Second stage of -carotene biosynthesis under high light in Dunaliella salina is regulated by NADPH oxidase homologs

Dunaliella salina has received much attention as a cell factory for the industrial production of the well-known antioxidant beta-carotene, but basic studies on the genetic background of the ROS signaling that mediates the biosynthesis and massive accumulation of beta-carotene are still lacking. In this study, we show that the second stage of beta-carotene accumulation in D. salina is mediated by ROS production by NADPH oxidase homologs. Photosystem II quickly lost its function when exposed to high light (HL), and almost no photosynthesis occurred even after 24 h. Interestingly, when ROS production was blocked in PSII, accumulated beta-carotene content was less than the control by 24 h. On the other hand, inhibition of ROS production by NADPH oxidase was a similar accumulation of beta-carotene until 24 h, but lower than the control after 24 h. We identified two NADPH oxidase homologs in D. salina and used quantitative PCR to show that the expression of these genes was closely linked to beta-carotene biosynthesis in the second stage. These results suggest that NADPH oxidase maintains beta-carotene synthesis by generating ROS signals instead of the photosynthetic system which lost function due to HL stress.