Membrane glycerolipid remodeling triggered by nitrogen and phosphorus starvation in Phaeodactylum tricornutum.

Diatoms constitute a major phylum of phytoplankton biodiversity in ocean water and freshwater ecosystems. They are known to respond to some chemical variations of the environment by the accumulation of triacylglycerol, but the relative changes occurring in membrane glycerolipids have not yet been studied. Our goal was first to define a reference for the glycerolipidome of the marine model diatom Phaeodactylum tricornutum, a necessary prerequisite to characterize and dissect the lipid metabolic routes that are orchestrated and regulated to build up each subcellular membrane compartment. By combining multiple analytical techniques, we determined the glycerolipid profile of P. tricornutum grown with various levels of nitrogen or phosphorus supplies. In different P. tricornutum accessions collected worldwide, a deprivation of either nutrient triggered an accumulation of triacylglycerol, but with different time scales and magnitudes. We investigated in depth the effect of nutrient starvation on the Pt1 strain (Culture Collection of Algae and Protozoa no. 1055/3). Nitrogen deprivation was the more severe stress, triggering thylakoid senescence and growth arrest. By contrast, phosphorus deprivation induced a stepwise adaptive response. The time scale of the glycerolipidome changes and the comparison with large-scale transcriptome studies were consistent with an exhaustion of unknown primary phosphorus-storage molecules (possibly polyphosphate) and a transcriptional control of some genes coding for specific lipid synthesis enzymes. We propose that phospholipids are secondary phosphorus-storage molecules broken down upon phosphorus deprivation, while nonphosphorus lipids are synthesized consistently with a phosphatidylglycerol-to-sulfolipid and a phosphatidycholine-to-betaine lipid replacement followed by a late accumulation of triacylglycerol.

Siderin from Toona ciliata (Meliaceae) as photosystem II inhibitor on spinach thylakoids.

Four natural products were isolated from plants of the Rutaceae and Meliaceae families and their effect on photosynthesis was tested. Siderin (1) inhibited both ATP synthesis and electron flow (basal, phosphorylating, and uncoupled) from water to methylviologen (MV); therefore, it acts as Hill reaction inhibitor in freshly lysed spinach thylakoids. Natural products 2-4 were inactive. Secondary metabolite 1 did not inhibit PSI electron transport. It inhibits partial reactions of PSII electron flow from water to 2,6-dichlorophenol indophenol (DCPIP), from water to sodium silicomolybdate, and partially inhibits electron flow from diphenylcarbazid (DPC) to DCPIP. These results established that the site of inhibition of 1 was at the donor and acceptor sides of PSII, between P(680) and Q(A). Chlorophyll a fluorescence measurements confirmed the behavior of the Toona ciliate coumarin 1 as P(680) to Q(A) inhibitor by the creation of silent centers. May be this is the mechanisms of action of 1 and is the way in which it develops a phytotoxic activity against photosynthesis.

Decreased Hill reaction rates and slow turnover of transitory starch in the obligate shade plant Panax quinquefolius L. (American ginseng).

To identify physiological processes that might limit photosynthesis in Panax quinquefolius L. (American ginseng) a comparison has been made with Panax ginseng C.A. Meyer (Korean ginseng), Pisum sativum L. (pea) and Spinacia oleracea L. (spinach). The quantum yield of oxygen evolution in intact leaves and isolated thylakoid membranes was found to be smaller in ginseng than in pea or spinach. However, the number of photosystem II (PSII) centers on a chlorophyll basis was found to be similar in all species. This suggests that ginseng thylakoid membranes possess relatively more inactive PSII centers than thylakoids of pea and spinach when grown under similar conditions. Unexpectedly, whole-chain electron transport from water to methyl viologen, and partial photosystem I reactions, demonstrated that electron transport rates to methyl viologen were anomalously low in P. quinquefolius and P. ginseng. Additionally, at elevated light intensities, intact leaves of P. quinquefolius were more susceptible to lipid peroxidation than pea leaves. In plants grown at a light intensity of 80 micro mol photons m(-2) s(-1) the levels of fructose and starch were higher in both ginseng species than in pea or spinach. Significantly, the level of starch in P. quinquefolius was relatively constant throughout the entire 12 h/12 h light/dark cycle and remained high after an extended dark time of 48 h. In addition, P. quinquefolius had lower activities of alpha-amylase and beta-amylase than P. ginseng, pea and Arabidopsis thaliana (L.) Heynh. The significance of the elevated levels of leaf starch in P. quinquefolius remains to be determined. However, the susceptibility of P. quinquefolius to photoinhibition may arise as a consequence of a reduced fraction of active PSII centers. This may result in the normal dissipative mechanisms in these plants becoming saturated at elevated, but moderate, light intensities.

Valinomycin sensitivity proves that light-induced thylakoid voltages result in millisecond phase of chlorophyll fluorescence transients.

Upon sudden exposure of plants to an actinic light of saturating intensity, the yield of chlorophyll fluorescence increases typically by 200-400% of the initial O-level. At least three distinct phases of these O-J-I-P transients can be resolved: O-J (0.05-5 ms), J-I (5-50 ms), and I-P (50-1000 ms). In thylakoid membranes, the J-I increase accounts for approximately 30% of the total fluorescence increase; in Photosystem II membranes, the J-I phase is always lacking. In the presence of the ionophore valinomycin, which is known to inhibit specifically the formation of membrane voltages, the magnitude of the J-I phase is clearly diminished; in the presence of valinomycin supplemented by potassium, the J-I phase is fully suppressed. We conclude that the light-driven formation of the thylakoid-membrane voltage results in an increase of the chlorophyll excited-state lifetime, a phenomenon explainable by the electric-field-induced shift of the free-energy level of the primary radical pair [Dau and Sauer, Biochim. Biophys. Acta 1102 (1992) 91]. The assignment of the J-I increase in the fluorescence yield enhances the potential of using O-J-I-P fluorescence transients for investigations on photosynthesis in intact organisms. A putative role of thylakoid voltages in protection of PSII against photoinhibitory damage is discussed.