Minisymposium 10: Photosynthesis: Light Adaptation

Abs # 22001: Zeaxanthin and the reversible xanthophyll cycle are components of the photosystem II damage and repair cycle in Dunaliella salina

Presenter:	Melis, Anastasios , melis@nature.berkeley.edu
Authors	Melis, Anastasios  (A)   Yokthongwattana, Kittisak  (A)   Jin, EonSeon  (A)   Polle, Juergen EW (A)
Affiliations:	(A): University of California, Berkeley

The photosynthetic apparatus organization and function was investigated in Dunaliella salina (green alga) wild type and a mutant (zea1), which was devoid of all b,b-epoxycarotenoids that are derived from the zeaxanthin epoxidation reaction. The zea1 mutant lacked antheraxanthin, violaxanthin and neoxanthin from its thylakoid membranes but constitutively accumulated zeaxanthin instead. The zea1 was also devoid of the so-called xanthophyll cycle, which, upon irradiance stress, reversibly converts violaxanthin to zeaxanthin via a de-epoxidation reaction. The zeaxanthin content was about 0.3x10^-16 mol Z/cell (Z/Chl=0.012/1) in the wild type and 6.9x10^-16 mol Z/cell (Z/Chl=0.24/1) in the zea1 mutant. In spite of the pronounced difference observed in the composition of b,b-epoxycarotenoids and amount of zeaxanthin between wild type and zea1, no discernible difference could be observed between the two strains in terms of photosynthesis and growth, organization of the photosynthetic apparatus, photoacclimation potential, sensitivity to photodamage, or recovery from photoinhibition. Wild type and zea1 were probed for the above parameters over a broad range of growth irradiance and upon light shift experiments (low-light to high-light shift and vice versa). A constitutive accumulation of zeaxanthin in the zea1 strain did not affect the acclimation of the photosynthetic apparatus to irradiance, as evidenced by indistinguishable irradiance-dependent adjustments in the chlorophyll antenna size and photosystem content of wild type and zea1. Additionally, a constitutive accumulation of zeaxanthin in the zea1 did not affect rates of photodamage or the recovery of the photosynthetic apparatus from photoinhibition. However, zeaxanthin in the wild type accumulated in parallel with the accumulation of photodamaged PSII centers in the chloroplast thylakoids and decayed in tandem with a chloroplast recovery from photoinhibition. These results suggested a role for zeaxanthin in the protection of photodamaged and disassembled PSII reaction centers, apparently needed while PSII is in the process of degradation and replacement of the D1/32 kD reaction center protein. Thylakoid membrane fractionation analyses revealed different localization domains for zeaxanthin under physiological and photoinhibition conditions. It is concluded that zeaxanthin and the reversible xanthophyll cycle are dynamically regulated components of the PSII damage and repair cycle in Dunaliella salina.