Molecular characterization of photomixotrophic tobacco cells resistant to protoporphyrinogen oxidase-inhibiting herbicides

Dose-dependent effects of 1O2 in chloroplasts are determined by its timing and localization of production

In plants, highly reactive singlet oxygen (1O2) is known to inhibit photosynthesis and to damage the cell as a cytotoxin. However, more recent studies have also proposed 1O2 as a signal. In plants under stress, not only 1O2 but also other reactive oxygen species (ROS) are generated simultaneously, thus making it difficult to link a particular response to the release of 1O2 and establish a signaling role for this ROS. This obstacle has been overcome by the identification of conditional mutants of Arabidopsis thaliana that selectively generate 1O2 and trigger various 1O2-mediated responses. In chloroplasts of these mutants, chlorophyll or its biosynthetic intermediates may act as a photosensitizer and generate 1O2. These 1O2-mediated responses are not only dependent on the dosage of 1O2 but also are determined by the timing and suborganellar localization of its production. This spatial- and temporal-dependent variability of 1O2-mediated responses emphasizes the importance of 1O2 as a highly versatile and short-lived signal that acts throughout the life cycle of a plant.

Promotion of cyclic electron transport around photosystem I during the evolution of NADP-malic enzyme-type C4 photosynthesis in the genus Flaveria

C4 plants display higher cyclic electron transport activity than C3 plants. This activity is suggested to be important for the production of ATPs required for C4 metabolism. To understand the process by which photosystem I (PSI) cyclic electron transport was promoted during C4 evolution, we conducted comparative analyses of the functionality of PSI cyclic electron transport among members of the genus Flaveria, which contains several C3, C3-C4 intermediate, C4-like and C4 species. The abundance of NDH-H, a subunit of NADH dehydrogenase-like complex, increased markedly in bundle sheath cells with the activity of the C4 cycle. By contrast, PROTON GRADIENT REGULATION5 (PGR5) and PGR5-LIKE1 increased in both mesophyll and bundle sheath cells in C4-like Flaveria palmeri and C4 species. Grana stacks were drastically reduced in bundle sheath chloroplasts of C4-like F. palmeri and C4 species; these species showed a marked increase in PSI cyclic electron transport activity. These results suggest that both the expression of proteins involved in PSI cyclic electron transport and changes in thylakoid structure contribute to the high activity of cyclic electron flow in NADP-malic enzyme-type C4 photosynthesis. We propose that these changes were important for the establishment of C4 photosynthesis from C3-C4 intermediate photosynthesis in Flaveria.

Defense/stress responses activated by chitosan in sycamore cultured cells

Chitosan (CHT) is a natural, non-toxic, and inexpensive compound obtained by partial alkaline deacetylation of chitin, the main component of the exoskeleton of crustaceans and other arthropods. The unique physiological and biological properties of CHT make this polymer useful for a wide range of industries. In agriculture, CHT is used to control numerous pre- and postharvest diseases on various horticultural commodities. In recent years, much attention has been devoted to CHT as an elicitor of defense responses in plants, which include raising of cytosolic Ca(2+), activation of MAP kinases, callose apposition, oxidative burst, hypersensitive response, synthesis of abscisic acid, jasmonate, phytoalexins, and pathogenesis-related proteins. In this work, we investigated the effects of different CHT concentrations on some defense/stress responses of sycamore (Acer pseudoplatanus L.) cultured cells. CHT induced accumulation of dead cells, and of cells with fragmented DNA, production of H(2)O(2) and nitric oxide, release of cytochrome c from the mitochondrion, accumulation of regulative 14-3-3 proteins in the cytosol and of HSP70 molecular chaperone binding protein in the endoplasmic reticulum, accompanied by marked modifications in the architecture of this cell organelle.

Effect of heat stress on actin cytoskeleton and endoplasmic reticulum of tobacco BY-2 cultured cells and its inhibition by Co2+

Temperature stress such as heat, cold, or freezing is a principal cause for yield reduction in crops. In particular, heat stress is very common and dangerous for plants since this stress can impact several plant and cellular functions. In spite of their role in sensing local stress and in controlling fundamental processes including PCD, the responses of cellular structures and organelles to heat stress are poorly investigated. In this work, we investigated the possible changes induced by mild heat stress, medium heat stress, and heat shock (HS; 5 min at 35 degrees C, 45 degrees C, or 50 degrees C, respectively) on actin cytoskeleton and endoplasmic reticulum (ER) of tobacco BY-2 cultured cells. While mild and medium heat stresses are ineffective, HS induces depolymerization of actin microfilaments and changes in ER morphology accompanied by accumulation of the HSP70 binding protein (BiP). These effects of HS are prevented by the inhibitor of ethylene production Co(2+). While the analyzed cell structures do not seem to be involved in the establishment of mild and medium heat stresses at least in this experimental system, the strong modifications induced by the treatment at 50 degrees C suggest that actin cytoskeleton and ER may be involved in the responses to HS. Besides, the inhibiting effect of Co(2+) suggests a role for ethylene as a regulative molecule in the responses to HS here observed.

