Monogalactosyldiacylglycerol deficiency in tobacco inhibits the cytochrome b6f-mediated intersystem electron transport process and affects the photostability of the photosystem II apparatus

Abstract

Monogalactosyldiacylglycerol (MGDG) is the most abundant lipid component of the thylakoid membrane. Although MGDG is believed to be important in sustaining the structure and function of the photosynthetic membrane, its exact role in photosynthesis in vivo requires further investigation. In this study, the transgenic tobacco plant M18, which has an MGDG deficiency of approximately 53%, and which contains many fewer thylakoid membranes and exhibits retarded growth and a chlorotic phenotype, was used to investigate the role of MGDG. Chlorophyll fluorescence analysis of the M18 line revealed that PSII activity was inhibited when the plants were exposed to light. The inactive linear electron transport found in M18 plants was mainly attributed to a block in the intersystem electron transport process that was revealed by P700 redox kinetics and PSI light response analysis. Immunoblotting and Blue Native SDS-PAGE analysis suggested that a reduction in the accumulation of cytochrome b6f in M18 plants is a direct structural effect of MGDG deficiency, and this is likely to be responsible for the inefficiency observed in intersystem electron transport. Although drastic impairments of PSII subunits were detected in M18 plants grown under normal conditions, further investigations of low-light-grown M18 plants indicated that the impairments are not direct structural effects. Instead, they are likely to result from the cumulative photodamage that occurs due to impaired photostability under long-term exposure to relatively high light levels. The study suggests that MGDG plays important roles in maintaining both the linear electron transport process and the photostability of the PSII apparatus.

Inducible Knockdown of MONOGALACTOSYLDIACYLGLYCEROL SYNTHASE1 Reveals Roles of Galactolipids in Organelle Differentiation in Arabidopsis Cotyledons

“…In green seedlings, the proportion of DGDG did not change, and thus the MGDG-DGDG ratio decreased. These findings agree with the previous reports on the Arabidopsis mgd1-1 mutant and the tobacco M18 line showing that the decrease in MGD1 activity primarily results in a loss of MGDG (Jarvis et al, 2000;Wu et al, 2013). A possible explanation for the decrease in MGDG-DGDG ratio by partial defects of the MGD1 function is that the decreased MGD1 activity results in an overbalance of DGDG biosynthesis relative to MGDG biosynthesis and consequent consumption of MGDG in the formation of DGDG, thus leading to a reduced distribution of MGDG to membrane bilayers.…”

“…We examined the effect of MGD1 suppression on chloroplast development in green and white cotyledons from DEX-treated L4 seedlings by transmission electron microscopy. Consistent with previous reports (Jarvis et al, 2000;Wu et al, 2013), chloroplasts in green cotyledons, with slightly reduced MGDG content (Fig. 2D), appeared a little less mature than those from DEX-untreated controls ( et al, 2007) and more severe than in the mgd1-1 knockdown mutant (Jarvis et al, 2000), agrees with their phenotype in galactolipid content, which indicates a direct impact of the MGD1-derived MGDG synthesis on the development of thylakoid membrane networks.…”

“…These defects result in impaired thylakoid membrane energization and decreased photoprotective capacity (Aronsson et al, 2008). Similar results were recently reported in an MGD1 knockdown mutant (M18) in tobacco (Nicotiana tabacum), which further revealed an involvement of MGD1-mediated MGDG biosynthesis in photosynthetic electron transport between PSII and PSI (Wu et al, 2013). However, the mgd1-2 knockout mutant accumulated only negligible amounts of both MGDG and DGDG, thus resulting in the absence of thylakoid membranes in leaf plastids and the complete dysfunction of photosynthetic activities (Kobayashi et al, 2007).…”

“…A partial loss of MGDG (approximately 50% of wildtype levels) results in decreased intersystem electron transport between PSII and PSI (Wu et al, 2013) and increased conductivity of thylakoid membranes (Aronsson et al, 2008), whereas the absence of MGDG (approximately 5% of wild-type levels) causes severe disorder in photosystem complexes and complete deficiency of photosynthetic electron transport (Kobayashi et al, 2013). Because the amiR-MGD1 L4 seedlings showed wide variation in MGDG content, from 15% to 75% of wild-type levels with DEX treatment (Fig.…”

“…Partial deficiency of MGDG by MGD1 knockdown leads to decreased amounts of thylakoid membranes with altered architecture (Jarvis et al, 2000;Wu et al, 2013), whereas crucial lack of both galactolipids by MGD1 knockout results in no or severely underdeveloped internal membrane structures in leaf plastids (Kobayashi et al, 2007). We examined the effect of MGD1 suppression on chloroplast development in green and white cotyledons from DEX-treated L4 seedlings by transmission electron microscopy.…”