Different Roles of α- and β-Branch Xanthophylls in Photosystem Assembly and Photoprotection

Abstract

Xanthophylls (oxygenated carotenoids) are essential components of the plant photosynthetic apparatus, where they act in photosystem assembly, light harvesting, and photoprotection. Nevertheless, the specific function of individual xanthophyll species awaits complete elucidation. In this work, we analyze the photosynthetic phenotypes of two newly isolated Arabidopsis mutants in carotenoid biosynthesis containing exclusively ␣-branch (chy1chy2lut5) or ␤-branch (chy1chy2lut2) xanthophylls. Both mutants show complete lack of qE, the rapidly reversible component of nonphotochemical quenching, and high levels of photoinhibition and lipid peroxidation under photooxidative stress. Both mutants are much more photosensitive than npq1lut2, which contains high levels of viola-and neoxanthin and a higher stoichiometry of light-harvesting proteins with respect to photosystem II core complexes, suggesting that the content in light-harvesting complexes plays an important role in photoprotection. In addition, chy1chy2lut5, which has lutein as the only xanthophyll, shows unprecedented photosensitivity even in low light conditions, reduced electron transport rate, enhanced photobleaching of isolated LHCII complexes, and a selective loss of CP26 with respect to chy1chy2lut2, highlighting a specific role of ␤-branch xanthophylls in photoprotection and in qE mechanism. The stronger photosystem II photoinhibition of both mutants correlates with the higher rate of singlet oxygen production from thylakoids and isolated lightharvesting complexes, whereas carotenoid composition of photosystem II core complex was not influential. In depth analysis of the mutant phenotypes suggests that ␣-branch (lutein) and ␤-branch (zeaxanthin, violaxanthin, and neoxanthin) xanthophylls have distinct and complementary roles in antenna protein assembly and in the mechanisms of photoprotection.Carotenoids are a group of C40 terpenoid compounds with a wide distribution in several biological taxa, ranging from archaea to bacteria, fungi, algae, and higher plants. Xanthophylls form a subgroup of oxygenated carotenoids, whose importance in the oxygenic photosynthesis is well known. Xanthophylls play essential roles in higher plant photosynthesis, as components of the photosynthetic apparatus of the chloroplast. In higher plants, ␤-carotene binds to reaction center subunits of both photosystem I (PSI) 4 and II (PSII), whereas xanthophylls are both accessory pigments and structural elements of light-harvesting complexes (Lhcs). Together with ␤-carotene, they act both as chromophores, absorbing light energy that is used in photosynthetic electron transport, and as photoprotectants of the photosynthetic apparatus from excess light and from the reactive oxygen species (ROS) that are generated during oxygenic photosynthesis. A remarkable characteristic of higher plant xanthophylls is that they show very similar spectral properties in the visible region. This evidence is apparently incoherent with the high conservation of their relative abundance across a range of pla...

The Arabidopsis nox Mutant Lacking Carotene Hydroxylase Activity Reveals a Critical Role for Xanthophylls in Photosystem I Biogenesis

“…Indeed, while 1 O 2 production from purified PSII core complexes is independent from the xanthophyll content, LHC from wild-type thylakoids produce far less 1 O 2 with respect to that from xanthophyll-depleted mutants (Dall'Osto et al, 2007b), implying that LHC proteins folded in the presence of sufficient xanthophyll levels act in preventing production and/or in scavenging 1 O 2 (Dall'Osto et al, 2010). One might wonder if the lack of antenna proteins alone could explain the impaired photoautotrophic growth in nox plants.…”

“…The photoprotective role of carotenoids in photosynthesis has been widely investigated (Havaux and Niyogi, 1999;Davison et al, 2002;Baroli et al, 2003;Dall'Osto et al, 2007b). Photoprotection is performed both by carotenes Telfer, 2005) and xanthophylls (Formaggio et al, 2001).…”

“…Since both a-and b-carotene can bind to photosynthetic core complexes, it might be asked whether photosensitivity could be ascribed to the accumulation of a-carotene into PSI and photosystem II (PSII) core complexes. To address this question, 1 O 2 production (see Supplemental Figures 3A to 3C online) and photobleaching kinetics under EL (see Supplemental Figures 3D to 3F online) were measured on PSII and PSI complexes isolated from either wild-type and lut5 leaves, which accumulate b-carotene and a-carotene, respectively (Dall'Osto et al, 2007b). b-Carotene substitution in lut5 did not affect 1 O 2 yield or photobleaching kinetics when compared with complexes isolated from the wild type at all light intensities tested.…”

“…Indeed, lack of lutein and/or Zea decreases the capacity for photoprotection in EL (Niyogi et al, 2001), while lack of Neo increases sensitivity to superoxide anions, formed in the Mehler's reaction (Dall'Osto et al, 2007a). Depletion of Viola, as in the chy1 chy2 lut5 mutant, caused extreme photosensitivity (Dall'Osto et al, 2007b). Lutein bound to site L1 of all LHC proteins is essential for protein folding and for chlorophyll triplet quenching (Croce et al, 1999;Formaggio et al, 2001).…”

“…Recent studies on carotenoid biosynthetic mutants of Arabidopsis thaliana have improved our understanding of the biosynthetic pathway (Pogson et al, 1998;Fiore et al, 2006;Kim and DellaPenna, 2006;North et al, 2007;Dall'Osto et al, 2007b;Kim et al, 2009) (see Supplemental Figure 1 online). b-Carotene, the precursor of b,b-xanthophylls, is formed from lycopene by the symmetrical action of b-cyclase, while the b,e-carotenoid a-carotene is formed by the combined action of both b-cyclase and e-cyclase on lycopene (DellaPenna and , thus making a bifurcation in the pathway between the b,band b,e-xanthophyll branches.…”