(3R,4R)-2,5-Dioxo-1-m-tolyl-3,4-diyl diacetate

In the enanti-omerically pure title compound, C(15)H(15)NO(6), the five-membered ring displays a twist conformation with the local axis through the N atom. The acetyl groups are perpendicular to the ring [dihedral angles 80.3 (1) and 89.3 (1)°] and project to opposite sides. The packing is governed by two weak C-H⋯O inter-actions, forming layers of mol-ecules parallel to the ab plane.

A balanced PGR5 level is required for chloroplast development and optimum operation of cyclic electron transport around photosystem I

PSI cyclic electron transport contributes markedly to photosynthesis and photoprotection in flowering plants. Although the thylakoid protein PGR5 (Proton Gradient Regulation 5) has been shown to be essential for the main route of PSI cyclic electron transport, its exact function remains unclear. In transgenic Arabidopsis plants overaccumulating PGR5 in the thylakoid membrane, chloroplast development was delayed, especially in the cotyledons. Although photosynthetic electron transport was not affected during steady-state photosynthesis, a high level of non-photochemical quenching (NPQ) was transiently induced after a shift of light conditions. This phenotype was explained by elevated activity of PSI cyclic electron transport, which was monitored in an in vitro system using ruptured chloroplasts, and also in leaves. The effect of overaccumulation of PGR5 was specific to the antimycin A-sensitive pathway of PSI cyclic electron transport but not to the NAD(P)H dehydrogenase (NDH) pathway. We propose that a balanced PGR5 level is required for efficient regulation of the rate of antimycin A-sensitive PSI cyclic electron transport, although the rate of PSI cyclic electron transport is probably also regulated by other factors during steady-state photosynthesis.

Arabidopsis ACCELERATED CELL DEATH2 modulates programmed cell death

The Arabidopsis thaliana chloroplast protein ACCELERATED CELL DEATH2 (ACD2) modulates the amount of programmed cell death (PCD) triggered by Pseudomonas syringae and protoporphyrin IX (PPIX) treatment. In vitro, ACD2 can reduce red chlorophyll catabolite, a chlorophyll derivative. We find that ACD2 shields root protoplasts that lack chlorophyll from light- and PPIX-induced PCD. Thus, chlorophyll catabolism is not obligatory for ACD2 anti-PCD function. Upon P. syringae infection, ACD2 levels and localization change in cells undergoing PCD and in their close neighbors. Thus, ACD2 shifts from being largely in chloroplasts to partitioning to chloroplasts, mitochondria, and, to a small extent, cytosol. ACD2 protects cells from PCD that requires the early mitochondrial oxidative burst. Later, the chloroplasts of dying cells generate NO, which only slightly affects cell viability. Finally, the mitochondria in dying cells have dramatically altered movements and cellular distribution. Overproduction of both ACD2 (localized to mitochondria and chloroplasts) and ascorbate peroxidase (localized to chloroplasts) greatly reduces P. syringae-induced PCD, suggesting a pro-PCD role for mitochondrial and chloroplast events. During infection, ACD2 may bind to and/or reduce PCD-inducing porphyrin-related molecules in mitochondria and possibly chloroplasts that generate reactive oxygen species, cause altered organelle behavior, and activate a cascade of PCD-inducing events.

Hybrid lethality of cultured cells of an interspecific F1 hybrid of Nicotiana gossei Domin and N. tabacum L

Cultured cells were established from the hypocotyl of F(1) hybrid seedlings of Nicotiana gossei Domin and N. tabacum L. The cultured cells started to die at 26 degrees C, but not at 37 degrees C, which is similar to what occurred in cells of the original hybrid plants. An increase in the number of cells without cytoplasmic strands and acidification of the cytoplasm followed by decomposition of the mitochondria and chloroplasts indicated that vacuolar collapse plays a central role in the execution of cell death. Oxygen but not light was required for cell death. Cellular levels of the superoxide anion and hydrogen peroxide temporarily increased during the early phase at 26 degrees C, while no such oxidative burst was observed at 37 degrees C. The reactive oxygen intermediates are potentially involved in the death of the hybrid cells.

