Composition of photosystem II antenna in light-harvesting complex II antisense tobacco plants at varying irradiances

J, Hase T, Jensen P, Hanke GT. Ferredoxin C2 is required for chlorophyll biosynthesis and accumulation of photosynthetic antennae in Arabidopsis

Ferredoxins (Fd) are small iron-sulphur proteins, with sub-types that have evolved for specific redox functions. Ferredoxin C2 (FdC2) proteins are essential Fd homologues conserved in all photosynthetic organisms and a number of different FdC2 functions have been proposed in angiosperms. Here we use RNAi silencing in Arabidopsis thaliana to generate a viable fdC2 mutant line with near-depleted FdC2 protein levels. Mutant leaves have ~50% less chlorophyll a and b, and chloroplasts have poorly developed thylakoid membrane structure. Transcriptomics indicates upregulation of genes involved in stress responses. Although fdC2 antisense plants show increased damage at photosystem II (PSII) when exposed to high light, PSII recovers at the same rate as wild type in the dark. This contradicts literature proposing that FdC2 regulates translation of the D1 subunit of PSII, by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates revealed a build-up of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. We localise FdC2 to the inner chloroplast envelope and show that the FdC2 RNAi line has a disproportionately lower protein abundance of antennae proteins, which are nuclear-encoded and must be refolded at the envelope after import.

Chlorophyll cycle regulates the construction and destruction of the light-harvesting complexes

Chlorophyll a and chlorophyll b are the major constituents of the photosynthetic apparatus in land plants and green algae. Chlorophyll a is essential in photochemistry, while chlorophyll b is apparently dispensable for their photosynthesis. Instead, chlorophyll b is necessary for stabilizing the major light-harvesting chlorophyll-binding proteins. Chlorophyll b is synthesized from chlorophyll a and is catabolized after it is reconverted to chlorophyll a. This interconversion system between chlorophyll a and chlorophyll b refers to the chlorophyll cycle. The chlorophyll b levels are determined by the activity of the three enzymes participating in the chlorophyll cycle, namely, chlorophyllide a oxygenase, chlorophyll b reductase, and 7-hydroxymethyl-chlorophyll reductase. This article reviews the recent progress on the analysis of the chlorophyll cycle and its enzymes. In particular, we emphasize the impact of genetic modification of chlorophyll cycle enzymes on the construction and destruction of the photosynthetic machinery. These studies reveal that plants regulate the construction and destruction of a specific subset of light-harvesting complexes through the chlorophyll cycle. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

The Arabidopsis TSPO-related protein is a stress and abscisic acid-regulated, endoplasmic reticulum-Golgi-localized membrane protein

The Arabidopsis gene At2g47770 encodes a membrane-bound protein designated AtTSPO (Arabidopsis thaliana TSPO-related). AtTSPO is related to the bacterial outer membrane tryptophan-rich sensory protein (TspO) and the mammalian mitochondrial 18-kDa translocator protein (18 kDa TSPO), members of the group of TspO/MBR domain-containing membrane proteins. In this study we show that AtTSPO is mainly detected in dry seeds, but can be induced in vegetative tissues by osmotic or salt stress or abscisic acid (ABA) treatment, corroborating available transcriptome data. Using subcellular fractionation, immunocytochemistry and fluorescent protein tagging approaches we present evidence that AtTSPO is targeted to the secretory pathway in plants. Induced or constitutively expressed AtTSPO can be detected in the endoplasmic reticulum and the Golgi stacks of plant cells. AtTSPO tagged with fluorescent protein in transgenic plants (Arabidopsis and tobacco) was mainly detected in the Golgi stacks of leaf epidermal cells. Constitutive expression of AtTSPO resulted in increased sensitivity to NaCl, but not to osmotic stress, and in reduced greening of cultured Arabidopsis cells under light growing conditions. Transgenic Arabidopsis plants overexpressing AtTSPO were more sensitive to ABA-induced growth inhibition, indicating that constitutive expression of AtTSPO may enhance ABA sensitivity. AtTSPO is rapidly downregulated during seed imbibition, and the ABA-dependent induction in plant is transient. Downregulation of AtTSPO seems to be boosted by treatment with aminolevulinic acid. Taken together, these results suggest that AtTSPO is a highly regulated protein, induced by abiotic stress to modulate, at least in part, transient intracellular ABA-dependent stress perception and/or signalling.

