Overaccumulation of tocopherol protects a Chamydomonas xanthophyll-deficient mutant from photo-oxidative stress

Zhirong Li1,2, Jay D. Keasling2,3, and Krishna K. Niyogi1,2

1) Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102, USA 2) Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 3) Department of Chemical Engineering, University of California, Berkeley, CA 94720, USA

Tocopherols (vitamin E) and carotenoids are the two most abundant groups of lipid-soluble antioxidants in the chloroplast. Carotenoids are well known for their roles in protecting against photo-oxidative stress, whereas the photoprotective functions of tocopherols have only recently been examined experimentally. In addition, little is known about the functional overlap of carotenoids and tocopherols in vivo. To investigate this possible overlap, Chlamydomonas reinhardtii strains were engineered to overproduce tocopherols by chloroplast transformation with non-codon-optimized and codon-optimized versions of homogentisate phytyltransferase (VTE2) from Synechocystis, and by nuclear transformation with VTE2 from C. reinhardtii, which resulted in a 1.6-fold, a 5- to10-fold, and more than 10-fold increase in total tocopherol content, respectively. To test if tocopherol overproduction can compensate for carotenoid deficiency in terms of antioxidant function, the VTE2 from C. reinhardtii was overexpressed in the npq1 lor1 double mutant by nuclear transformation to generate npq1 lor1 strains in which tocopherols were overaccumulated. Following transfer to high light, the new strains showed resistance for up to 2 days and higher efficiency of photosystem II. The new strains were also much more resistant to other oxidative stress. These results suggest overlapping function of tocopherols and carotenoids in protection against photo-oxidative stress.

e-mail address of presenting author: zhirong@berkeley.edu