Proteases and cellular regulation in plants

The CUL3A-LFH1-UBC15 ubiquitin ligase complex mediates SHORT VEGETATIVE PHASE degradation to accelerate flowering at high ambient temperature

Ambient temperature affects flowering time in plants, and the MADS-box transcription factor SHORT VEGETATIVE PHASE (SVP) plays a crucial role in the response to changes in ambient temperature. SVP protein stability is regulated by the 26S proteasome pathway and decreases at high ambient temperature, but the details of SVP degradation are unclear. Here, we show that SVP degradation at high ambient temperature is mediated by the CULLIN3-RING E3 ubiquitin ligase (CRL3) complex in Arabidopsis thaliana. We identified a previously uncharacterized protein that interacts with SVP at high ambient temperature and contains a BTB/POZ domain. We named this protein LATE FLOWERING AT HIGH TEMPERATURE 1 (LFH1). Single mutants of LFH1 or CULLIN3A (CUL3A) showed late flowering specifically at 27°C. LFH1 protein levels increased at high ambient temperature. We found that LFH1 interacts with CUL3A in the cytoplasm and is important for SVP-CUL3A complex formation. Mutations in CUL3A and/or LFH1 led to increased SVP protein stability at high ambient temperature, suggesting that the CUL3-LFH1 complex functions in SVP degradation. Screening E2 ubiquitin-conjugating enzymes (UBCs) using RING-BOX PROTEIN 1 (RBX1), a component of the CRL3 complex, as bait identified UBC15. ubc15 mutants also showed late flowering at high ambient temperature. In vitro and in vivo ubiquitination assays using recombinant CUL3A, LFH1, RBX1, and UBC15 showed that SVP is highly ubiquitinated in an ATP-dependent manner. Collectively, these results indicate that the degradation of SVP at high ambient temperature is mediated by a CRL3 complex comprising CUL3A, LFH1, and UBC15.

Combined nitrogen and drought stress leads to overlapping and unique proteomic responses in potato

Nitrogen deficient and drought-tolerant or sensitive potatoes differ in proteomic responses under combined (NWD) and individual stresses. The sensitive genotype 'Kiebitz' exhibits a higher abundance of proteases under NWD. Abiotic stresses such as N deficiency and drought affect the yield of Solanum tuberosum L. tremendously. Therefore, it is of importance to improve potato genotypes in terms of stress tolerance. In this study, we identified differentially abundant proteins (DAPs) in four starch potato genotypes under N deficiency (ND), drought stress (WD), or combined stress (NWD) in two rain-out shelter experiments. The gel-free LC-MS analysis generated a set of 1177 identified and quantified proteins. The incidence of common DAPs in tolerant and sensitive genotypes under NWD indicates general responses to this stress combination. Most of these proteins were part of the amino acid metabolism (13.9%). Three isoforms of S-adenosyl methionine synthase (SAMS) were found to be lower abundant in all genotypes. As SAMS were found upon application of single stresses as well, these proteins appear to be part of the general stress response in potato. Interestingly, the sensitive genotype 'Kiebitz' showed a higher abundance of three proteases (subtilase, carboxypeptidase, subtilase family protein) and a lower abundance of a protease inhibitor (stigma expressed protein) under NWD stress compared to control plants. The comparably tolerant genotype 'Tomba', however, displayed lower abundances of proteases. This indicates a better coping strategy for the tolerant genotype and a quicker reaction to WD when previously stressed with ND.

Overexpression of seagrass DnaJ gene ZjDjB1 enhances the thermotolerance of transgenic arabidopsis thaliana

Seagrass meadows are one of the most important marine resources that grow along the coast. They provide habitat and a food source for animals. They also protect the coast, fix sediment and purify seawater. In the current period of global climate change, anomalies in coastal water temperatures are increasing. A sudden increase in water temperature owing to a heat wave can have a profound effect on seagrass. Zostera japonica is a type of intertidal seagrasses, which is exposed to the air at low tide. High temperatures in the summer often lead to a decline in seagrass meadows. DnaJ proteins, also known as J proteins, are a family of conserved chaperone proteins. They are designated as J proteins because they contain a highly conserved J domain. They function as chaperones of heat shock proteins in organisms. In this study, the role of DnaJ protein (ZjDjB1) of Z. japonica under heat stress was studied. ZjDjB1 was localized to the cytoplasm and nucleus. The overexpression of ZjDjB1 in Arabidopsis thaliana results in an increase in thermotolerance and a decrease in the accumulation of reactive oxygen species and also a reduction in membrane damage. ZjDjB1 may achieve this goal by maintaining a low activity of proteolytic enzymes.

Comparative transcriptome analysis of Zea mays in response to petroleum hydrocarbon stress

The use of plants for the improvement of soils contaminated with hydrocarbons has been a primary research focus in phytoremediation studies. Obtaining insights regarding genes that are differentially induced by petroleum hydrocarbon stress and understanding plant response mechanisms against petroleum hydrocarbons at molecular level is essential for developing better phytoremediation strategies to remove these hazardous contaminants. The purpose of this study was to analyze the transcriptomal profile changes under hydrocarbon stress in maize plants and identify the genes associated with the phytoremediative capacity. Zea mays GeneChips were used to analyze the global transcriptome profiles of maize treated with different concentrations of petroleum hydrocarbons. In total, 883, 1281, and 2162 genes were differentially induced or suppressed in the comparisons of 0 (control) vs. 1% crude petroleum, 1 vs. 5% crude petroleum, and 0 vs. 5% crude petroleum, respectively. The differentially expressed genes were functionally associated with the osmotic stress response mechanism, likely preventing the uptake of water from the roots, and the phytoremediative capacity of plants, e.g., secretory pathway genes. The results presented here show the regulatory mechanisms in the response to petroleum hydrocarbon pollution in soil. Our study provides global gene expression data of Z. mays in response to petroleum hydrocarbon stress that could be useful for further studies investigating the biodegradation mechanism in maize and other plants.