Transgenic tobacco plants overexpressing chloroplastic ferredoxin-NADP(H) reductase display normal rates of photosynthesis and increased tolerance to oxidative stress

Ferredoxin-NADP(H) reductase (FNR) catalyzes the last step of photosynthetic electron transport in chloroplasts, driving electrons from reduced ferredoxin to NADP+. This reaction is rate limiting for photosynthesis under a wide range of illumination conditions, as revealed by analysis of plants transformed with an antisense version of the FNR gene. To investigate whether accumulation of this flavoprotein over wild-type levels could improve photosynthetic efficiency and growth, we generated transgenic tobacco (Nicotiana tabacum) plants expressing a pea (Pisum sativum) FNR targeted to chloroplasts. The alien product distributed between the thylakoid membranes and the chloroplast stroma. Transformants grown at 150 or 700 micromol quanta m(-2) s(-1) displayed wild-type phenotypes regardless of FNR content. Thylakoids isolated from plants with a 5-fold FNR increase over the wild type displayed only moderate stimulation (approximately 20%) in the rates of electron transport from water to NADP+. In contrast, when donors of photosystem I were used to drive NADP+ photoreduction, the activity was 3- to 4-fold higher than the wild-type controls. Plants expressing various levels of FNR (from 1- to 3.6-fold over the wild type) failed to show significant differences in CO2 assimilation rates when assayed over a range of light intensities and CO2 concentrations. Transgenic lines exhibited enhanced tolerance to photooxidative damage and redox-cycling herbicides that propagate reactive oxygen species. The results suggest that photosynthetic electron transport has several rate-limiting steps, with FNR catalyzing just one of them.

Translational regulation of light-harvesting complex expression during photoacclimation to high-light in Chlamydomonas reinhardtii

When challenged with excess light, the green alga Chlamydomonas reinhardtii responds, in part, by down-regulating light harvesting capacity at Photosystem II while concomitantly reorganising cellular metabolism to increase sink capacity. We examined the role of translational control during different stages of photoacclimation by analysing polysome profiles of two different light-harvesting complex (LHC) genes encoding a major LHCII component (Lhcbm) and CP29 (Lhcb4) plus iron superoxide dismutase (FeSOD), and through measurement of protein synthesis by in vivo labelling. Within two hours following transfer of low-light (LL) acclimated cultures into high-light (HL), Lhcbm transcripts are off-loaded from polysomes indicating a decline in translational initiation. Lhcbm translational repression at two hours was specific since FeSOD mRNA remained associated with polysomes and in vivo labelling showed an increase in overall protein synthesis. After 4 hours HL exposure, however, a global disassembly of polysomes was detected suggesting a general decline in translational initiation. Interestingly, the decrease in polysomes coincides with maximal FeSOD transcript levels emphasizing that transcript profiles do not always accurately reflect gene expression. Within 8 hours after LL to HL shift, polysomes reform and all transcripts examined are loaded back onto polysomes. Disassembly of polysomes was mimicked by hydrogen peroxide application to non-shifted cultures suggesting that production of hydrogen peroxide due to HL-stress may effect the general decline in polysomes observed during HL acclimation.

Abundantly and rarely expressed Lhc protein genes exhibit distinct regulation patterns in plants

We have analyzed gene regulation of the Lhc supergene family in poplar (Populus spp.) and Arabidopsis (Arabidopsis thaliana) using digital expression profiling. Multivariate analysis of the tissue-specific, environmental, and developmental Lhc expression patterns in Arabidopsis and poplar was employed to characterize four rarely expressed Lhc genes, Lhca5, Lhca6, Lhcb7, and Lhcb4.3. Those genes have high expression levels under different conditions and in different tissues than the abundantly expressed Lhca1 to 4 and Lhcb1 to 6 genes that code for the 10 major types of higher plant light-harvesting proteins. However, in some of the datasets analyzed, the Lhcb4 and Lhcb6 genes as well as an Arabidopsis gene not present in poplar (Lhcb2.3) exhibited minor differences to the main cooperative Lhc gene expression pattern. The pattern of the rarely expressed Lhc genes was always found to be more similar to that of PsbS and the various light-harvesting-like genes, which might indicate distinct physiological functions for the rarely and abundantly expressed Lhc proteins. The previously undetected Lhcb7 gene encodes a novel plant Lhcb-type protein that possibly contains an additional, fourth, transmembrane N-terminal helix with a highly conserved motif. As the Lhcb4.3 gene seems to be present only in Eurosid species and as its regulation pattern varies significantly from that of Lhcb4.1 and Lhcb4.2, we conclude it to encode a distinct Lhc protein type, Lhcb8.

