Monogalactosyldiacylglycerol Deficiency in Arabidopsis Affects Pigment Composition in the Prolamellar Body and Impairs Thylakoid Membrane Energization and Photoprotection in Leaves  

Abstract

Monogalactosyldiacylglycerol (MGDG) is the major lipid constituent of chloroplast membranes and has been proposed to act directly in several important plastidic processes, particularly during photosynthesis. In this study, the effect of MGDG deficiency, as observed in the monogalactosyldiacylglycerol synthase1-1 (mgd1-1) mutant, on chloroplast protein targeting, phototransformation of pigments, and photosynthetic light reactions was analyzed. The targeting of plastid proteins into or across the envelope, or into the thylakoid membrane, was not different from wild-type in the mgd1 mutant, suggesting that the residual amount of MGDG in mgd1 was sufficient to maintain functional targeting mechanisms. In dark-grown plants, the ratio of bound protochlorophyllide (Pchlide, F656) to free Pchlide (F631) was increased in mgd1 compared to the wild type. Increased levels of the photoconvertible pigment-protein complex (F656), which is photoprotective and suppresses photooxidative damage caused by an excess of free Pchlide, may be an adaptive response to the mgd1 mutation. Leaves of mgd1 suffered from a massively impaired capacity for thermal dissipation of excess light due to an inefficient operation of the xanthophyll cycle; the mutant contained less zeaxanthin and more violaxanthin than wild type after 60 min of high-light exposure and suffered from increased photosystem II photoinhibition. This is attributable to an increased conductivity of the thylakoid membrane at high light intensities, so that the proton motive force is reduced and the thylakoid lumen is less acidic than in wild type. Thus, the pH-dependent activation of the violaxanthin de-epoxidase and of the PsbS protein is impaired.

The galactolipid monogalactosyldiacylglycerol (MGDG) contributes to photosynthesis-related processes inArabidopsis thaliana

“…21,22 After excitation at 440 nm, mgd1-1 mutants display distinctly higher fluorescence emission peaks corresponding to photoactive Pchlide than wild type counterparts and (hence) higher photoactive:non-photoactive Pchlide ratios. 13 These changes may be photoprotective responses that favour formation of photoactive Pchlide and optimize the plants' opportunities to use light for chlorophyll production, enabling the transformation of etioplasts into chloroplasts.Interestingly,the xanthophyll cycle, another photoprotective mechanism, is impaired in mgd1-1. 13 Normally, the xanthophyll cycle pigment violaxanthin is de-epoxidized into antheraxanthin, and then into zeaxanthin, by the enzyme VDE (Fig.…”

“…13 These changes may be photoprotective responses that favour formation of photoactive Pchlide and optimize the plants' opportunities to use light for chlorophyll production, enabling the transformation of etioplasts into chloroplasts.…”

“…11,12 The data recently collected for mgd1-1 do not support MGDG's involvement in protein targeting, since (inter alia) the level of MGDG in mgd1-1 mutants is clearly sufficient for efficient targeting. 13 Further, the galactolipid associated with the TOC complex 12 is digalactosyldiacylglycerol (DGDG) and the digalactosyldiacylglycerol synthase 1 (dgd1) mutant, 14 which has ~10% of wild-type levels of DGDG, has impaired import efficiency. 15,16 Hence, this may indicate that DGDG is relatively more important for chloroplast import than MGDG.…”

“…13 Normally, the xanthophyll cycle pigment violaxanthin is de-epoxidized into antheraxanthin, and then into zeaxanthin, by the enzyme VDE (Fig. 1), which is dependent on MGDG.…”

“…[24][25][26] Thus, since mgd1-1 mutants have reduced total amounts of xanthophyll and chlorophyll pigments, but increased chlorophyll a/b ratios, their photosynthesis capacity is unsurprisingly reduced, even though the organization of their electron transport chains is not strongly affected by the MGDG deficiency. 13 During short-term high light stress, antheraxanthin and zeaxanthin are thought to facilitate dissipation of excess light energy in the PSII antenna bed by non-photochemical quenching. 27,28 Upon high light stress the pH decreases, triggering photoprotective mechanisms via changes in the PSII antenna system.…”