Chloroplast shrinkage and increased photophosphorylation in vitro upon illuminating intact plants of Pisum sativum

Role of Ions in the Regulation of Light-Harvesting

Regulation of photosynthetic light harvesting in the thylakoids is one of the major key factors affecting the efficiency of photosynthesis. Thylakoid membrane is negatively charged and influences both the structure and the function of the primarily photosynthetic reactions through its electrical double layer (EDL). Further, there is a heterogeneous organization of soluble ions (K+, Mg2+, Cl-) attached to the thylakoid membrane that, together with fixed charges (negatively charged amino acids, lipids), provides an electrical field. The EDL is affected by the valence of the ions and interferes with the regulation of "state transitions," protein interactions, and excitation energy "spillover" from Photosystem II to Photosystem I. These effects are reflected in changes in the intensity of chlorophyll a fluorescence, which is also a measure of photoprotective non-photochemical quenching (NPQ) of the excited state of chlorophyll a. A triggering of NPQ proceeds via lumen acidification that is coupled to the export of positive counter-ions (Mg2+, K+) to the stroma or/and negative ions (e.g., Cl-) into the lumen. The effect of protons and anions in the lumen and of the cations (Mg2+, K+) in the stroma are, thus, functionally tightly interconnected. In this review, we discuss the consequences of the model of EDL, proposed by Barber (1980b) Biochim Biophys Acta 594:253-308) in light of light-harvesting regulation. Further, we explain differences between electrostatic screening and neutralization, and we emphasize the opposite effect of monovalent (K+) and divalent (Mg2+) ions on light-harvesting and on "screening" of the negative charges on the thylakoid membrane; this effect needs to be incorporated in all future models of photosynthetic regulation by ion channels and transporters.

Below-ambient levels of UV induce chloroplast structural change and alter starch metabolism

Electromagnetic radiation (EMR) in the 400-700 nm bandwidth of photosynthetically active radiation (PAR) has been established as an important source of energy for photosynthesis and environmental signals regulating many aspects of green-plant life. Above-ambient levels of UV-B radiation (290-320 nm) under high-PAR conditions have been shown to elicit responses in chloroplasts of Brassica napus similar to those of chloroplasts at low-PAR exposure (W. Fagerberg and J. Bornman, Physiol. Plant. 101: 833-844, 1997). The question arises as to whether UV at normal levels can also evoke similar responses. Here we provide evidence that even below-ambient levels of UV-B (1/28 ambient; Durham, N.H., U.S.A., 1200 hours, March) were capable of inducing an increase in thylakoid surface area relative to the chloroplast volume typical of a low-PAR response (shade response) in sunflowers. This response occurred even though leaves were concurrently exposed to PAR levels that normally induce a "sun" or high-PAR response in the absence of UV-B. Subambient levels of UV-B were also associated with a decrease in chloroplast and starch volume. Exposure to levels of UV-A 1/10 of ambient appeared to enhance the high-PAR response of the chloroplast, characterized by an increase in the amounts of stored starch, an increase in chloroplast volume density ratio values, and a decrease in thylakoid surface area density ratios relative to the high-light controls. These effects were opposite to those seen in UV-B-exposed tissue. In a general sense, subambient levels of UV-B evoked a response similar to that elicited by low-PAR irradiance, while subambient UV-A elicited responses similar to those typical of high-PAR irradiance. The fact that below-ambient levels of UV altered a normal chloroplast structural response to PAR provides evidence that UV may be an important environmental signal for plants.

Effects of lysine, threonine and methionine on light-driven protein synthesis in isolated pea (Pisum sativum L.) chloroplasts

Light-driven incorporation of [(14)C]leucine (LEU) into protein in isolated pea chloroplasts was inhibited by 0.1 and 1 mM lysine (LYS), 0.1-10 mM threonine (THR) and 1 mM methionine (MET). Equimolar combinations of LYS plus THR were inhibitory at both 0.1 and 0.5 mM. Incorporation of [(14)C] aspartic acid (ASP) and [(3)H]tyrosine (TYR) was also reduced by 1 mM LYS or THR. In the cases tested, LYS and/or THR inhibitions were partially or fully reversed by 0.1 mM MET. [(35)S]MET incorporation was unaffected or stimulated by LYS and THR. These data are consistent with the hypothesis that MET is biosynthesized in isolated chloroplasts and that its synthesis is regulated by LYS and/or THR. Of 16 other amino acids tested at 1 mM, isoleucine, phenylalanine, tryptophan, tyrosine and valine inhibited protein synthesis.

[The formation of polyphosphates in ankistrodesmus braunii by photophosphorylation in red and in far-red light under nitrogen]

A simplified extraction procedure was used to study the effect of red and far-red light (683 and 712 nm) on the incorporation of (32)P and on the distribution pattern of various phosphate fractions in 8 min-experiments under nitrogen. Nitrogen was used to avoid competition for energy-rich phosphate bonds between CO2-fixation and the formation of polyphosphates. The total incorporation of (32)P was higher in red than in far-red light, and so was the percentage of the acidsoluble organic phosphate. In the absence of oxygen the incorporation in the dark was rather low and mainly confined to orthophosphate. The ratio between the amount of labelling of polyphosphates and that of acid-soluble organic phosphates was higher in far-red light and in the dark than in red light.DCMU, even in a nitrogen atmosphere, produced a servre inhibition in red light. This inhibition increased with increasing light intensity. The labelling of organic phosphates was more affected than that of polyphosphates, while orthophosphate incorporation was least inhibited. In far-red light, DCMU exerted little influence except at a rather high light intensity, showing that cyclic photophosphorylation was proceeding alone.Antimycin A, on the other hand, was almost ineffective in strong red light, but produced a serious inhibition in far-red light. In red light of medium intensity, antimycin effected some inhibition, although much less than DCMU. Under these conditions the effect of the two inhibitors was additive when they were applied together. Labelling of polyphosphates was more sensitive to antimycin A than labelling of acid-soluble organic phosphates.It may be concluded from the data presented that far-red light produces conditions for pure cyclic photophosphorylation, whereas a large proportion of the photophosphorylation taking place in red light in the absence of CO2 and exogenous oxygen might be regarded as "pseudocyclic". The distribution pattern of the phosphorylated fractions under the different conditions suggests that polyphosphate formation in the light is favoured but not exclusively effected by cyclic photophosphorylation.