Effects of chlorophyllide a oxygenase overexpression on light acclimation in Arabidopsis thaliana

Land plants change the compositions of light-harvesting complexes (LHC) and chlorophyll (Chl) a/b ratios in response to the variable light environments which they encounter. In this study, we attempted to determine the mechanism which regulates Chl a/b ratios and whether the changes in Chl a/b ratios are essential in regulation of LHC accumulation during light acclimation. We hypothesized that changes in the mRNA levels for chlorophyll a oxygenase (CAO) involved in Chl b biosynthesis are an essential part of light response of Chl a/b ratios and LHC accumulation. We also examined the light-intensity dependent response of CAO-overexpression and wild-type Arabidopsis thaliana plants. When wild-type plants were acclimated from low-light (LL) to high-light (HL) conditions, CAO mRNA levels decreased and the Chl a/b ratio increased. In transgenic plants overexpressing CAO, the Chl a/b ratio remained low under HL conditions; thereby suggesting that changes in the CAO mRNA levels are necessary for those in Chl a/b ratios upon light acclimation. Under HL conditions, the accumulation of Lhcb1, Lhcb3 and Lhcb6 was enhanced in plants overexpressing CAO. On the contrary, in a CAO-deficient mutant, chlorina 1-1, theaccumulation of Lhcb1, Lhcb2, Lhcb3, Lhcb6 and Lhca4 was reduced. In comparison to wild-type, beta-carotene levels were reduced in CAO-overexpressing plants, while they were elevated in chlorina 1-1 mutants. These results imply that the transcriptional control of CAO is a part of the regulatory mechanism for the accumulation of a distinct set of LHC proteins upon light acclimation.

Post-transcriptional light regulation of nuclear-encoded genes

A significant number of studies have detected a post-transcriptional component in the light responses of nuclear genes. As yet there are few in-depth studies of the mechanism(s) involved, and it seems likely some additional examples have been missed. For instance, transcriptional responses have sometimes been inferred on the basis of experiments with translational fusions containing both the promoter and 5' UTR of the test gene, but we now know that elements within the 5' UTR can mediate post-transcriptional light responses. Similarly, because of possible changes in translation rates and protein turnover, the common assumption that mRNA levels directly dictate protein levels is tenuous at best. It is no longer permissible to assume that the biological effect of a gene is a simple function of its transcription. Thus it is likely that with careful experimental design, reports of nuclear-encoded post-transcriptional gene regulation will become increasingly prevalent.

Antisense inhibition of the photosynthetic antenna proteins CP29 and CP26: implications for the mechanism of protective energy dissipation

The specific roles of the chlorophyll a/b binding proteins CP29 and CP26 in light harvesting and energy dissipation within the photosynthetic apparatus have been investigated. Arabidopsis was transformed with antisense constructs against the genes encoding the CP29 or CP26 apoprotein, which gave rise to several transgenic lines with remarkably low amounts of the antisense target proteins. The decrease in the level of CP24 protein in the CP29 antisense lines indicates a physical interaction between these complexes. Analysis of chlorophyll fluorescence showed that removal of the proteins affected photosystem II function, probably as a result of changes in the organization of the light-harvesting antenna. However, whole plant measurements showed that overall photosynthetic rates were similar to those in the wild type. Both antisense lines were capable of the qE type of nonphotochemical fluorescence quenching, although there were minor changes in the capacity for quenching and in its induction kinetics. High-light-induced violaxanthin deepoxidation to zeaxanthin was not affected, although the pool size of these pigments was decreased slightly. We conclude that CP29 and CP26 are unlikely to be sites for nonphotochemical quenching.