The Tomato Hybrid Proline-rich Protein regulates the abscission zone competence to respond to ethylene signals

The Tomato Hybrid Proline-rich Protein (THyPRP) gene was specifically expressed in the tomato (Solanum lycopersicum) flower abscission zone (FAZ), and its stable antisense silencing under the control of an abscission zone (AZ)-specific promoter, Tomato Abscission Polygalacturonase4, significantly inhibited tomato pedicel abscission following flower removal. For understanding the THyPRP role in regulating pedicel abscission, a transcriptomic analysis of the FAZ of THyPRP-silenced plants was performed, using a newly developed AZ-specific tomato microarray chip. Decreased expression of THyPRP in the silenced plants was already observed before abscission induction, resulting in FAZ-specific altered gene expression of transcription factors, epigenetic modifiers, post-translational regulators, and transporters. Our data demonstrate that the effect of THyPRP silencing on pedicel abscission was not mediated by its effect on auxin balance, but by decreased ethylene biosynthesis and response. Additionally, THyPRP silencing revealed new players, which were demonstrated for the first time to be involved in regulating pedicel abscission processes. These include: gibberellin perception, Ca²⁺-Calmodulin signaling, Serpins and Small Ubiquitin-related Modifier proteins involved in post-translational modifications, Synthaxin and SNARE-like proteins, which participate in exocytosis, a process necessary for cell separation. These changes, occurring in the silenced plants early after flower removal, inhibited and/or delayed the acquisition of the competence of the FAZ cells to respond to ethylene signaling. Our results suggest that THyPRP acts as a master regulator of flower abscission in tomato, predominantly by playing a role in the regulation of the FAZ cell competence to respond to ethylene signals.

Programmed cell death in plants: A chloroplastic connection

Programmed cell death (PCD) is an integral cellular program by which targeted cells culminate to demise under certain developmental and pathological conditions. It is essential for controlling cell number, removing unwanted diseased or damaged cells and maintaining the cellular homeostasis. The details of PCD process has been very well elucidated and characterized in animals but similar understanding of the process in plants has not been achieved rather the field is still in its infancy that sees some sporadic reports every now and then. The plants have 2 energy generating sub-cellular organelles- mitochondria and chloroplasts unlike animals that just have mitochondria. The presence of chloroplast as an additional energy transducing and ROS generating compartment in a plant cell inclines to advocate the involvement of chloroplasts in PCD execution process. As chloroplasts are supposed to be progenies of unicellular photosynthetic organisms that evolved as a result of endosymbiosis, the possibility of retaining some of the components involved in bacterial PCD by chloroplasts cannot be ruled out. Despite several excellent reviews on PCD in plants, there is a void on an update of information at a place on the regulation of PCD by chloroplast. This review has been written to provide an update on the information supporting the involvement of chloroplast in PCD process and the possible future course of the field.

Genome-wide digital transcript analysis of putative fruitlet abscission related genes regulated by ethephon in litchi

The high level of physiological fruitlet abscission in litchi (Litchi chinensis Sonn.) causes severe yield loss. Cell separation occurs at the fruit abscission zone (FAZ) and can be triggered by ethylene. However, a deep knowledge of the molecular events occurring in the FAZ is still unknown. Here, genome-wide digital transcript abundance (DTA) analysis of putative fruit abscission related genes regulated by ethephon in litchi were studied. More than 81 million high quality reads from seven ethephon treated and untreated control libraries were obtained by high-throughput sequencing. Through DTA profile analysis in combination with Gene Ontology and KEGG pathway enrichment analyses, a total of 2730 statistically significant candidate genes were involved in the ethephon-promoted litchi fruitlet abscission. Of these, there were 1867 early-responsive genes whose expressions were up- or down-regulated from 0 to 1 d after treatment. The most affected genes included those related to ethylene biosynthesis and signaling, auxin transport and signaling, transcription factors (TFs), protein ubiquitination, ROS response, calcium signal transduction, and cell wall modification. These genes could be clustered into four groups and 13 subgroups according to their similar expression patterns. qRT-PCR displayed the expression pattern of 41 selected candidate genes, which proved the accuracy of our DTA data. Ethephon treatment significantly increased fruit abscission and ethylene production of fruitlet. The possible molecular events to control the ethephon-promoted litchi fruitlet abscission were prompted out. The increased ethylene evolution in fruitlet would suppress the synthesis and polar transport of auxin and trigger abscission signaling. To the best of our knowledge, it is the first time to monitor the gene expression profile occurring in the FAZ-enriched pedicel during litchi fruit abscission induced by ethephon on the genome-wide level. This study will contribute to a better understanding for the molecular regulatory mechanism of fruit abscission in litchi.

Quality control of Photosystem II: the molecular basis for the action of FtsH protease and the dynamics of the thylakoid membranes

The reaction center-binding D1 protein of Photosystem II is damaged by excessive light, which leads to photoinhibition of Photosystem II. The damaged D1 protein is removed immediately by specific proteases, and a metalloprotease FtsH located in the thylakoid membranes is involved in the proteolytic process. According to recent studies on the distribution and organization of the protein complexes/supercomplexes in the thylakoid membranes, the grana of higher plant chloroplasts are crowded with Photosystem II complexes and light-harvesting complexes. For the repair of the photodamaged D1 protein, the majority of the active hexameric FtsH proteases should be localized in close proximity to the Photosystem II complexes. The unstacking of the grana may increase the area of the grana margin and facilitate easier access of the FtsH proteases to the damaged D1 protein. These results suggest that the structural changes of the thylakoid membranes by light stress increase the mobility of the membrane proteins and support the quality control of Photosystem II.

Assessment of the geographic origins of pinewood nematode isolates via single nucleotide polymorphism in effector genes

The pinewood nematode, Bursaphelenchus xylophilus, is native to North America but it only causes damaging pine wilt disease in those regions of the world where it has been introduced. The accurate detection of the species and its dispersal routes are thus essential to define effective control measures. The main goals of this study were to analyse the genetic diversity among B. xylophilus isolates from different geographic locations and identify single nucleotide polymorphism (SNPs) markers for geographic origin, through a comparative transcriptomic approach. The transcriptomes of seven B. xylophilus isolates, from Continental Portugal (4), China (1), Japan (1) and USA (1), were sequenced in the next generation platform Roche 454. Analysis of effector gene transcripts revealed inter-isolate nucleotide diversity that was validated by Sanger sequencing in the genomic DNA of the seven isolates and eight additional isolates from different geographic locations: Madeira Island (2), China (1), USA (1), Japan (2) and South Korea (2). The analysis identified 136 polymorphic positions in 10 effector transcripts. Pairwise comparison of the 136 SNPs through Neighbor-Joining and the Maximum Likelihood methods and 5-mer frequency analysis with the alignment-independent bilinear multivariate modelling approach correlated the SNPs with the isolates geographic origin. Furthermore, the SNP analysis indicated a closer proximity of the Portuguese isolates to the Korean and Chinese isolates than to the Japanese or American isolates. Each geographic cluster carried exclusive alleles that can be used as SNP markers for B. xylophilus isolate identification.

