Absorption and Transfer of Light and Photoreduction Activities of Spinach Chloroplasts under Calcium Deficiency: Promotion by Cerium

Chloroplasts were isolated from spinach cultured in calcium-deficient, cerium-chloride-administered calcium-present Hoagland’s media or that of calcium-deficient Hoagland’s media and demonstrated the effects of cerium on distribution of light energy between photosystems II and I and photochemical activities of spinach chloroplast grown in calcium-deficient media. It was observed that calcium deprivation significantly inhibited light absorption, energy transfer from LHCII to photosystemII, excitation energy distribution from PSI to PSII, and transformation from light energy to electron energy and oxygen evolution of chloroplasts. However, cerium treatment to calcium-deficient chloroplasts could obviously improve light absorption and excitation energy distribution from photosystem I to photosystem II and increase activity of whole chain electron transport, photosystems II and I DCPIP photoreduction, and oxygen evolution of chloroplasts. The results suggested that cerium under calcium deficiency condition could substitute for calcium in chloroplasts, maintain the stability of chloroplast membrane, and improve photosynthesis of spinach chloroplast, but the mechanisms still need further study.     

Light regulated translational activators: identification of chloroplast gene specific mRNA binding proteins.

Genetic analysis has revealed a set of nuclear-encoded factors that regulate chloroplast mRNA translation by interacting with the 5' leaders of chloroplastic mRNAs. We have identified and isolated proteins that bind specifically to the 5' leader of the chloroplastic psbA mRNA, encoding the photosystem II reaction center protein D1. Binding of these proteins protects a 36 base RNA fragment containing a stem-loop located upstream of the ribosome binding site. Binding of these proteins to the psbA mRNA correlates with the level of translation of psbA mRNA observed in light- and dark-grown wild type cells and in a mutant that lacks D1 synthesis in the dark. The accumulation of at least one of these psbA mRNA-binding proteins is dependent upon chloroplast development, while its mRNA-binding activity appears to be light modulated in developed chloroplasts. These nuclear encoded proteins are prime candidates for regulators of chloroplast protein synthesis and may play an important role in coordinating nuclear-chloroplast gene expression as well as provide a mechanism for regulating chloroplast gene expression during development in higher plants.     

Fluorescence and Delayed Light Emission from Mesophyll and Bundle Sheath Cells in Leaves of Normal and Salt-Treated Panicum miliaceum

A modified fluorescence microscope system was used to measure chlorophyll fluorescence and delayed light emission from mesophyll and bundle sheath cells in situ in fresh-cut sections from leaves of Panicum miliaceum L. The fluorescence rise in 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU)-treated leaves and the slow fluorescence kinetics in untreated leaves show that mesophyll chloroplasts have larger photosystem II unit sizes than do bundle sheath chloroplasts. The larger photosystem II units imply more efficient noncyclic electron transport in mesophyll chloroplasts. Quenching of slow fluorescence also differs between the cell types with mesophyll chloroplasts showing complex kinetics and bundle sheath chloroplasts showing a relatively simple decline. Properties of the photosynthetic system were also investigated in leaves from plants grown in soil containing elevated NaCl levels. As judged by changes in both fluorescence kinetics in DCMU-treated leaves and delayed light emission in leaves not exposed to DCMU, salinity altered photosystem II in bundle sheath cells but not in mesophyll cells. This result may indicate different ionic distributions in the two cell types or, alternatively, different responses of the two chloroplast types to environmental change.     

Alteration in the acceptor side of photosystem II of chloroplast by high light

The effect of high light on the acceptor side of photosystem II of chloroplasts and core particles of spinach was studied. BothV _max and apparentK _m for DCIP were altered in photoinhibited photosystem II core particles. The double reciprocal plot analysis as a function of actinic light showed increased slope in chloroplasts photoinhibited in the presence of DCMU. Exposure of chloroplasts to high light in the presence of DCMU did not protect the chloroplast against high light induced decrease in F_m, level. Further the high light stress induced decrease inF _m level was not restored by the addition of DCMU. These results suggest that the high light stress induced damage to chloroplast involves alteration in the binding site forQ _B on the DI protein on the acceptor side of photosystem II     

Chemical modification studies of chloroplast membranes. Water oxidation inhibition by diazonium-benzenesulfonic acid