A small decrease of plastid transketolase activity in antisense tobacco transformants has dramatic effects on photosynthesis and phenylpropanoid metabolism

Transketolase (TK) catalyzes reactions in the Calvin cycle and the oxidative pentose phosphate pathway (OPPP) and produces erythrose-4-phosphate, which is a precursor for the shikimate pathway leading to phenylpropanoid metabolism. To investigate the consequences of decreased TK expression for primary and secondary metabolism, we transformed tobacco with a construct containing an antisense TK sequence. The results were as follows: (1) a 20 to 40% reduction of TK activity inhibited ribulose-1,5-bisphosphate regeneration and photosynthesis. The inhibition of photosynthesis became greater as irradiance increased across the range experienced in growth conditions (170 to 700 micromol m(-2) sec(-1)). TK almost completely limited the maximum rate of photosynthesis in saturating light and saturating CO(2). (2) Decreased expression of TK led to a preferential decrease of sugars, whereas starch remained high until photosynthesis was strongly inhibited. One of the substrates of TK (fructose-6-phosphate) is the starting point for starch synthesis, and one of the products (erythrose-4-phosphate) inhibits phosphoglucose isomerase, which catalyzes the first reaction leading to starch. (3) A 20 to 50% decrease of TK activity led to decreased levels of aromatic amino acids and decreased levels of the intermediates (caffeic acid and hydroxycinnamic acids) and products (chlorogenic acid, tocopherol, and lignin) of phenylpropanoid metabolism. (4) There was local loss of chlorophyll and carotene on the midrib when TK activity was inhibited by >50%, spreading onto minor veins and lamina in severely affected transformants. (5) OPPP activity was not strongly inhibited by decreased TK activity. These results identify TK activity as an important determinant of photosynthetic and phenylpropanoid metabolism and show that the provision of precursors by primary metabolism colimits flux into the shikimate pathway and phenylpropanoid metabolism.

A small decrease of plastid transketolase activity in antisense tobacco transformants has dramatic effects on photosynthesis and phenylpropanoid metabolism

Transketolase (TK) catalyzes reactions in the Calvin cycle and the oxidative pentose phosphate pathway (OPPP) and produces erythrose-4-phosphate, which is a precursor for the shikimate pathway leading to phenylpropanoid metabolism. To investigate the consequences of decreased TK expression for primary and secondary metabolism, we transformed tobacco with a construct containing an antisense TK sequence. The results were as follows: (1) a 20 to 40% reduction of TK activity inhibited ribulose-1,5-bisphosphate regeneration and photosynthesis. The inhibition of photosynthesis became greater as irradiance increased across the range experienced in growth conditions (170 to 700 micromol m(-2) sec(-1)). TK almost completely limited the maximum rate of photosynthesis in saturating light and saturating CO(2). (2) Decreased expression of TK led to a preferential decrease of sugars, whereas starch remained high until photosynthesis was strongly inhibited. One of the substrates of TK (fructose-6-phosphate) is the starting point for starch synthesis, and one of the products (erythrose-4-phosphate) inhibits phosphoglucose isomerase, which catalyzes the first reaction leading to starch. (3) A 20 to 50% decrease of TK activity led to decreased levels of aromatic amino acids and decreased levels of the intermediates (caffeic acid and hydroxycinnamic acids) and products (chlorogenic acid, tocopherol, and lignin) of phenylpropanoid metabolism. (4) There was local loss of chlorophyll and carotene on the midrib when TK activity was inhibited by >50%, spreading onto minor veins and lamina in severely affected transformants. (5) OPPP activity was not strongly inhibited by decreased TK activity. These results identify TK activity as an important determinant of photosynthetic and phenylpropanoid metabolism and show that the provision of precursors by primary metabolism colimits flux into the shikimate pathway and phenylpropanoid metabolism.

A small chloroplast-encoded protein as a novel architectural component of the light-harvesting antenna

A small conserved open reading frame in the plastid genome, ycf9, encodes a putative membrane protein of 62 amino acids. To determine the function of this reading frame we have constructed a knockout allele for targeted disruption of ycf9. This allele was introduced into the tobacco plastid genome by biolistic transformation to replace the wild-type ycf9 allele. Homoplasmic ycf9 knockout plants displayed no phenotype under normal growth conditions. However, under low light conditions, their growth rate was significantly reduced as compared with the wild-type, due to a lowered efficiency of the light reaction of photosynthesis. We show that this phenotype is caused by the deficiency in a pigment-protein complex of the light-harvesting antenna of photosystem II and hence by a reduced efficiency of photon capture when light availability is limiting. Our results indicate that, in contrast to the current view, light-harvesting complexes do not only consist of the classical pigment-binding proteins, but may contain small structural subunits in addition. These subunits appear to be crucial architectural factors for the assembly and/or maintenance of stable light-harvesting complexes.