Quiescent and necrotrophic lifestyle choice during postharvest disease development

Insidious fungal infections by postharvest pathogens remain quiescent during fruit growth until, at a particular phase during fruit ripening and senescence, the pathogens switch to the necrotrophic lifestyle and cause decay. During ripening, fruits undergo physiological processes, such as activation of ethylene biosynthesis, cuticular changes, and cell-wall loosening-changes that are accompanied by a decline of antifungal compounds, both those that are preformed and those that are inducible secondary metabolites. Pathogen infection of the unripe host fruit initiates defensive signal-transduction cascades, culminating in accumulation of antifungal proteins that limit fungal growth and development. In contrast, development of the same pathogens during fruit ripening and storage activates a substantially different signaling network, one that facilitates aggressive fungal colonization. This review focuses on responses induced by the quiescent pathogens of postharvest diseases in unripe host fruits. New genome-scale experimental approaches have begun to delineate the complex and multiple networks of host and pathogen responses activated to maintain or to facilitate the transition from the quiescent to the necrotrophic lifestyle.

Caseinolytic and milk-clotting activities from Moringa oleifera flowers

This work reports the detection and characterization of caseinolytic and milk-clotting activities from Moringa oleifera flowers. Proteins extracted from flowers were precipitated with 60% ammonium sulphate. Caseinolytic activity of the precipitated protein fraction (PP) was assessed using azocasein, as well as α(s)-, β- and κ-caseins as substrates. Milk-clotting activity was analysed using skim milk. The effects of heating (30-100°C) and pH (3.0-11.0) on enzyme activities were determined. Highest caseinolytic activity on azocasein was detected after previous incubation of PP at pH 4.0 and after heating at 50°C. Milk-clotting activity, detected only in the presence of CaCl(2), was highest at incubation of PP at pH 3.0 and remained stable up to 50°C. The pre-treatment of milk at 70°C resulted in highest clotting activity. Enzyme assays in presence of protease inhibitors indicated the presence of aspartic, cysteine, serine and metallo proteases. Aspartic proteases appear to be the main enzymes involved in milk-clotting activity. PP promoted extensive cleavage of κ-casein and low level of α(s)- and β-caseins hydrolysis. The milk-clotting activity indicates the application of M. oleifera flowers in dairy industry.

Proteomic analysis of ripening tomato fruit infected by Botrytis cinerea

Botrytis cinerea, a model necrotrophic fungal pathogen that causes gray mold as it infects different organs on more than 200 plant species, is a significant contributor to postharvest rot in fresh fruit and vegetables, including tomatoes. By describing host and pathogen proteomes simultaneously in infected tissues, the plant proteins that provide resistance and allow susceptibility and the pathogen proteins that promote colonization and facilitate quiescence can be identified. This study characterizes fruit and fungal proteins solubilized in the B. cinerea-tomato interaction using shotgun proteomics. Mature green, red ripe wild type and ripening inhibited (rin) mutant tomato fruit were infected with B. cinerea B05.10, and the fruit and fungal proteomes were identified concurrently 3 days postinfection. One hundred eighty-six tomato proteins were identified in common among red ripe and red ripe-equivalent ripening inhibited (rin) mutant tomato fruit infected by B. cinerea. However, the limited infections by B. cinerea of mature green wild type fruit resulted in 25 and 33% fewer defense-related tomato proteins than in red and rin fruit, respectively. In contrast, the ripening stage of genotype of the fruit infected did not affect the secreted proteomes of B. cinerea. The composition of the collected proteins populations and the putative functions of the identified proteins argue for their role in plant-pathogen interactions.

Manipulation of monoubiquitin improves salt tolerance in transgenic tobacco

Ubiquitin (Ub) is regarded as a stress protein involved in many stress responses. In this paper, sense and antisense transgenic tobacco plants, as well as the wild type and vector control, were used to study the role of Ub in salt tolerance of plants. In sense Ta-Ub2 transgenic tobacco plants, there was higher expression of Ub protein conjugates than in the wild type and vector control, but the reverse trend was observed in antisense Nt-Ub1 transgenic plants. The germination rate of tobacco seed, growth status and photosynthesis of the tobacco plants suggested that over-expressing Ub promoted the growth of transgenic tobacco plants and enhanced their salt tolerance, but the opposite effect was seen in plants with repressed Ub expression. Changes in antioxidant capacity may be one of the mechanisms underlying Ub-regulated salt tolerance. Furthermore, improved tolerance to a combination of stresses was also observed in the sense transgenic tobacco plants. These findings imply that Ub is involved in the tolerance of plants to abiotic stress.

Functional roles of effectors of plant-parasitic nematodes

Plant pathogens have evolved a variety of different strategies that allow them to successfully infect their hosts. Plant-parasitic nematodes secrete numerous proteins into their hosts. These proteins, called effectors, have various functions in the plant cell. The most studied effectors to date are the plant cell wall degrading enzymes, which have an interesting evolutionary history since they are believed to have been acquired from bacteria or fungi by horizontal gene transfer. Extensive genome, transcriptome and proteome studies have shown that plant-parasitic nematodes secrete many additional effectors. The function of many of these is less clear although during the last decade, several research groups have determined the function of some of these effectors. Even though many effectors remain to be investigated, it has already become clear that they can have very diverse functions. Some are involved in suppression of plant defences, while others can specifically interact with plant signalling or hormone pathways to promote the formation of nematode feeding sites. In this review, the most recent progress in the understanding of the function of plant-parasitic nematode effectors is discussed.

Production of plant proteases in vivo and in vitro--a review

In the latest two decades, the interest received by plant proteases has increased significantly. Plant enzymes such as proteases are widely used in medicine and the food industry. Some proteases, like papain, bromelain and ficin are used in various processes such as brewing, meat softening, milk-clotting, cancer treatment, digestion and viral disorders. These enzymes can be obtained from their natural source or through in vitro cultures, in order to ensure a continuous source of plant enzymes. The focus of this review will be the production of plant proteases both in vivo and in vitro, with particular emphasis on the different types of commercially important plant proteases that have been isolated and characterized from naturally grown plants. In vitro approaches for the production of these proteases is also explored, focusing on the techniques that do not involve genetic transformation of the plants and the attempts that have been made in order to enhance the yield of the desired proteases.