Development of PPO inhibitor-resistant cultures and crops

Recent progress in the development of protoporphyrinogen oxidase (PPO, Protox) inhibitor-resistant plant cell cultures and crops is reviewed, with emphasis on the molecular and cellular aspects of this topic. PPO herbicide-resistant maize plants have been reported, along with the isolation of plant PPO genes and the isolation of herbicide-resistant mutants. At the same time, PPO inhibitor-resistant rice plants have been developed by expression of the Bacillus subtilis PPO gene via targeting the gene into either chloroplast or cytoplasm. Other attempts to develop PPO herbicide-resistant plants include conventional tissue culture methods, expression of modified co-factors of the protoporphyrin IX binding subunit proteins, over-expression of wild-type plant PPO gene, and engineering of P-450 monooxygenases to degrade the PPO inhibitor.

Development of protoporphyrinogen oxidase as an efficient selection marker for Agrobacterium tumefaciens-mediated transformation of maize

In this article, we report the isolation of plant protoporphyrinogen oxidase (PPO) genes and the isolation of herbicide-tolerant mutants. Subsequently, an Arabidopsis double mutant (Y426M + S305L) was used to develop a selectable marker system for Agrobacterium tumefaciens-mediated transformation of maize (Zea mays) and to obtain multiple events tolerant to the PPO family of herbicides. Maize transformants were produced via butafenacil selection using a flexible light regime to increase selection pressure. Butafenacil selection per se did not change transgene copy number distribution relative to other selectable marker systems, but the most tolerant events identified in the greenhouse were more likely to contain multiple copies of the introduced mutant PPO gene. To date, more than 2,500 independent transgenic maize events have been produced using butafenacil selection. The high frequency of A. tumefaciens-mediated transformation via PPO selection enabled us to obtain single-copy transgenic maize lines tolerant to field levels of butafenacil.

Kinetic and physical characterisation of recombinant wild-type and mutant human protoporphyrinogen oxidases

The effects of various protoporphyrinogen oxidase (PPOX) mutations responsible for variegate porphyria (VP), the roles of the arginine-59 residue and the glycines in the conserved flavin binding site, in catalysis and/or cofactor binding, were examined. Wild-type recombinant human PPOX and a selection of mutants were generated, expressed, purified and partially characterised. All mutants had reduced PPOX activity to varying degrees. However, the activity data did not correlate with the ability/inability to bind flavin. The positive charge at arginine-59 appears to be directly involved in catalysis and not in flavin-cofactor binding alone. The K(m)s for the arginine-59 mutants suggested a substrate-binding problem. T(1/2) indicated that arginine-59 is required for the integrity of the active site. The dominant alpha-helical content was decreased in the mutants. The degree of alpha-helix did not correlate linearly with T(1/2) nor T(m) values, supporting the suggestion that arginine-59 is important for catalysis at the active site. Examination of the conserved dinucleotide-binding sequence showed that substitution of glycine in codon 14 was less disruptive than substitutions in codons 9 and 11. Ultraviolet melting curves generally showed a two-state transition suggesting formation of a multi-domain structure. All mutants studied were more resistant to thermal denaturation compared to wild type, except for R168C.

Impaired expression of the plastidic ferrochelatase by antisense RNA synthesis leads to a necrotic phenotype of transformed tobacco plants

Protoporphyrin IX is the last common intermediate of tetrapyrrole biosynthesis. The chelation of a Mg2+ ion by magnesium chelatase and of a ferrous ion by ferrochelatase directs protoporphyrin IX towards the formation of chlorophyll and heme, respectively. A full length cDNA clone encoding a ferrochelatase was identified from a Nicotiana tabacum cDNA library. The encoded protein consists of 497 amino acid residues with a molecular weight of 55.4 kDa. In vitro import of the protein into chloroplasts and its location in stroma and thylakoids confirm its close relationship to the previously described Arabidopsis thaliana plastid-located ferrochelatase (FeChII). A 1700-bp tobacco FeCh cDNA sequence was expressed in Nicotiana tabacum cv. Samsun NN under the control of the CaMV 35S promoter in antisense orientation allowing investigation into the consequences of selective reduction of the plastidic ferrochelatase activity for protoporphyrin IX channeling in chloroplasts and for interactions between plastidic and mitochondrial heme synthesis. Leaves of several transformants showed a reduced chlorophyll content and, during development, a light intensity-dependent formation of necrotic leaf lesions. In comparison with wild-type plants the total ferrochelatase activity was decreased in transgenic lines leading to an accumulation of photosensitizing protoporphyrin IX. Ferrochelatase activity was reduced only in plastids but not in mitochondria of transgenic plants. By means of the specifically diminished ferrochelatase activity consequences of the selective inhibition of protoheme formation for the intracellular supply of heme can be investigated in the future.

