Abstract
In green plants, the xanthophyll carotenoid zeaxanthin is synthesized transiently under conditions of excess light energy and participates in photoprotection. In the Arabidopsis lut2 npq2 double mutant, all xanthophylls were replaced constitutively by zeaxanthin, the only xanthophyll whose synthesis was not impaired. The relative proportions of the different chlorophyll antenna proteins were strongly affected with respect to the wild-type strain. The major antenna, LHCII, did not form trimers, and its abundance was strongly reduced as was CP26, albeit to a lesser extent. In contrast, CP29, CP24, LHCI proteins, and the PSI and PSII core complexes did not undergo major changes. PSII-LHCII supercomplexes were not detectable while the PSI-LHCI supercomplex remained unaffected. The effect of zeaxanthin accumulation on the stability of the different Lhc proteins was uneven: the LHCII proteins from lut2 npq2 had a lower melting temperature as compared with the wild-type complex while LHCI showed increased resistance to heat denaturation. Consistent with the loss of LHCII, light-state 1 to state 2 transitions were suppressed, the photochemical efficiency in limiting light was reduced and photosynthesis was saturated at higher light intensities in lut2 npq2 leaves, resulting in a photosynthetic phenotype resembling that of high light-acclimated leaves. Zeaxanthin functioned in vivo as a light-harvesting accessory pigment in lut2 npq2 chlorophyll antennae. As a whole, the in vivo data are consistent with the results obtained by using recombinant Lhc proteins reconstituted in vitro with purified zeaxanthin. While PSII photoinhibition was similar in wild type and lut2 npq2 exposed to high light at low temperature, the double mutant was much more resistant to photooxidative stress and lipid peroxidation than the wild type. The latter observation is consistent with an antioxidant and lipid protective role of zeaxanthin in vivo.
Highlights
When vascular plants or green algae are suddenly exposed to high light, the diepoxide xanthophyll violaxanthin is rapidly
The constitutive presence of zeaxanthin in the chlorophyll antennae has been reported in a number of mutants of Arabidopsis thaliana, which provide an interesting source for analyzing the effects of this carotenoid on the organization and function of the photosynthetic apparatus (14 –16)
Leaves of the Arabidopsis lut2 npq2 double mutant are completely deficient in the xanthophylls neoxanthin, violaxanthin, antheraxanthin, and lutein (Table I)
Summary
When vascular plants or green algae are suddenly exposed to high light, the diepoxide xanthophyll violaxanthin is rapidly. Upon return to light limiting conditions, zeaxanthin is epoxidized back to violaxanthin by a zeaxanthin epoxidase enzyme localized on the stromal side of the thylakoid membranes. It is well established that zeaxanthin synthesis and lumen acidification in high light act synergistically to convert PSII from a state of maximum efficiency of light harvesting to a state of high energy dissipation in the form of heat (4 – 6) The latter phenomenon is usually measured as a nonphotochemical quenching of chlorophyll fluorescence (NPQ). In the Arabidopsis aba mutants, zeaxanthin epoxidase is not functional, causing accumulation of zeaxanthin at the expense of the epoxy-xanthophylls antheraxanthin, violaxanthin, and neoxanthin [17, 18]. In contrast to the case of Lhcb proteins forming the PSII antenna system, the stability of the Lhca proteins of PSI appeared to be enhanced by zeaxanthin
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