An ordered EST catalogue and gene expression profiles of cassava (Manihot esculenta) at key growth stages

A cDNA library was constructed from the root tissues of cassava variety Huanan 124 at the root bulking stage. A total of 9,600 cDNA clones from the library were sequenced with single-pass from the 5'-terminus to establish a catalogue of expressed sequence tags (ESTs). Assembly of the resulting EST sequences resulted in 2,878 putative unigenes. Blastn analysis showed that 62.6% of the unigenes matched with known cassava ESTs and the rest had no 'hits' against the cassava database in the integrative PlantGDB database. Blastx analysis showed that 1,715 (59.59%) of the unigenes matched with one or more GenBank protein entries and 1,163 (40.41%) had no 'hits'. A cDNA microarray with 2,878 unigenes was developed and used to analyze gene expression profiling of Huanan 124 at key growth stages including seedling, formation of root system, root bulking, and starch maturity. Array data analysis revealed that (1) the higher ratio of up-regulated ribosome-related genes was accompanied by a high ratio of up-regulated ubiquitin, proteasome-related and protease genes in cassava roots; (2) starch formation and degradation simultaneously occur at the early stages of root development but starch degradation is declined partially due to decrease in UDP-glucose dehydrogenase activity with root maturity; (3) starch may also be synthesized in situ in roots; (4) starch synthesis, translocation, and accumulation are also associated probably with signaling pathways that parallel Wnt, LAM, TCS and ErbB signaling pathways in animals; (5) constitutive expression of stress-responsive genes may be due to the adaptation of cassava to harsh environments during long-term evolution.

Isolation of genes differentially expressed during development and ripening of Fragaria chiloensis fruit by suppression subtractive hybridization

Fragaria chiloensis, the native Chilean strawberry, is noted for its good fruit quality characters. However, it is a highly perishable fruit due to its rapid softening. With the aim to screen for genes differentially expressed during development and ripening of strawberry fruit, the subtractive suppressive hybridization (SSH) methodology was employed. Six libraries were generated contrasting transcripts from four different developmental stages. A set of 1807 genes was isolated and characterized. In our EST collection, approximately 90% of partial cDNAs showed significant similarity to proteins with known or unknown function registered in databases. Among them, proteins related to protein fate were identified in a large green fruit library and protein related with cellular transport, cell wall-related proteins, and transcription regulators were identified in a ripe fruit library. Thirteen genes were analyzed by qRT-PCR during development and ripening of the Chilean strawberry fruit. The information generated in this study provides new clues to aid the understanding of the ripening process in F. chiloensis fruit.

Ubiquitination of LHY by SINAT5 regulates flowering time and is inhibited by DET1

Ubiquitin is a small polypeptide and ubiquitination is the post-translational modification by ubiquitin protein, resulting in degradation of target proteins by the 26S proteasome complex. Here, we found that E3 ubiquitin ligase SINAT5, an Arabidopsis homologue of the Drosophila SINA RING-finger protein, interacts directly with LHY, a component of the circadian oscillator, and DET1, a negative regulator of light-regulated gene expression. We also found that SINAT5 has E3 ubiquitination activity for LHY but not for DET1. Interestingly, LHY ubiquitination by SINAT5 was inhibited by DET1. Late flowering of sinat5 mutants indicates that flowering time can be controlled by DET1 through regulation of LHY stability by SINAT5.

Tubers from potato lines expressing a tomato Kunitz protease inhibitor are substantially equivalent to parental and transgenic controls

Recombinant protease inhibitors represent useful tools for the development of insect-resistant transgenic crops, but questions have been raised in recent years about the impact of these proteins on endogenous proteases and chemical composition of derived food products. In this study, we performed a detailed compositional analysis of tubers from potato lines expressing the broad-spectrum inhibitor of Ser and Asp proteases, tomato cathepsin D inhibitor (SlCDI), to detect possible unintended effects on tuber composition. A compositional analysis of key nutrients and toxic chemicals was carried out with tubers of SlCDI-expressing and control (comparator) lines, followed by a two-dimensional gel electrophoresis (2-DE) proteomic profiling of total and allergenic proteins to detect eventual effects at the proteome level. No significant differences were observed among control and SlCDI-expressing lines for most chemicals assayed, in line with the very low abundance of SlCDI in tubers. Likewise, proteins detected after 2-DE showed no quantitative variation among the lines, except for a few proteins in some control and test lines, independent of slcdi transgene expression. Components of the patatin storage protein complex and Kunitz protease inhibitors immunodetected after 2-DE showed unaltered deposition patterns in SlCDI-expressing lines, clearly suggesting a null impact of slcdi on the intrinsic allergenic potential of potato tubers. These data suggest, overall, a null impact of slcdi expression on tuber composition and substantial equivalence between comparator and SlCDI-expressing tubers despite reported effects on leaf protein catabolism. They also illustrate the usefulness of proteomics as a tool to assess the authenticity of foods derived from novel-generation transgenic plants.

DOR: a link between an F-box protein and guard cell ABA signaling

Guard cells are a model system for studying signal transduction. F-box proteins, representing one of the largest gene families in Arabidopsis, have been shown to be involved in many developmental and physiological processes, including stress responses. However, it is unclear if there is a direct link between an F-box protein and the guard cell ABA signaling. DOR is a guard cell-preferential F-box protein, and our results suggested that it likely forms two negative feedback regulatory loops for the ABA-induced stomatal closure under drought conditions in Arabidopsis. These findings have a potential impact on genetically modifying drought stress responses in plants.

Biochemical and proteomic analysis of 'Dixiland' peach fruit (Prunus persica) upon heat treatment

Shipping of peaches to distant markets and storage require low temperature; however, cold storage affects fruit quality causing physiological disorders collectively termed 'chilling injury' (CI). In order to ameliorate CI, different strategies have been applied before cold storage; among them heat treatment (HT) has been widely used. In this work, the effect of HT on peach fruit quality as well as on carbon metabolism was evaluated. When fruit were exposed to 39 degrees C for 3 d, ripening was delayed, with softening inhibition and slowing down of ethylene production. Several differences were observed between fruit ripening at ambient temperature versus fruit that had been heat treated. However, the major effects of HT on carbon metabolism and organoleptic characteristics were reversible, since normal fruit ripening was restored after transferring heated peaches to ambient temperature. Positive quality features such as an increment in the fructose content, largely responsible for the sweetness, and reddish coloration were observed. Nevertheless, high amounts of acetaldehyde and low organic acid content were also detected. The differential proteome of heated fruit was characterized, revealing that heat-induced CI tolerance may be acquired by the activation of different molecular mechanisms. Induction of related stress proteins in the heat-exposed fruits such as heat shock proteins, cysteine proteases, and dehydrin, and repression of a polyphenol oxidase provide molecular evidence of candidate proteins that may prevent some of the CI symptoms. This study contributes to a deeper understanding of the cellular events in peach under HT in view of a possible technological use aimed to improve organoleptic and shelf-life features.

