The psbO gene for 33-kDa precursor polypeptide of the oxygen-evolving complex in Arabidopsis thaliana--nucleotide sequence and control of its expression

Leaf plasticity to light intensity in Italian cypress (Cupressus sempervirens L.): adaptability of a Mediterranean conifer cultivated in the Alps

Italian cypress (Cupressus sempervirens L.) is native to the eastern Mediterranean, an area characterised by hot, dry summers and mild winters. Over the centuries, however, the species has been introduced into more northerly regions, a long way from its native range. The current, generally warmer climatic conditions brought about by global warming have favoured its cultivation in even more northerly areas in the Alps and other European alpine regions. Given that not only temperature, but also light availability are limiting factors for the spread of cypress in these environments, it is important to ascertain how this species copes with low light conditions. The photosynthetic characteristics of cypress leaves collected from different portions of the crown with contrasting light availability were evaluated by several methods. Chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoid (Car) content was found to be higher in shade leaves than in sun leaves when measured on a fresh mass basis, although enzymatic activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) and nitrate reductase were lower in shade leaves. When the efficiency of PSII was measured by chlorophyll fluorescence, a marked reduction in F(m) was found in shade leaves, while F(o) remained unchanged. The use of exogenous electron donors diphenyl carbazide (DPC) and NH(2)OH actually improved the photosynthetic efficiency of shade leaves, and the same effect was found when PSII electron transport activity was measured as O(2) evolution. Altogether, these results seem to indicate lesser photosynthetic efficiency in shade leaves, probably an impairment on the donor side of the PSII. At the same time, analysis by SDS-PAGE revealed differences in the polypeptide composition of the thylakoid membranes of sun and shade leaves: the bands corresponding to 23 kDa, 28-25 kDa and 33 kDa polypeptides were less intense in the thylakoid membranes extracted from shade leaves. These results were further confirmed by an immunological study showing that the content of the 33 kDa protein, corresponding to the extrinsic PSII protein PsbO, was significantly diminished in shade leaves. The high plasticity of cypress leaves appears to be an advantageous trait in the plant's response to variations in environmental conditions, including global change. Implications for the management of this Mediterranean species at the northern edge of its distribution are discussed.

LIGHT-REGULATED PHOTOSYNTHETIC GENE EXPRESSION AND PHOSPHORIBULOKINASE ENZYME ACTIVITY IN THE HETEROKONT ALGA VAUCHERIA LITOREA (XANTHOPHYCEAE) AND ITS SYMBIOTIC MOLLUSKAN PARTNER ELYSIA CHLOROTICA (GASTROPODA)(1)

Photosynthesis is composed of tightly coupled reactions requiring finely tuned nucleocytosolic-plastid interaction. Herein, we examined the influence of light on select photosynthetic gene expression and enzyme activity in the plastid-containing mollusk (sea slug) Elysia chlorotica and its heterokont algal prey Vaucheria litorea C. Agardh. Transcript levels of nuclear photosynthetic genes (psbO and prk) were significantly lower in E. chlorotica compared with V. litorea, whereas plastid photosynthesis genes (psaA and rbcL) were more comparable, although still lower in the animal. None of the genes responded similarly to changes in light conditions over a 24 h period in the sea slug compared with the alga. Activity of the nuclear-encoded photosynthetic enzyme phosphoribulokinase (PRK) exhibited redox regulation in vitro in crude extracts of both organisms sequentially treated with oxidizing and reducing agents. However, PRK was differentially affected in vivo by redox and light versus dark treatment in V. litorea, but not in E. chlorotica. Overall, these results support the active transcription of algal nuclear and plastid genes in E. chlorotica, as well as sustained activity of a nuclear-encoded plastid enzyme, even after several months of starvation (absence of algal prey). The apparent absence of tight transcriptional regulation and redox control suggests that essential nuclear-encoded regulatory factors in V. litorea are probably not present in the sea slug. These findings are discussed relative to light regulation of photosynthetic gene expression in the green and red algal lineages and in the context of the sea slug/algal plastid kleptoplastic association.

