Assembly of the photosynthetic apparatus

Biogenesis of photosynthetic complexes in the chloroplast of Chlamydomonas reinhardtii requires ARSA1, a homolog of prokaryotic arsenite transporter and eukaryotic TRC40 for guided entry of tail-anchored proteins

as1, for antenna size mutant 1, was obtained by insertion mutagenesis of the unicellular green alga Chlamydomonas reinhardtii. This strain has a low chlorophyll content, 8% with respect to the wild type, and displays a general reduction in thylakoid polypeptides. The mutant was found to carry an insertion into a homologous gene, prokaryotic arsenite transporter (ARSA), whose yeast and mammal counterparts were found to be involved in the targeting of tail-anchored (TA) proteins to cytosol-exposed membranes, essential for several cellular functions. Here we present the characterization in a photosynthetic organism of an insertion mutant in an ARSA-homolog gene. The ARSA1 protein was found to be localized in the cytosol, and yet its absence in as1 leads to a small chloroplast and a strongly decreased chlorophyll content per cell. ARSA1 appears to be required for optimal biogenesis of photosynthetic complexes because of its involvement in the accumulation of TOC34, an essential component of the outer chloroplast membrane translocon (TOC) complex, which, in turn, catalyzes the import of nucleus-encoded precursor polypeptides into the chloroplast. Remarkably, the effect of the mutation appears to be restricted to biogenesis of chlorophyll-binding polypeptides and is not compensated by the other ARSA homolog encoded by the C. reinhardtii genome, implying a non-redundant function.

DAC is involved in the accumulation of the cytochrome b6/f complex in Arabidopsis

The biogenesis and assembly of photosynthetic multisubunit protein complexes is assisted by a series of nucleus-encoded auxiliary protein factors. In this study, we characterize the dac mutant of Arabidopsis (Arabidopsis thaliana), which shows a severe defect in the accumulation of the cytochrome b(6)/f complex, and provide evidence suggesting that the efficiency of cytochrome b(6)/f complex assembly is affected in the mutant. DAC is a thylakoid membrane protein with two predicted transmembrane domains that is conserved from cyanobacteria to vascular plants. Yeast (Saccharomyces cerevisiae) two-hybrid and coimmunoprecipitation analyses revealed a specific interaction between DAC and PetD, a subunit of the cytochrome b(6)/f complex. However, DAC was found not to be an intrinsic component of the cytochrome b(6)/f complex. In vivo chloroplast protein labeling experiments showed that the labeling rates of the PetD and cytochrome f proteins were greatly reduced, whereas that of the cytochrome b(6) protein remained normal in the dac mutant. DAC appears to be a novel factor involved in the assembly/stabilization of the cytochrome b(6)/f complex, possibly through interaction with the PetD protein.

PSII photochemistry in vegetative buds and needles of Norway spruce (Picea abies L. Karst.) probed by OJIP chlorophyll a fluorescence measurement

Vegetative buds represent developmental stage of Norway spruce (Picea abies L. Karst.) needles where chloroplast biogenesis and photosynthetic activity begin. We used the analyses of polyphasic chlorophyll a fluorescence rise (OJIP) to compare photosystem II (PSII) functioning in vegetative buds and fully photosynthetically active mature current-year needles. Considerably decreased performance index (PIABS) in vegetative buds compared to needles pointed to their low photosynthetic efficiency. Maximum quantum yield of PSII (Fv/Fm) in buds was slightly decreased but above limited value for functionality indicating that primary photochemistry of PSII is not holdback of vegetative buds photosynthetic activity. The most significant difference observed between investigated developmental stages was accumulation of reduced primary quinine acceptor of PSII (QA-) in vegetative buds, as a result of its limited re-oxidation by passing electrons to secondary quinone acceptor, QB. We suggest that reduced electron transfer from QA- to QB could be the major limiting factor of photosynthesis in vegetative buds.