Seven in absentia proteins affect plant growth and nodulation in Medicago truncatula

Protein ubiquitination is a posttranslational regulatory process essential for plant growth and interaction with the environment. E3 ligases, to which the seven in absentia (SINA) proteins belong, determine the specificity by selecting the target proteins for ubiquitination. SINA proteins are found in animals as well as in plants, and a small gene family with highly related members has been identified in the genome of rice (Oryza sativa), Arabidopsis (Arabidopsis thaliana), Medicago truncatula, and poplar (Populus trichocarpa). To acquire insight into the function of SINA proteins in nodulation, a dominant negative form of the Arabidopsis SINAT5 was ectopically expressed in the model legume M. truncatula. After rhizobial inoculation of the 35S:SINAT5DN transgenic plants, fewer nodules were formed than in control plants, and most nodules remained small and white, a sign of impaired symbiosis. Defects in rhizobial infection and symbiosome formation were observed by extensive microscopic analysis. Besides the nodulation phenotype, transgenic plants were affected in shoot growth, leaf size, and lateral root number. This work illustrates a function for SINA E3 ligases in a broad spectrum of plant developmental processes, including nodulation.

Physiological characterization and identification of genes differentially expressed in response to drought induced by PEG 6000 in Populus canadensis leaves

We report here about the physiological and molecular responses of Populus canadensis (clone Dorskamp) to drought. The stress was applied to young rooted cuttings by PEG 6000 application over 30 days. This stress induces a decrease in predawn leaf water potential. After 10 days of stress, there was a decrease in stomatal conductance and a slight retardation of leaf growth, but the osmotic potential remained constant. Using the differential display technique, we searched for genes differentially expressed in response to drought at this date. Thirty-six differentially expressed leaf cDNAs were detected between stressed and control conditions. Thirty-four cDNAs clones were successfully cloned and 23 were found to share high identity with Arabidopsis thaliana and Populus trichocarpa genes. The transcriptional regulation of 21 genes was examined by reverse RNA dot blot, confirming an increase in expression for 16 of them after 10 days of treatment. Among these 16 genes, most of them are involved in a different cellular metabolic pathway. These differentially expressed genes are also involved and/or regulated by other treatments such as salt, withholding water or auxin application. The maintenance of growth observed during the first 10 days of the stress period could be due to the regulation of these genes and can be a common response between herbaceous plants and trees.

Kunitz trypsin inhibitor: an antagonist of cell death triggered by phytopathogens and fumonisin b1 in Arabidopsis

Programmed cell death (PCD) is a central regulatory process in both plant development and in plant responses to pathogens. PCD requires a coordinate activation of pro-apoptotic factors such as proteases and suppressors inhibiting and modulating these processes. In plants, various caspase-like cysteine proteases as well as serine proteases have been implicated in PCD. Here, we show that a serine protease (Kunitz trypsin) inhibitor (KTI1) of Arabidopsis acts as a functional KTI when produced in bacteria and in planta. Expression of AtKTI1 is induced late in response to bacterial and fungal elicitors and to salicylic acid. RNAi silencing of the AtKTI1 gene results in enhanced lesion development after infiltration of leaf tissue with the PCD-eliciting fungal toxin fumonisin B1 (FB1) or the avirulent bacterial pathogen Pseudomonas syringae pv tomato DC3000 carrying avrB (Pst avrB). Overexpression of AtKTI1 results in reduced lesion development after Pst avrB and FB1 infiltration. Interestingly, RNAi silencing of AtKTI1 leads to enhanced resistance to the virulent pathogen Erwinia carotovora subsp. carotovora SCC1, while overexpression of AtKTI1 leads to higher susceptibility towards this pathogen. Together, these data indicate that AtKTI1 is involved in modulating PCD in plant-pathogen interactions.

Evolution of the type III secretion system and its effectors in plant-microbe interactions

Many bacterial plant pathogens require the type III secretion system (T3SS) and its effector proteins (T3SEs) to invade and extract nutrients from their hosts successfully. While the molecular function of this system is being studied intensively, we know comparatively little about the evolutionary and ecological pressures governing its fate over time, and even less about the detailed mechanisms underlying and driving complex T3SS-mediated coevolutionary dynamics. In this review we summarize our current understanding of how host-pathogen interactions evolve, with a particular focus on the T3SS of bacterial plant pathogens. We explore the evolutionary origins of the T3SS relative to the closely related flagellar system, and investigate the evolutionary pressures on this secretion and translocation apparatus. We examine the evolutionary forces acting on T3SEs, and compare the support for vertical descent with modification of these virulence-associated systems (pathoadaptation) vs horizontal gene transfer. We address the evolutionary origins of T3SEs from the perspective of both the evolutionary mechanisms that generate new effectors, and the mobile elements that may be the source of novel genetic material. Finally, we propose a number of questions raised by these studies, which may serve to guide our thinking about these complex processes.

Auxin in action: signalling, transport and the control of plant growth and development

Hormones have been at the centre of plant physiology research for more than a century. Research into plant hormones (phytohormones) has at times been considered as a rather vague subject, but the systematic application of genetic and molecular techniques has led to key insights that have revitalized the field. In this review, we will focus on the plant hormone auxin and its action. We will highlight recent mutagenesis and molecular studies, which have delineated the pathways of auxin transport, perception and signal transduction, and which together define the roles of auxin in controlling growth and patterning.

Type III effector proteins: doppelgangers of bacterial virulence

Bacterial pathogens have co-evolved with their hosts in their ongoing quest for advantage in the resulting interaction. These intimate associations have resulted in remarkable adaptations of prokaryotic virulence proteins and their eukaryotic molecular targets. An important strategy used by microbial pathogens of animals to manipulate host cellular functions is structural mimicry of eukaryotic proteins. Recent evidence demonstrates that plant pathogens also use structural mimicry of host factors as a virulence strategy. Nearly all virulence proteins from phytopathogenic bacteria have eluded functional annotation on the basis of primary amino-acid sequence. Recent efforts to determine their three-dimensional structures are, however, revealing important clues about the mechanisms of bacterial virulence in plants.

A bacterial inhibitor of host programmed cell death defenses is an E3 ubiquitin ligase

The Pseudomonas syringae protein AvrPtoB is translocated into plant cells, where it inhibits immunity-associated programmed cell death (PCD). The structure of a C-terminal domain of AvrPtoB that is essential for anti-PCD activity reveals an unexpected homology to the U-box and RING-finger components of eukaryotic E3 ubiquitin ligases, and we show that AvrPtoB has ubiquitin ligase activity. Mutation of conserved residues involved in the binding of E2 ubiquitin-conjugating enzymes abolishes this activity in vitro, as well as anti-PCD activity in tomato leaves, which dramatically decreases virulence. These results show that Pseudomonas syringae uses a mimic of host E3 ubiquitin ligases to inactivate plant defenses.