The water-soluble chemical modifier, diazonium benzene-sulfonic acid, significantly inhibited photosystem II-dependent water oxidation (oxygen evolution) when the compound was reacted with chloroplast membranes in the light but not in the dark. The photochemistry of photosystem II was not affected by the diazonium treatment, shown by complete restoration of photosystem II-dependent electron flow from the alternate electron donor diphenylcarbazide.Paralleling the inhibition of oxygen evolution the illuminated chloroplasts bound significantly more diazonium reagent than did chloroplasts treated in the dark. Both the inhibition of oxygen evolution and the increased binding of the diazonium to the membranes were dependent on photosystem II electron flux, which could not be replaced by photosystem I cyclic electron flow. A dark base to acid or acid to base transition resulted in a similar inhibition of water oxidation and increased diazonium binding.The results suggest a membrane conformational change associated with photosystem II electron flow that exposes otherwise buried diazo reactive groups at the external grana membrane surface.     

Deletion of FtsH11 protease has impact on chloroplast structure and function in Arabidopsis thaliana when grown under continuous light

The membrane‐integrated metalloprotease FtsH11 of Arabidopsis thaliana is proposed to be dual‐targeted to mitochondria and chloroplasts. A bleached phenotype was observed in ftsh11 grown at long days or continuous light, pointing to disturbances in the chloroplast. Within the chloroplast, FtsH11 was found to be located exclusively in the envelope. Two chloroplast‐located proteins of unknown function (Tic22‐like protein and YGGT‐A) showed significantly higher abundance in envelope membranes and intact chloroplasts of ftsh11 and therefore qualify as potential substrates for the FtsH11 protease. No proteomic changes were observed in the mitochondria of 6‐week‐old ftsh11 compared with wild type, and FtsH11 was not immunodetected in these organelles. The abundance of plastidic proteins, especially of photosynthetic proteins, was altered even during standard growth conditions in total leaves of ftsh11. At continuous light, the amount of photosystem I decreased relative to photosystem II, accompanied by a drastic change of the chloroplast morphology and a drop of non‐photochemical quenching. FtsH11 is crucial for chloroplast structure and function during growth in prolonged photoperiod.     

Gene-specific trans-regulatory functions of magnesium for chloroplast mRNA stability in higher plants

In higher plant chloroplasts the accumulation of plastid-encoded mRNAs during leaf maturation is regulated via gene-specific mRNA stabilization. The half-lives of chloroplast RNAs are specifically affected by magnesium ions. psbA mRNA (D1 protein of photosystem II), rbcL mRNA (large subunit of ribulose-1,5-bisphosphate carboxylase), 16 S rRNA, and tRNA(His) gain stability at specific magnesium concentrations in an in vitro degradation system from spinach chloroplasts. Each RNA exhibits a typical magnesium concentration-dependent stabilization profile. It shows a cooperative response of the stability-regulated psbA mRNA and a saturation curve for the other RNAs. The concentration of free Mg(2+) rises during chloroplast development within a range sufficient to mediate gene-specific mRNA stabilization in vivo as observed in vitro. We suggest that magnesium ions are a trans-acting factor mediating differential mRNA stability.     

Rapid, ATP-dependent degradation of a truncated D1 protein in the chloroplast.

The D1 protein constitutes one of the reaction center subunits of photosystem II and turns over rapidly due to photooxidative damage. Here, we studied the degradation of a truncated D1 protein. A plasmid with a precise deletion in the reading frame of the psbA gene encoding D1 was introduced into the chloroplast of Chlamydomonas reinhardtii. A homoplasmic mutant containing the desired gene was able to synthesize the truncated form of the polypeptide, but could not accumulate significant levels of it. As a consequence, other central photosystem II subunits did not assemble within the thylakoid membrane. In vivo pulse-chase experiments showed that the abnormal D1 protein is rapidly degraded in the light. Degradation was delayed in the light in the presence of an uncoupler, or when cells were incubated in the dark. Pulse-chase experiments performed in vitro indicate that an ATP and metal-dependent protease is responsible for the breakdown process. The paper describes the first in vivo and in vitro functional test for ATP-dependent degradation of a defect polypeptide in chloroplasts. The possible involvement of proteases similar to those removing abnormal proteins in prokaryotic organisms is discussed on the basis of proteases recently identified in chloroplasts.