A functional component of the transcriptionally active chromosome complex, Arabidopsis pTAC14, interacts with pTAC12/HEMERA and regulates plastid gene expression

The SET domain-containing protein, pTAC14, was previously identified as a component of the transcriptionally active chromosome (TAC) complexes. Here, we investigated the function of pTAC14 in the regulation of plastid-encoded bacterial-type RNA polymerase (PEP) activity and chloroplast development. The knockout of pTAC14 led to the blockage of thylakoid formation in Arabidopsis (Arabidopsis thaliana), and ptac14 was seedling lethal. Sequence and transcriptional analysis showed that pTAC14 encodes a specific protein in plants that is located in the chloroplast associated with the thylakoid and that its expression depends on light. In addition, the transcript levels of all investigated PEP-dependent genes were clearly reduced in the ptac14-1 mutants, while the accumulation of nucleus-encoded phage-type RNA polymerase-dependent transcripts was increased, indicating an important role of pTAC14 in maintaining PEP activity. pTAC14 was found to interact with pTAC12/HEMERA, another component of TACs that is involved in phytochrome signaling. The data suggest that pTAC14 is essential for proper chloroplast development, most likely by affecting PEP activity and regulating PEP-dependent plastid gene transcription in Arabidopsis together with pTAC12.

AtECB2, a pentatricopeptide repeat protein, is required for chloroplast transcript accD RNA editing and early chloroplast biogenesis in Arabidopsis thaliana

Chloroplast biogenesis is a complex process in higher plants. Screening chloroplast biogenesis mutants, and elucidating their molecular mechanisms, will provide insight into the process of chloroplast biogenesis. In this paper, we obtained an early chloroplast biogenesis mutant atecb2 that displayed albino cotyledons and was seedling lethal. Microscopy observations revealed that the chloroplast of atecb2 mutants lacked an organized thylakoid membrane. The AtECB2 gene, which is highly expressed in cotyledons and seedlings, encodes a pentatricopeptide repeat protein (PPR) with a C-terminal DYW domain. The AtECB2 protein is localized in the chloroplast, and contains a conserved HxEx(n)CxxC motif that is similar to the activated site of cytidine deaminase. The AtECB2 mutation affects the expression pattern of plastid-encoded genes. Immunoblot analyses showed that the levels of photosynthetic proteins decreased substantially in atecb2 mutants. Inspection of all reported plastid RNA editing sites revealed that one editing site, accD, is not edited in atecb2 mutants. Therefore, the AtECB2 protein must regulate the RNA editing of this site, and the dysfunctional AccD protein from the unedited RNA molecules could lead to the mutated phenotype. All of these results indicate that AtECB2 is required for chloroplast transcript accD RNA editing and early chloroplast biogenesis in Arabidopsis thaliana.

Towards understanding the functional difference between the two PsbO isoforms in Arabidopsis thaliana--insights from phenotypic analyses of psbo knockout mutants

The extrinsic PsbO subunit of the water-oxidizing photosystem II (PSII) complex is represented by two isoforms in Arabidopsis thaliana, namely PsbO1 and PsbO2. Recent analyses of psbo1 and psbo2 knockout mutants have brought insights into their roles in photosynthesis and light stress. Here we analyzed the two psbo mutants in terms of PsbOs expression pattern, organization of PSII complexes and GTPase activity. Both PsbOs are present in wild-type plants, and their expression is mutually controlled in the mutants. Almost all PSII complexes are in the monomeric form not only in the psbo1 but also in the psbo2 mutant grown under high-light conditions. This results either from an enhanced susceptibility of PSII to photoinactivation or from malfunction of the repair cycle. Notably, the psbo1 mutant displays such problems even under growth-light conditions. These results together with the finding that PsbO2 has a threefold higher GTPase activity than PsbO1 have significance for the turnover of the PSII D1 subunit in Arabidopsis.