Inducible trans-activation of plastid transgenes: expression of the R. eutropha phb operon in transplastomic tobacco

Deleterious effects of constitutive transgene expression can occur if gene products are harmful to the transformed plant. Constraints such as growth inhibition and male sterility have been observed in plastid transformants containing the phb operon encoding the genes required for the production of the polyester polyhydroxybutyric acid (PHB). In order to induce PHB synthesis in tobacco in a well-timed manner, we have constructed a trans-activation system to regulate transcription of the phb operon in plastids. This system consists of a nuclear-located, ethanol-inducible T7RNA polymerase (T7RNAP) which is targeted to plastids harboring the phb operon under control of T7 regulatory elements. Following treatment with 5% ethanol, moderate induction of PHB synthesis was found. PHB amounts reached 1,383 ppm in dry weight, and an overall background activity of 171 ppm was measured in uninduced tissues. On the transcriptional level, T7RNAP induction was proven and we found that the phb operon is transcribed into at least two mRNAs. Without ethanol induction, development of flowers and fertile seeds was possible. Thus, the main problem of inhibitory transgene expression was solved. Our results show that this inducible trans-activation system could serve as an alternative to constitutive expression of transgenes in the plastome.

Proteome analysis ofArabidopsis thaliana by two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionisation-time of flight mass spectrometry

In the present study we show results of a large-scale proteome analysis of the recently sequenced plant Arabidopsis thaliana. On the basis of a previously published sequential protein extraction protocol, we prepared protein extracts from eight different A. thaliana tissues (primary leaf, leaf, stem, silique, seedling, seed, root, and inflorescence) and analysed these by two-dimensional gel electrophoresis. A total of 6000 protein spots, from three of these tissues, namely primary leaf, silique and seedling, were excised and the contained proteins were analysed by matrix assisted laser desorption/ionisation time of flight mass spectrometry peptide mass fingerprinting. This resulted in the identification of the proteins contained in 2943 spots, which were found to be products of 663 different genes. In this report we present and discuss the methodological and biological results of our plant proteome analysis.

A novel plant cysteine protease has a dual function as a regulator of 1-aminocyclopropane-1-carboxylic Acid synthase gene expression

The hormone ethylene influences plant growth, development, and some defense responses. The fungal elicitor Ethylene-Inducing Xylanase (EIX) elicits ethylene biosynthesis in tomato (Lycopersicon esculentum) and tobacco (Nicotiana tabacum) leaves by induction of 1-aminocyclopropane-1-caboxylic acid synthase (Acs) gene expression. A minimal promoter element in the LeAcs2 gene required for EIX responsiveness was defined by deletion analysis in transgenic tomato plants. The sequence between -715 and -675 of the tomato Acs2 gene was found to be essential for induction by EIX. A Cys protease (LeCp) was isolated that specifically binds to this cis element in vitro. Ectopic expression of LeCp in tomato leaves induced the expression of Acs2. Moreover, chromatin immunoprecipitation showed that LeCp binds in vivo to the Acs promoter. We propose a mechanism for the dual function of the LeCp protein. The protease acts enzymatically in the cytoplasm. Then, upon signaling, a small ubiquitin-related modifier protein binds to it, enabling entrance into the nucleus, where it acts as a transcription factor. Thus, LeCp can be considered a dual-function protein, having enzymatic activity and, upon elicitor signaling, exhibiting transcriptional factor activity that induces LeAcs2 expression.

High heterogeneity within the ribosomal proteins of the Arabidopsis thaliana 80S ribosome

Proteomic studies have addressed the composition of plant chloroplast ribosomes and 70S ribosomes from the unicellular organism Chlamydomonas reinhardtii But comprehensive characterization of cytoplasmic 80S ribosomes from higher plants has been lacking. We have used two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS) to analyse the cytoplasmic 80S ribosomes from the model flowering plant Arabidopsis thaliana. Of the 80 ribosomal protein families predicted to comprise the cytoplasmic 80S ribosome, we have confirmed the presence of 61; specifically, 27 (84%) of the small 40S subunit and 34 (71%) of the large 60S subunit. Nearly half (45%) of the ribosomal proteins identified are represented by two or more distinct spots in the 2-DE gel indicating that these proteins are either post-translationally modified or present as different isoforms. Consistently, MS-based protein identification revealed that at least one-third (34%) of the identified ribosomal protein families showed expression of two or more family members. In addition, we have identified a number of non-ribosomal proteins that co-migrate with the plant 80S ribosomes during gradient centrifugation suggesting their possible association with the 80S ribosomes. Among them, RACK1 has recently been proposed to be a ribosome-associated protein that promotes efficient translation in yeast. The study, thus provides the basis for further investigation into the function of the other identified non-ribosomal proteins as well as the biological meaning of the various ribosomal protein isoforms.

Catalase deficiency drastically affects gene expression induced by high light in Arabidopsis thaliana

In plants, hydrogen peroxide (H(2)O(2)) plays a major signaling role in triggering both a defense response and cell death. Increased cellular H(2)O(2) levels and subsequent redox imbalances are managed at the production and scavenging levels. Because catalases are the major H(2)O(2) scavengers that remove the bulk of cellular H(2)O(2), altering their levels allows in planta modulation of H(2)O(2) concentrations. Reduced peroxisomal catalase activity increased sensitivity toward both ozone and photorespiratory H(2)O(2)-induced cell death in transgenic catalase-deficient Arabidopsis thaliana. These plants were used as a model system to build a comprehensive inventory of transcriptomic variations, which were triggered by photorespiratory H(2)O(2) induced by high-light (HL) irradiance. In addition to an H(2)O(2)-dependent and -independent type of transcriptional response during light stress, microarray analysis on both control and transgenic catalase-deficient plants, exposed to 0, 3, 8, and 23 h of HL, revealed several specific regulatory patterns of gene expression. Thus, photorespiratory H(2)O(2) has a direct impact on transcriptional programs in plants.

The 26S proteasome in garlic (Allium sativum): purification and partial characterization

The 26S proteasome (multicatalytic protease complex, MPC) was purified from fresh garlic cloves (Allium sativum) to near homogeneity by ion exchange chromatography on DEAE-sephacel, gel filtration on Sepharose-4B, and glycerol density gradient centrifugation. Two alpha-type (20S proteasome "catalytic core") subunits were identified by the direct sequencing of peptide fragments (mass fingerprint analysis, Mass Spectrometry Lab, Stanford University) or the sequencing of a cloned cDNA generated using a garlic cDNA library as the template; these subunits were found to have a high homology to those from other plants. Polyacrylamide gel electrophoresis under denaturing conditions separated the garlic MPC into multiple polypeptides having molecular masses in the range of 21-35 (components of the 20S catalytic core) and 55-100 kDa (components of the 19S regulatory units). The banding pattern of the garlic MCP is similar to that of spinach and rat liver with minor differences in some components; however, polyclonal antibodies against mammalian proteasomes failed to significantly stain the enzyme from garlic. This is the first work to identify the garlic proteasome.

