Gauri S. Singhal

Changes in the Activities of Anti-Oxidant Enzymes during Exposure of Intact Wheat Leaves to Strong Visible Light at Different Temperatures in the Presence of Protein Synthesis Inhibitors.

Changes in activities of the enzymes involved in the metabolism of active oxygen species were followed in homogenates prepared from wheat leaves (Triticum aestivum L.) exposed to strong visible light (600 W m-2). The activities of superoxide dismutase (SOD), ascorbate peroxidase, and monodehydroascorbate reductase increased significantly on prolonged illumination of the leaves, indicating an increase in the rate of generation of active oxygen species. This increase was further exacerbated when high light stress was combined with low temperature (8[deg]C). Our results indicate that the increase in activities of SOD and ascorbate peroxidase involved de novo protein synthesis that was sensitive to the nuclear-directed protein synthesis inhibitor cycloheximide. The activity of catalase, on the other hand, decreased on exposure to strong light, which could be due to its photolability, particularly at lower temperatures. Ascorbate and total carotenoid contents also increased on light treatment of the leaves. The induction of the enzymes except for catalase and increase in the levels of ascorbate and total carotenoids in response to the stress conditions indicate that they play an important role in the protection of higher plants from the damaging effects of toxic active species.

Interrelationship between Salt and Light Stress on Primary Processes of Photosynthesis

Summary Influence of salt stress and its interaction with light stress on primary photosynthetic processes were investigated in wheat ( Triticum aestivum L. var. HD 2329). Salt stress by itself had no significant effect on electron transport activity and F v /F m ratio suggesting that the efficiency of the photochemistry of PSII is not affected. Decrease in F m due to salt stress may influence reduction of Q A Our results on fluorescence indicate that salt stress predisposes photoinhibition of plants and also reduces its ability to recover from photoinhibition.

Involvement of active oxygen species in photoinhibition of photosystem II: Protection of photosynthetic efficiency and inhibition of lipid peroxidation by superoxide dismutase and catalase

Abstract Superoxide dismutase and catalase were observed to prevent the loss of photosynthetic electron transport activity and the variable chlorophyll fluorescence in wheat photosystem II (PSII) preparations exposed to strong visible light at 25 and 5 °C. A simultaneous increase in the rate of lipid peroxidation in PSII membranes was also observed which was significantly inhibited in the presence of superoxide dismutase and catalase. Our results strongly suggest the generation of active species such as O 2 − , H 2 O 2 and OH · in the PSII complex during photoinhibition at ambient temperature and low temperatures. The protection of PSII functions against strong-light-induced damage by the antioxidant enzymes was, however, not complete, which might suggest the existence of an oxygen-independent mechanism operating simultaneously.

UVB-induced photodamage to phycobilisomes of Synechococcus sp. PCC 7942

The effect of UVB irradiation on the phycobilisomes (PBSs) of Synechococcus sp. PCC 7942 cells was studied. The sucrose density-gradient-isolated PBSs from in vivo UVB-treated (280-320 nm) cells showed a strong decrease in beta-phycocyanin (beta PC) and alpha-phycocyanin (alpha PC) polypeptides, in addition to a decrease in the linker polypeptides LCM 75 (linker connecting the core to the thylakoid membranes), LR 33 (linker in the rod structure), LRC 31.5 (linker connecting the rod to the core) and LRC 29. In vitro UVB treatment of gradient-isolated intact PBSs for 1 h had no effect on any of the constituent polypeptides, and only after 2 h was a degradation of LCM 75 and LR 33 and a decrease in beta PC evident. Further investigation of phycobiliproteins (4 h of UVB irradiation) using polyclonal antibody directed against purified whole PBSs revealed that, in vivo, there was a gradual decline in the levels of LCM 75, LR 33, LRC 31.5, LRC 29, beta PC and alpha PC.

Effect of phophatidylcholine and phosphatidylethanolamine enrichment on the structure and function of yeast membrane

The phospholipid composition of yeast plasma membrane was manipulated by two different methods: (i) by using two auxotrophic strains KA101 (cho1) and MC13 (Cho+) which required phospholipid bases for growth and (ii) by supplementing Saccharomyces cerevisiae (3059) cells with high concentration of choline or ethanolamine. It was possible to enrich the plasma membrane with phosphatidylcholine (PC) or phosphatidylethanolamine (PE) by both methods. The uptake of amino acids, e.g., glycine, glutamic acid, leucine, lysine methionine, phenylalanine, proline and serine, was significantly reduced in PC- or PE-enriched cells. However, the extent of reduction in transport was variable among different strains. A fluorescent probe, 1-anilino-8-naphthalene sulfonate (ANS), was used to monitor the structural changes induced by altered phospholipid composition. It was observed that the relative fluorescence intensity of bound ANS was decreased as a consequence of PC or PE enrichment. The decrease in fluorescence was probably associated with reduced number of available binding sites (n) and increased apparent dissociation constant (Kd). Furthermore, our results also suggest that a critical level of PE or PC is required for proper functioning of yeast membrane.