Analysis of Arabidopsis PsbQA gene expression in transgenic tobacco reveals differential role of its promoter and transcribed region in organ-specific and light-mediated regulation

Arabidopsis PsbQ, encoding a 16 kDa protein of the oxygen-evolving complex, is regulated by light and is expressed preferentially in leaf tissues. To analyze the components required for light-regulated and organ-specific expression of PsbQA, several promoter constructs were generated and expressed in tobacco. The 2.2 kb promoter could confer organ-specific expression of the reporter gene, whereas regulatory elements for light-dependent induction could not be located within this promoter and the transcribed region extending up to a second exon, represented by a genomic fragment encompassing the gene. The genomic fragment representing the transcribed region, however, could confer light regulation even on a constitutive promoter, as observed by steady-state mRNA analysis in T0 and T1 tobacco plants. The results obtained have led to the conclusion that regulatory elements for organ-specificity mainly reside in the promoter region whereas the transcribed region of the gene has an important role in light regulation.

Plant molecular biology and biotechnology research in the post-recombinant DNA era

After the beginning of the recombinant DNA era in the mid-1970s, researchers in India started to make use of the new technology to understand the structure of plant genes and regulation of their expression. The outcome started to appear in print in early the 1980s and genes for histones, tubulin, photosynthetic membrane proteins, phototransduction components, organelles and those regulated differentially by developmental and extrinsic signals were sequenced and characterized. Some genes of biotechnological importance like those encoding an interesting seed protein and the enzyme glyoxalase were also isolated. While work on the characterization of genome structure and organization was started quite early, it remained largely focused on the identification of DNA markers and genetic variability. In this context, the work on mustard, rice and wheat is worth mentioning. In the year 2000, India became a member of the international consortium to sequence entire rice genome. Several laboratories have also given attention to regulated expression of plastid and nuclear genes as well as to isolate target-specific promoters or design promoters with improved potential. Simultaneously, transgenic systems for crops like mustard, rice, wheat, cotton, legumes and several vegetables have been established. More recently, genes of agronomic importance like those for insect resistance, abiotic stress tolerance, nutritional improvement and male sterility, isolated in India or abroad, have been utilized for raising transgenics for crop improvement. Some of these transgenics have already shown their potential in containment facility or limited field trials conducted under the stipulated guidelines. Plant molecular biology and biotechnology are thus clearly poised to make an impact on research in basic biology and agriculture in the near future.

Characterization of an Arabidopsis thaliana mutant with impaired psbO, one of two genes encoding extrinsic 33-kDa proteins in photosystem II

A 33-kDa protein component of the oxygen-evolving complex in photosystem II is essential for photosynthesis, and it has been believed that mutants with deletion of this 33-kDa protein are not found in higher plants. We report here the first isolation of an Arabidopsis thaliana mutant with a defect in one of the genes for the 33-kDa proteins, psbO, and an intact gene (psbO2). This mutant showed considerable growth retardation, suggesting that there is a functional difference between psbO and psbO2.

Silencing of a gene encoding a protein component of the oxygen-evolving complex of photosystem II enhances virus replication in plants

It has been suggested that, in addition to viral proteins, host proteins are involved in RNA virus replication. In this study the RNA helicase domain of the Tobacco mosaic virus (TMV) replicase proteins was used as bait in the yeast two-hybrid system to identify tobacco proteins with a putative role in TMV replication. Two host proteins were characterized. One protein (designated #3) belongs to a protein family of ATPases associated with various activities (AAA), while the second host protein (designated #13) is the 33K subunit of the oxygen-evolving complex of photosystem II. Using Tobacco rattle virus vectors, genes #3 and #13 were silenced in Nicotiana benthamiana, after which the plants were challenged by TMV infection. Silencing of gene #13 resulted in a 10-fold increase of TMV accumulation, whereas silencing of gene #3 caused a twofold reduction of TMV accumulation. Additionally, silencing of genes #3 and #13 decreased and increased, respectively, the accumulation of two other viruses. Similar to silencing of gene #13, inhibition of photosystem II by application of an herbicide increased TMV accumulation several fold. Infection of N. benthamiana with TMV resulted in a decrease of #13 mRNA levels. Silencing of gene #13 may reflect a novel strategy of TMV to suppress basal host defense mechanisms. The two-hybrid screenings did not identify tobacco proteins involved in helicase domain-induced N-mediated resistance.