The possible role of transhexadecenoic acid and phosphatidylglycerol in light reactions of photosynthesis

Genetically-determined variations in photosynthesis indicate roles for specific fatty acid species in chilling responses

Using a population of recombinant inbred lines (RILs) cowpea (Vigna unguiculata. L. Walp), we tested for co-linkages between lipid contents and chilling responses of photosynthesis. Under low-temperature conditions (19°C/13°C, day/night), we observed co-linkages between quantitative trait loci intervals for photosynthetic light reactions and specific fatty acids, most strikingly, the thylakoid-specific fatty acid 16:1^Δ3trans found exclusively in phosphatidylglycerol (PG 16:1t). By contrast, we did not observe co-associations with bulk polyunsaturated fatty acids or high-melting-point-PG (sum of PG 16:0, PG 18:0 and PG 16:1t) previously thought to be involved in chilling sensitivity. These results suggest that in cowpea, chilling sensitivity is modulated by specific lipid interactions rather than bulk properties. We were able to recapitulate the predicted impact of PG 16:1t levels on photosynthetic responses at low temperature using mutants and transgenic Arabidopsis lines. Because PG 16:1t synthesis requires the activity of peroxiredoxin-Q, which is activated by H₂ O₂ and known to be involved in redox signalling, we hypothesise that the accumulation of PG 16:1t occurs as a result of upstream effects on photosynthesis that alter redox status and production of reactive oxygen species.

Is phosphatidylglycerol essential for terrestrial life?

Lipids are of increasing importance in understanding biological systems. Lipids carrying an anionic charge are noted in particular for their electrostatic interactions with both proteins and divalent cations. However, the biological, analytical, chemical and biophysical data of such species are rarely considered together, limiting our ability to assess the true role of such lipids in vivo. In this review, evidence from a range of studies about the lipid phosphatidylglycerol is considered. This evidence supports the conclusions that this lipid is ubiquitous in living systems and generally of low abundance but probably fundamental for terrestrial life. Possible reasons for this are discussed and further questions posed.

The role of phospholipids in regulating photosynthetic electron transport activities: Treatment of thylakoids with phospholipase C

The involvement of phospholipids in the regulation of photosynthetic electron transport activities was studied by incubating isolated pea thylakoids with phospholipase C to remove the head-group of phospholipid molecules. The treatment was effective in eliminating 40-50% of chloroplast phospholipids and resulted in a drastic decrease of photosynthetic electron transport. Measurements of whole electron transport (H2O→methylviologen) and Photosystem II activity (H2O→p-benzoquinone) demonstrated that the decrease of electron flow was due to the inactivation of Photosystem II centers. The variable part of fluorescence induction measured in the absence of electron acceptor was decreased by the progress of phospholipase C hydrolysis and part of the signal could be restored on addition of 3-(3',4'-dicholorophenyl)-1,1-dimethylurea. The B and Q bands of thermoluminescence corresponding to S2S3QB (-) and S2S3QA (-) charge recombination, respectively, was also decreased with a concomitant increase of the C band, which originated from the tyrosine D(+)QA (-) charge recombination. These results suggest that phospholipid molecules play an important role in maintaining the membrane organization and thus maintaining the electron transport activity of Photosystem II complexes.

Structural and functional consequences of galactolipids on thylakoid membrane organization

Photosynthetic membranes of higher plant chloroplasts are composed primarily of polar, but uncharged, galactolipids unlike most mammalian membranes which contain large amounts of phosphatidylcholine. It is unclear what role(s) the galactolipids play in maintaining the differentiated thylakoid membranes, or in stabilizing the photosynthetically active enzyme complexes. Some of the membrane complexes show no lipid selectivity for maintaining structural or functional integrity. Others are poisoned or dissociated in the presence of high concentrations of a trace lipid class. The efficiency of energy transfer and the reconstitution of protein complexes into liposomes are dependent on the lipid class employed. The lipids are asymmetrically arranged along and across the thylakoid membranes but not as distinctly as the proteins.