Effect of phosphatidylserine enrichment on amino acid transport in yeast.

A 1.5- to 3.5-fold accumulation of phosphatidylserine was observed when Candida albicans and Saccharomyces cerevisiae cells were grown in the presence of hydroxylamine, a known inhibitor of phosphatidylserine decarboxylase. However, as compared to S. cerevisiae cells, the levels of phosphatidylcholine and phosphatidylethanolamine were much lower in C. albicans cells. The enrichment of phosphatidylserine selectively affected the transport of several amino acids.

Regulation of Plastid Gene Expression by High Temperature during Light Induced Chloroplast Development

Effect of high temperature on the plastid gene expression during the light induced chloroplast development in etiolated seedlings was determined by Northern hybridisation using cloned DNA fragments of wheat chloroplast genome. Based on their response to high temperature, plastid genes were grouped into three categories: (1) plastid genes whose expression was not affected by high temperature (genes for rRNA, ribosomal proteins, tRNAs, and some genes coding for putative NADH dehydrogenase); (2) plastid genes whose expression increased at high temperature (genes coding for α-subunit of RNA polymerase and some unidentified transcripts, and (3) plastid genes whose expression decreased at high temperature (genes coding for proteins involved in photosynthetic process). Loss of a number of primary transcripts originating from operons consisting of genes that code for proteins involved in the photosynthetic process was observed. The expression of all the light inducible plastid genes was inhibited suggesting that the light inducibility property was lost at high temperature.

Ultraviolet-B Induced Changes in Ultrastructure and D1/D2 Proteins in Cyanobacteria Synechococcus sp. PCC 7942

Effects of ultraviolet-B (UV-B) irradiation on ultrastructure, total cellular protein, and PS2 proteins D1 and D2 of Synechococcus sp. PCC 7942 cells was studied. The scanning electron micrographs showed UV-B radiation induced bending of the cells. The transmission electron micrographs revealed disorganization and shift in thylakoid lamellar structure to one side of the cell. The cellular phycocyanin/chlorophyll ratio decreased with increasing UV-B treatment and due to this the colour of cells turned light-green. No apparent change in total cellular proteins was evident, but the contents of two major proteins of PS2, D1 and D2, showed decline due to UV-B irradiation, although to different extent.

Photosynthetic activity and peroxidation of thylakoid lipids during photoinhibition and high temperature treatment of isolated wheat chloroplasts

Summary Interactive effects of strong visible light and high temperature on isolated wheat ( Triticum aestivum ) chloroplasts were investigated by imposing them one after another and simultaneously. Both of the stresses were observed to impair photosystem II mediated electron transport activity. A concomitant reduction in chlorophyll fluorescence parameters and increase in thylakoid lipid peroxidation were also observed. Peroxidation of thylakoid lipids suggests the involvement of active oxygen species, which could be generated during photoinhibition. Our results demonstrate that the extent of damage to chloroplasts varied quantitatively upon reversing the order of imposition of strong light and high temperature stresses. Loss of electron transport activity, variable chlorophyll fluorescence (F v ) and increase in thylakoid lipid peroxidation were greater when photoinhibition treatment was given before heat stress. The reason for this differential damage upon changing the order of the two stresses is discussed on the basis of the, primary target site and the amount of potentially damaging active oxygen species generated under stress conditions.

Effect of Irradiance on the Thermal Stability of Thylakoid Membrane Isolated from Acclimated Wheat Leaves

Thermal stability of thylakoid membranes isolated from acclimated and non-acclimated wheat (Triticum aestivum L. cv. HD 2329) leaves under irradiation was studied. Damage to the photosynthetic electron transport activity was more pronounced in thylakoid membranes isolated from non-acclimated leaves as compared to thylakoid membrane isolated from acclimated wheat leaves at 35 °C. The loss of D1 protein was faster in non-acclimated thylakoid membrane as compared to acclimated thylakoid membranes at 35 °C. However, the effect of elevated temperature on the 33 kDa protein associated with oxygen evolving complex in these two types of thylakoid membranes was minimal. Trypsin digestion of the 33 kDa protein in the thylakoid membranes isolated from control and acclimated seedlings suggested that re-organisation of 33 kDa protein occurs before its release during high temperature treatment.