in H. pluvialis under stress conditions. The present study revealed the influence of stress conditions on the expression of MEP pathway genes and changes in pigment profiles.