Transcript profiling of early lateral root initiation

At the onset of lateral root initiation in Arabidopsis thaliana, the phytohormone auxin activates xylem pole pericycle cells for asymmetric cell division. However, the molecular events leading from auxin to lateral root initiation are poorly understood, in part because the few responsive cells in the process are embedded in the root and are thus difficult to access. A lateral root induction system, in which most xylem pole pericycle cells were synchronously activated by auxin transport inhibition followed by auxin application, was used for microarray transcript profiling. Of 4,600 genes analyzed, 906 significantly differentially regulated genes were identified that could be grouped into six major clusters. Basically, three major patterns were discerned representing induced, repressed, and transiently expressed genes. Analysis of the coregulated genes, which were specific for each time point, provided new insight into the molecular regulation and signal transduction preceding lateral root initiation in Arabidopsis. The reproducible expression profiles during a time course allowed us to define four stages that precede the cell division in the pericycle. These early stages were characterized by G1 cell cycle block, auxin perception, and signal transduction, followed by progression over G1/S transition and G2/M transition. All these processes took place within 6 h after transfer from N-1-naphthylphthalamic acid to 1-naphthalene acetic acid. These results indicate that this lateral root induction system represents a unique synchronized system that allows the systematic study of the developmental program upstream of the cell cycle activation during lateral root initiation.

Putative protease inhibitor gene discovery and transcript profiling during fruit development and leaf damage in grapefruit (Citrus paradisi Macf.)

Seven putative protease inhibitor (PPI) cDNAs, representing four protein families, were isolated from a grapefruit (Citrus paradisi Macf. Cv. Marsh) immature fruit flavedo cDNA library. Cloned open reading frames encoded proteins with similarity to, and protein signatures for: legume Kuntiz inhibitors (lkiL-1, lkiL-2, lkiL-3), potato trypsin inhibitor I (ptiIL-1), serpins (serpL-1), cystatins (cystL-1), and gamma thionins (gthL-1). Response of transcript abundance to fruit development and leaf wounding was determined for all but lkiL-1 using real-time RT-PCR. Immature leaves had the highest transcript levels for all PPIs. The gthL-1 transcript in immature leaves was the most abundant transcript but was absent from healthy mature leaves. In fruit flavedo, transcripts for all PPIs were most abundant in youngest fruit (<15 mm dia. fruit), and declined during development, but displayed different patterns of developmental change. Mechanical or Diaprepes root weevil (DRW) feeding damage to leaves caused a <10-fold reduction or had no effect on transcript level with the exception of gthL-1 which, as a result of damage, increased >50-fold in mature leaves and decreased >1400-fold in immature leaves. This developmental control of transcript response to wounding in a woody perennial is opposite of what has been observed for defensive proteinase inhibitors (PIs) in other plants (typically herbaceous and/or annual plants), where younger leaves typically invoke a higher defensive proteinase inhibitor transcript accumulation than older tissues. Except for gthL-1, the PPI transcripts were minimally responsive or unresponsive to wounding. Changes in PPI transcript levels suggest diverse roles for the products of these genes in citrus, with only gthL-1 responding in a defense-like manner.

The diverse roles of ubiquitin and the 26S proteasome in the life of plants

A tightly regulated and highly specific system for the degradation of individual proteins is essential for the survival of all organisms. In eukaryotes, this is achieved by the tagging of proteins with ubiquitin and their subsequent recognition and degradation by the 26S proteasome. In plants, genetic analysis has identified many genes that regulate developmental pathways. Subsequent analysis of these genes has implicated ubiquitin and the 26S proteasome in the control of diverse developmental processes, and indicates that proteolysis is a crucial regulatory step throughout the life cycle of plants.

The S8 serine, C1A cysteine and A1 aspartic protease families in Arabidopsis

The Arabidopsis thaliana genome has over 550 protease sequences representing all five catalytic types: serine, cysteine, aspartic acid, metallo and threonine (MEROPS peptidase database, http://merops.sanger.ac.uk/), which probably reflect a wide variety of as yet unidentified functions performed by plant proteases. Recent indications that the 26S proteasome, a T1 family-threonine protease, is a regulator of light and hormone responsive signal transduction highlight the potential of proteases to participate in many aspects of plant growth and development. Recent discoveries that proteases are required for stomatal distribution, embryo development and disease resistance point to wider roles for four additional multigene families that include some of the most frequently studied (yet poorly understood) plant proteases: the subtilisin-like, serine proteases (family S8), the papain-like, cysteine proteases (family C1A), the pepsin-like, aspartic proteases (family A1) and the plant matrixin, metalloproteases (family M10A). In this report, 54 subtilisin-like, 30 papain-like and 59 pepsin-like proteases from Arabidopsis, are compared with S8, C1A and A1 proteases known from other plant species at the functional, phylogenetic and gene structure levels. Examples of structural conservation between S8, C1A and A1 genes from rice, barley, tomato and soybean and those from Arabidopsis are noted, indicating that some common, essential plant protease roles were established before the divergence of monocots and eudicots. Numerous examples of tandem duplications of protease genes and evidence for a variety of restricted expression patterns suggest that a high degree of specialization exists among proteases within each family. We propose that comprehensive analysis of the functions of these genes in Arabidopsis will firmly establish serine, cysteine and aspartic proteases as regulators and effectors of a wide range of plant processes.

A comprehensive analysis of hydrogen peroxide-induced gene expression in tobacco

Hydrogen peroxide plays a central role in launching the defense response during stress in plants. To establish a molecular profile provoked by a sustained increase in hydrogen peroxide levels, catalase-deficient tobacco plants (CAT1AS) were exposed to high light (HL) intensities over a detailed time course. The expression kinetics of >14000 genes were monitored by using transcript profiling technology based on cDNA-amplified fragment length polymorphism. Clustering and sequence analysis of 713 differentially expressed transcript fragments revealed a transcriptional response that mimicked that reported during both biotic and abiotic stresses, including the up-regulation of genes involved in the hypersensitive response, vesicular transport, posttranscriptional processes, biosynthesis of ethylene and jasmonic acid, proteolysis, mitochondrial metabolism, and cell death, and was accompanied by a very rapid up-regulation of several signal transduction components. Expression profiling corroborated by functional experiments showed that HL induced photoinhibition in CAT1AS plants and that a short-term HL exposure of CAT1AS plants triggered an increased tolerance against a subsequent severe oxidative stress.