Influence of winter and summer growth conditions on leaf membrane lipids of Pisum sativum L

The lipid composition and degree of unsaturation of fatty acids has been measured for membranes isolated from leaves of Pisum sativum grown under either summer or winter conditions. Although the lipid-class composition was not significantly changed for the two growth conditions the degree of unsaturation of the fatty acids was greater for winter than summer plants. The difference in unsaturation was evident with all lipid classes of the non-thylakoid membranes including the galactolipids of the chloroplast envelope. In contrast, both the relative amounts of lipid classes and degree of saturation were not greatly changed for summer and winter thylakoids with the exception that phosphatidylglycerol had a greater linolenic acid (18:3) content for the thylakoids of winter grown leaves. However, a striking difference was found for the total acyl lipid to chlorophyll ratio for thylakoids isolated from summer or winter plants, with the former producing a lower ratio than the latter growth conditions. The above changes in lipid composition of chloroplast membranes are discussed in terms of optimizing their functional activities under the different growth conditions.

Transmembrane distribution of phospholipids and their involvement in electron transport, as revealed by phospholipids A2 treatment of spinach thylakoids

Thylakoid membranes were treated with either pancreatic or snake venom phospholipase A2, and the residual phospholipid content of these membranes was determined and compared to the rates of Photosystem II and/or Photosystem I electron transports. The hydrolysis curves of both phosphatidylglycerol and phosphatidylcholine displayed a first, rapid phase which was almost temperature-insensitive, followed by a second, slower phase which depended strongly on the temperature. When pancreatic phospholipase A2 had access either to the outer face or to both faces of the thylakoid membrane, either only part of or all the phospholipids, respectively, could be hydrolysed. These results were interpreted as indicating an asymmetric distribution of phospholipids across the thylakoid membrane, phosphatidylglycerol and phosphatidylcholine being preferentially located in the outer and the inner layer, respectively. When acting on uncoupled thylakoid membranes, phospholipase A2 exerted an inhibitory effect on Photosystem II activity and a stimulatory effect on Photosystem I activity. The involvement of phosphatidylcholine and of phosphatidylglycerol in electron transport activities of Photosystem II and of Photosystem I are discussed with special reference to the role of the external and internal pools of these phospholipids.

Role of lipids in functions of photosynthetic membranes revealed by treatment with lipolytic acyl hydrolase

1. Thylakoid membranes and subchloroplast particles I and II enriched in photosystem I and photosystem II, respectively, were treated with a potato lipolytic acyl hydrolase. 2. In the thylakoid membrane fraction, this treatment inhibited electron flows involving both photosystems and the associated photophosphorylations. However, electron flows involving either photosystem I or photosystem II were still preserved. The treatment of thylakoid membranes by lipolytic acyl hydrolase brought about a temporal convergence of different events such as maximal activity of reduced dichloroindophenol-supported electron flows, complete inhibition of photophosphorylations and electron transport activities through photosystem II + I, onset of the decay N,N,N',N-tetramethyl-rho-phenylenediamine-supported activity of photosystem activity and of the stimulation of photosystem activity (from reduced dichloroindophenol to NADP+ by exogenous plastocyanin. 3. Lipolytic acyl hydrolase catalyzed a limited hydrolysis of each lipid but in a stepwise manner, the galactolipids being attacked before the ionic lipids. The extent of the hydrolysis was not more than 50% for each lipid. Most of the hydrolytic process occurred before any significant change in photochemical activities could be observed. 4. In subchloroplast particles I, a treatment by lipolytic acyl hydrolase did not greatly affect the electron transport whilst lipid hydrolysis was almost complete. 5. In subchloroplast particles II, neither the electron flow activities nor lipid content were significantly altered by lipolytic acyl hydrolase. 6. The sites of lipolytic acyl hydrolase action appeared to be localized between plastoquinones and P700. It is suggested that it is not possible to establish a quantitative and/or temporal correlation between the extent of lipid hydrolysis and the inhibition of photochemical activities. 7. The profile of the hydrolysis of lipids in thylakoid membranes suggests that ionic lipids are less accessible to lipolytic acyl hydrolase than galactolipids.