Redox Regulation of Photosynthetic Genes. In: Aro E, Andersson B, editors. Regulation of Photosynthesis. Dordrecht: Springer Netherlands; 2001. pp. 85-107. [DOI: 10.1007/0-306-48148-0_5]

Physiological and molecular responses to variation of light intensity in rubber Tree (Hevea brasiliensis Muell. Arg.)

Light is one of most important factors to plants because it is necessary for photosynthesis. In this study, physiological and gene expression analyses under different light intensities were performed in the seedlings of rubber tree (Hevea brasiliensis) clone GT1. When light intensity increased from 20 to 1000 µmol m(-2) s(-1), there was no effect on the maximal quantum yield of photosystem II (PSII) photochemistry (Fv/Fm), indicating that high light intensity did not damage the structure and function of PSII reaction center. However, the effective photochemical quantum yield of PSII (Y(II)), photochemical quenching coefficient (qP), electron transfer rate (ETR), and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae (qL) were increased significantly as the light intensity increased, reached a maximum at 200 µmol m(-2) s(-1), but decreased from 400 µmol m(-2) s(-1). These results suggested that the PSII photochemistry showed an optimum performance at 200 µmol m(-2) s(-1) light intensity. The chlorophyll content was increased along with the increase of light intensity when it was no more than 400 µmol m(-2) s(-1). Since increasing light intensity caused significant increase in H2O2 content and decreases in the per unit activity of antioxidant enzymes SOD and POD, but the malondialdehyde (MDA) content was preserved at a low level even under high light intensity of 1000 µmol m(-2) s(-1), suggesting that high light irradiation did not induce membrane lipid peroxidation in rubber tree. Moreover, expressions of antioxidant-related genes were significantly up-regulated with the increase of light intensity. They reached the maximum expression at 400 µmol m(-2) s(-1), but decreased at 1000 µmol m(-2) s(-1). In conclusion, rubber tree could endure strong light irradiation via a specific mechanism. Adaptation to high light intensity is a complex process by regulating antioxidant enzymes activities, chloroplast formation, and related genes expressions in rubber tree.

Two independent light signals cooperate in the activation of the plastid psbD blue light-responsive promoter in Arabidopsis

The psbD blue light-responsive promoter (BLRP), whose activation has been considered to require strong blue light, is recognized only by SIG5 among six sigma factors of plastid RNA polymerase in Arabidopsis. We found SIG5 transcript accumulation was rapidly induced after a 30-min induction time by blue light (470 nm) with an intensity threshold of 5 micromol m(-2)s(-1) through cryptochromes. Besides this weak blue light, the psbD BLRP activation required the stronger light such as 50 micromol m(-2)s(-1) irrespective of blue or red light (660 nm). Thus, the two independent light signalings, the cryptochrome-mediated signaling to induce SIG5 transcription and the stronger light-dependent signaling, cooperate to activate the psbD BLRP.

Decreased content of leaf ferredoxin changes electron distribution and limits photosynthesis in transgenic potato plants

A complete ferredoxin (Fd) cDNA clone was isolated from potato (Solanum tuberosum L. cv Desiree) leaves. By molecular and immunoblot analysis, the gene was identified as the leaf-specific Fd isoform I. Transgenic potato plants were constructed by introducing the homologous potato fed 1 cDNA clone as an antisense construct under the control of the constitutive cauliflower mosaic virus 35S promoter. Stable antisense lines with Fd contents between 40% and 80% of the wild-type level were selected by northern- and western-blot analysis. In short-term experiments, the distribution of electrons toward their stromal acceptors was altered in the mutant plants. Cyclic electron transport, as determined by the quantum yields of photosystems I and II, was enhanced. The CO2 assimilation rate was decreased, but depending on the remaining Fd content, some lines showed photoinhibition. The leaf protein content remained largely constant, but the antisense plants had a lower total chlorophyll content per unit leaf area and an increased chlorophyll a/b ratio. In the antisense plants, the redox state of the quinone acceptor A in photosystem II (QA) was more reduced than that of the wild-type plants under all experimental conditions. Because the plants with lower Fd amounts reacted as if they were grown under a higher light intensity, the possibility that the altered chloroplast redox state affects light acclimation is discussed.

Redox and light regulation of gene expression in photosynthetic prokaryotes

All photosynthetic organisms control expression of photosynthesis genes in response to alterations in light intensity as well as to changes in cellular redox potential. Light regulation in plants involves a well-defined set of red- and blue-light absorbing photoreceptors called phytochrome and cryptochrome. Less understood are the factors that control synthesis of the plant photosystem in response to changes in cellular redox. Among a diverse set of photosynthetic bacteria the best understood regulatory systems are those synthesized by the photosynthetic bacterium Rhodobacter capsulatus. This species uses the global two-component signal transduction cascade, RegB and RegA, to anaerobically de-repress anaerobic gene expression. Under reducing conditions, the phosphate on RegB is transferred to RegA, which then activates genes involved in photosynthesis, nitrogen fixation, carbon fixation, respiration and electron transport. In the presence of oxygen, there is a second regulator known as CrtJ, which is responsible for repressing photosynthesis gene expression. CrtJ responds to redox by forming an intramolecular disulphide bond under oxidizing, but not reducing, growth conditions. The presence of the disulphide bond stimulates DNA binding activity of the repressor. There is also a flavoprotein that functions as a blue-light absorbing anti-repressor of CrtJ in the related bacterial species Rhodobacter sphaeroides called AppA. AppA exhibits a novel long-lived photocycle that is initiated by blue-light absorption by the flavin. Once excited, AppA binds to CrtJ thereby inhibiting the repressor activity of CrtJ. Various mechanistic aspects of this photocycle will be discussed.

A novel plant protein undergoing light-induced phosphorylation and release from the photosynthetic thylakoid membranes

The characteristics of a phosphoprotein with a relative electrophoretic mobility of 12 kDa have been unknown during two decades of studies on redox-dependent protein phosphorylation in plant photosynthetic membranes. Digestion of this protein from spinach thylakoid membranes with trypsin and subsequent tandem nanospray-quadrupole-time-of-flight mass spectrometry of the peptides revealed a protein sequence that did not correspond to any previously known protein. Sequencing of the corresponding cDNA uncovered a gene for a precursor protein with a transit peptide followed by a strongly basic mature protein with a molecular mass of 8,640 Da. Genes encoding homologous proteins were found on chromosome 3 of Arabidopsis and rice as well as in ESTs from 20 different plant species, but not from any other organisms. The protein can be released from the membrane with high salt and is also partially released in response to light-induced phosphorylation of thylakoids, in contrast to all other known thylakoid phosphoproteins, which are integral to the membrane. On the basis of its properties, this plant-specific protein is named thylakoid soluble phosphoprotein of 9 kDa (TSP9). Mass spectrometric analyses revealed the existence of non-, mono-, di-, and triphosphorylated forms of TSP9 and phosphorylation of three distinct threonine residues in the central part of the protein. The phosphorylation and release of TSP9 from the photosynthetic membrane on illumination favor participation of this basic protein in cell signaling and regulation of plant gene expression in response to changing light conditions.