Chloroplast biogenesis - correlation between structure and function

Chloroplast biogenesis is a multistage process leading to fully differentiated and functionally mature plastids. Complex analysis of chloroplast biogenesis was performed on the structural and functional level of its organization during the photoperiodic plant growth after initial growth of seedlings in the darkness. We correlated, at the same time intervals, the structure of etioplasts transforming into mature chloroplasts with the changes in the photosynthetic protein levels (selected core and antenna proteins of PSI and PSII) and with the function of the photosynthetic apparatus in two plant species: bean (Phaseolus vulgaris L.) and pea (Pisum sativum L). We selected these plant species since we demonstrated previously that the mature chloroplasts differ in the thylakoid organization. We showed that the protein biosynthesis as well as photosynthetic complexes formation proceeds gradually in both plants in spite of periods of darkness. We found that both steady structural differentiation of the bean chloroplast and reformation of prolamellar bodies in pea were accompanied by a gradual increase of the photochemical activity in both species. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Carotenoid deficiency triggers autophagy in the model green alga Chlamydomonas reinhardtii

All aerobic organisms have developed sophisticated mechanisms to prevent, detect and respond to cell damage caused by the unavoidable production of reactive oxygen species (ROS). Plants and algae are able to synthesize specific pigments in the chloroplast called carotenoids to prevent photo-oxidative damage caused by highly reactive by-products of photosynthesis. In this study we used the unicellular green alga Chlamydomonas reinhardtii to demonstrate that defects in carotenoid biosynthesis lead to the activation of autophagy, a membrane-trafficking process that participates in the recycling and degradation of damaged or toxic cellular components. Carotenoid depletion caused by either the mutation of phytoene synthase or the inhibition of phytoene desaturase by the herbicide norflurazon, resulted in a strong induction of autophagy. We found that high light transiently activates autophagy in wild-type Chlamydomonas cells as part of an adaptation response to this stress. Our results showed that a Chlamydomonas mutant defective in the synthesis of specific carotenoids that accumulate during high light stress exhibits constitutive autophagy. Moreover, inhibition of the ROS-generating NADPH oxidase partially reduced the autophagy induction associated to carotenoid deficiency, which revealed a link between photo-oxidative damage, ROS accumulation and autophagy activation in Chlamydomonas cells with a reduced carotenoid content.

Defects in leaf carbohydrate metabolism compromise acclimation to high light and lead to a high chlorophyll fluorescence phenotype in Arabidopsis thaliana

BACKGROUND

We have studied the impact of carbohydrate-starvation on the acclimation response to high light using Arabidopsis thaliana double mutants strongly impaired in the day- and night path of photoassimilate export from the chloroplast. A complete knock-out mutant of the triose phosphate/phosphate translocator (TPT; tpt-2 mutant) was crossed to mutants defective in (i) starch biosynthesis (adg1-1, pgm1 and pgi1-1; knock-outs of ADP-glucose pyrophosphorylase, plastidial phosphoglucomutase and phosphoglucose isomerase) or (ii) starch mobilization (sex1-3, knock-out of glucan water dikinase) as well as in (iii) maltose export from the chloroplast (mex1-2).

RESULTS

All double mutants were viable and indistinguishable from the wild type when grown under low light conditions, but--except for sex1-3/tpt-2--developed a high chlorophyll fluorescence (HCF) phenotype and growth retardation when grown in high light. Immunoblots of thylakoid proteins, Blue-Native gel electrophoresis and chlorophyll fluorescence emission analyses at 77 Kelvin with the adg1-1/tpt-2 double mutant revealed that HCF was linked to a specific decrease in plastome-encoded core proteins of both photosystems (with the exception of the PSII component cytochrome b559), whereas nuclear-encoded antennae (LHCs) accumulated normally, but were predominantly not attached to their photosystems. Uncoupled antennae are the major cause for HCF of dark-adapted plants. Feeding of sucrose or glucose to high light-grown adg1-1/tpt-2 plants rescued the HCF- and growth phenotypes. Elevated sugar levels induce the expression of the glucose-6-phosphate/phosphate translocator2 (GPT2), which in principle could compensate for the deficiency in the TPT. A triple mutant with an additional defect in GPT2 (adg1-1/tpt-2/gpt2-1) exhibited an identical rescue of the HCF- and growth phenotype in response to sugar feeding as the adg1-1/tpt-2 double mutant, indicating that this rescue is independent from the sugar-triggered induction of GPT2.