Polyester synthesis in transplastomic tobacco (Nicotiana tabacum L.): significant contents of polyhydroxybutyrate are associated with growth reduction

The pathway for synthesis of polyhydroxybutyrate (PHB), a polyester produced by three bacterial enzymes, was transferred to the tobacco plastid genome by the biolistic transformation method. The polycistronic phb operon encoding this biosynthetic pathway was cloned into plastome transformation vectors. Following selection and regeneration, the content and structure of plant-produced hydroxybutyrate was analysed by gas chromatography. Significant PHB synthesis was limited to the early stages of in vitro culture. Within the transformants, PHB synthesis levels were highly variable. In the early regeneration stage, single regenerates reached up to 1.7% PHB in dry weight. At least 70% of plant-produced hydroxybutyric acid was proven to be polymer with a molecular mass of up to 2,500 kDa. PHB synthesis levels of the transplastomic lines were decreasing when grown autotrophically but their phb transcription levels remained stable. Transcription of the three genes is divided into two transcripts with phbB being transcribed separately from phbC and phbA. In mature plants even low amounts of PHB were associated with male sterility. Fertility was only observed in a mutant carrying a defective phb operon. These results prove successful expression of the entire PHB pathway in plastids, concomitant, however, with growth deficiency and male sterility.

A novel C-terminal proteolytic processing of cytosolic pyruvate kinase, its phosphorylation and degradation by the proteasome in developing soybean seeds

Cytosolic pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40) is an important glycolytic enzyme, but the post-translational regulation of this enzyme is poorly understood. Sequence analysis of the soybean seed enzyme suggested the potential for two phosphorylation sites: site-1 (FVRKGS220DLVN) and site-2 (VLTRGGS407TAKL). Sequence- and phosphorylation state-specific antipeptide antibodies established that cytosolic pyruvate kinase (PyrKinc) is phosphorylated at both sites in vivo. However, by SDS-PAGE, the phosphorylated polypeptides were found to be smaller (20-51 kDa) than the full length (55 kDa). Biochemical separations of seed proteins by size exclusion chromatography and sucrose-density gradient centrifugation revealed that the phosphorylated polypeptides were associated with 26S proteasomes. The 26S proteasome particle in developing seeds was determined to be of approximately 1900 kDa. In vitro, the 26S proteasome degraded associated PyrKinc polypeptides, and this was blocked by proteasome-specific inhibitors such as MG132 and NLVS. By immunoprecipitation, we found that some part of the phosphorylated PyrKinc was conjugated to ubiquitin and shifted to high molecular mass forms in vivo. Moreover, recombinant wild-type PyrKinc was ubiquitinated in vitro to a much greater extent than the S220A and S407A mutant proteins, suggesting a link between phosphorylation and ubiquitination. In addition, during seed development, a progressive accumulation of a C-terminally truncated polypeptide of approximately 51 kDa was observed that was in parallel with a loss of the full-length 55 kDa polypeptide. Interestingly, the C-terminal 51 kDa truncation showed not only pyruvate kinase activity but also activation by aspartate. Collectively, the results suggest that there are two pathways for PyrKinc modification at the post-translational level. One involves partial C-terminal truncation to generate a 51 kDa pyruvate kinase subunit which might have altered regulatory properties and the other involves phosphorylation and ubiquitin conjugation that targets the protein to the 26S proteasome for complete degradation.

Biochemical identification of proteasome-associated endonuclease activity in sunflower

Proteasomes have been purified from sunflower hypocotyles. They elute with a molecular mass of 600 kDa from gel filtration columns and two-dimensional gel electrophoresis indicates that the complex contains at least 20 different protein subunits. Peptide microsequencing revealed the presence of four subunits homologous to subunits Beta2, Beta6, Alpha5 and Alpha6 of plant proteasomes. These proteasomes have chymotrypsin-like activity and the highly purified fraction of this complex is associated with an endonuclease activity hydrolyzing Tobacco mosaic virus RNA and Lettuce mosaic virus RNA with a cleavage pattern showing fragments of well-defined size. This is the first evidence of a RNA endonuclease activity associated with plant proteasomes.

The COP9 signalosome: regulating plant development through the control of proteolysis

The COP9 signalosome (CSN) is a multiprotein complex that was initially identified in plants as a repressor of photomorphogenesis. It is now known to play major roles in several other developmental pathways, from auxin response to flower development. Furthermore, the COP9 signalosome shares homologies with the lid sibcomplex of the proteasome and is evolutionarily conserved from fission yeast to humans. It is important for the proper development of virtually all higher eukaryotes. In recent years, significant progress has been made in unraveling the molecular, cellular, and physiological mode of action of the COP9 signalosome. This review discusses our current understanding of the COP9 signalosome function with particular emphasis on its recently defined role in modulating a wide variety of cellular processes by regulating specific protein degradation events.

Two different novel cis-acting elements of erd1, a clpA homologous Arabidopsis gene function in induction by dehydration stress and dark-induced senescence

Many plant genes have been shown to be induced by water stress and function in stress tolerance. The erd1 gene has been shown to be upregulated in response to both water stress and etiolation. Promoter studies using the erd1 promoter region fused to the luciferase (LUC) reporter gene in Arabidopsis thaliana were performed to identify the putative cis elements involved. Results indicated that the cis elements, responsible for gene expression during dehydration and etiolation, are separately located in two discrete portions of the erd1 promoter. Base substitution analysis showed that a 14-bp region from -599 to -586, and a myc recognition motif from -466 to -461 are necessary for the induction of LUC activity in dehydrated plants. On the other hand, base substitution analysis revealed that both an abscisic acid responsive element (ABRE)-like sequence (from -199 to -195) and an ACGT sequence (from -155 to -152) are required for an etiolation-induced increase in LUC activity. LUC activity measurements from etiolated transgenic plants incubated in either water, N6-benzyleadenine (BA), or a 1% sucrose solution found that while BA was able to delay the increase in LUC activity seen in water-treated plants, no increase in LUC activity was seen in plants incubated in sucrose. These results indicate that the erd1 promoter contains two different regulatory systems that are involved in upregulation by dehydration stress and dark-induced senescence.

Stability in vitro of the 69K movement protein of Turnip yellow mosaic virus is regulated by the ubiquitin-mediated proteasome pathway

Plant viruses move to adjacent cells with the use of virus-encoded cell-to-cell movement proteins. Using proteins produced by in vitro translation, we present evidence that the '69K' movement protein of Turnip yellow mosaic virus (TYMV) is recognized as a substrate for the attachment of polyubiquitin chains and for subsequent rapid and selective proteolysis by the proteasome, the ATP-dependent proteolytic system present in reticulocyte lysate. Truncation of the 69K protein suggests the existence of two degradation signals within its sequence. We propose that selective degradation of virus movement proteins may contribute to the previously reported transient nature of their accumulation during infection.