CONCLUSIONS

We propose that cytosolic carbohydrate availability modulates acclimation to high light in A. thaliana. It is conceivable that the strong relationship between the chloroplast and nucleus with respect to a co-ordinated expression of photosynthesis genes is modified in carbohydrate-starved plants. Hence carbohydrates may be considered as a novel component involved in chloroplast-to-nucleus retrograde signaling, an aspect that will be addressed in future studies.

ABI4 and its role in chloroplast retrograde communication

The acquisition of plastids is a landmark event in plant evolution. The proper functionality of these organelles depends on strict and continuous communication between the plastids and the nucleus to precisely adjust gene expression in response to the organelle's requirements. Signals originating from the plastids impact the expression of a variety of nuclear genes, and this retrograde communication is essential to couple the nuclear expression of plastid-localized products with organelle gene expression and, ultimately, functionality. Major advances have been made in this field over the past few years with the characterization of independent retrograde signaling pathways and the identification of some of their components. One such factor is the nuclear transcriptional factor ABI4 (ABA-INSENTIVE 4). ABI4, together with the plastid PPR GUN1 protein, has been proposed to function as a node of convergence for multiple plastid retrograde signaling pathways. ABI4 is conserved among plants and also plays important roles in various critical developmental and metabolic processes. ABI4 is a versatile regulator that positively and negatively modulates the expression of many genes, including other transcriptional factors. However, its mode of action during plastid retrograde signaling is not fully understood. In this review, we describe the current evidence that supports the participation of ABI4 in different retrograde communication pathways. ABI4 is regulated at the transcriptional and post-transcriptional level. A known regulator of ABI4 includes the PTM transcription factor, which moves from the chloroplast to the nucleus. This transcription factor is a candidate for the transmission of retrograde signals between the plastid and ABI4.

Multistep assembly of chloroplast NADH dehydrogenase-like subcomplex A requires several nucleus-encoded proteins, including CRR41 and CRR42, in Arabidopsis

Chloroplast NADH dehydrogenase-like complex (NDH) mediates photosystem I cyclic electron transport and chlororespiration in thylakoids. Recently, substantial progress has been made in understanding the structure of NDH, but our knowledge of its assembly has been limited. In this study, a series of interactive proteomic analyses identified several stroma-localized factors required for the assembly of a stroma-protruding arm of NDH (subcomplex A). In addition to further characterization of the previously identified CHLORORESPIRATORY REDUCTION1 (CRR1), CRR6, and CRR7, two novel stromal proteins, CRR41 and CRR42, were discovered. Arabidopsis thaliana mutants lacking these proteins are specifically defective in the accumulation of subcomplex A. A total of 10 mutants lacking subcomplex A, including crr27/cpn60β4, which is specifically defective in the folding of NdhH, and four mutants lacking NdhL-NdhO subunits, were extensively characterized. We propose a model for subcomplex A assembly: CRR41, NdhO, and native NdhH, as well as unknown factors, are first assembled to form an NDH subcomplex A assembly intermediate (NAI500). Subsequently, NdhJ, NdhM, NdhK, and NdhI are incorporated into NAI500 to form NAI400. CRR1, CRR6, and CRR42 are involved in this process. CRR7 is likely to be involved in the final step, in which the fully assembled NAI, including NdhN, is inserted into thylakoids.