Resistance of a soybean cell line to oxyfluorfen by overproduction of mitochondrial protoporphyrinogen oxidase

The diphenyl ether herbicide oxyfluorfen (2-chloro-4-trifluoromethylphenyl 3-ethoxy-4-nitrophenyl ether) inhibits protoporphyrinogen oxidase (Protox) which catalyzes the oxidation of protoporphyrinogen IX (Protogen) to protoporphyrin IX (Proto IX), the last step of the common pathway to chlorophyll and haeme biosynthesis. We have selected an oxyfluorfen-resistant soybean cell line by stepwise selection methods, and the resistance mechanism has been investigated. No growth inhibition was observed in resistant cells at a concentration of 10(-7) M oxyfluorfen, a concentration at which normal cells did not survive. While the degree of inhibition of total extractable Protox by oxyfluorfen was the same in both cell types, the enzyme activity in the mitochondrial fraction from non-treated resistant cells was about nine-fold higher than that from normal cells. Northern analysis of mitochondrial Protox revealed that the concentration of mitochondrial Protox mRNA was much higher in resistant cells than that in normal cells. There were no differences in the absorption and metabolic breakdown of oxyfluorfen. The growth of resistant cells was also insensitive to oxadiazon [5-tert-butyl-3-(2,4-dichloro-5-isopropoxyphenyl)-1,3,4-oxadiazol-2-(3H)- one], the other chemical class of Protox inhibitor. Therefore, the resistance of the selected soybean cell line to oxyfluorfen is probably mainly due to the overproduction of mitochondrial Protox.

Dual targeting of spinach protoporphyrinogen oxidase II to mitochondria and chloroplasts by alternative use of two in-frame initiation codons

Protoporphyrinogen oxidase (Protox) is the final enzyme in the common pathway of chlorophyll and heme biosynthesis. Two Protox isoenzymes have been described in tobacco, a plastidic and a mitochondrial form. We isolated and sequenced spinach Protox cDNA, which encodes a homolog of tobacco mitochondrial Protox (Protox II). Alignment of the deduced amino acid sequence between Protox II and other tobacco mitochondrial Protox homologs revealed a 26-amino acid N-terminal extension unique to the spinach enzyme. Immunoblot analysis of spinach leaf extract detected two proteins with apparent molecular masses of 57 and 55 kDa in chloroplasts and mitochondria, respectively. In vitro translation experiments indicated that two translation products (59 and 55 kDa) are produced from Protox II mRNA, using two in-frame initiation codons. Transport experiments using green fluorescent protein-fused Protox II suggested that the larger and smaller translation products (Protox IIL and IIS) target exclusively to chloroplasts and mitochondria, respectively.

Recombinant maize protoporphyrinogen IX oxidase expressed in Escherichia coli forms complexes with GroEL and DnaK chaperones

The clone corresponding to maize plastidic protoporphyrinogen IX oxidase (PPO) has been isolated by functional complementation and inserted into a pET16b vector for expression in Escherichia coli. Recombinant PPO was purified by standard affinity chromatography using a metal chelating resin. Two contaminants copurified with recombinant PPO and were identified as GroEL and DnaK. Since chaperone binding to hydrophobic regions of the protein is regulated by ATP availability, an ATP washing step was introduced prior to elution of the recombinant protein from an affinity column. This washing step selectively removed both chaperones and allowed the recovery of pure PPO. Coexpression of PPO and GroELS resulted in a sixfold increase of soluble PPO yield, suggesting that bacterial chaperones could be limiting during the folding of the heterologous protein. However, a portion of PPO was still found in the insoluble fraction. Buffer containing the GroEL and DnaK enabled resuspension of PPO from the insoluble fraction but failed to enhance refolding of the denaturated protein. Attempts to increase the amount of soluble PPO using a thioredoxin-PPO fusion protein were not successful. Initial characterization of the recombinant PPO found that it possessed a high V(max), an elevated affinity for substrate, and an elevated sensitivity to PPO inhibitor herbicides compared to previous reports.

Molecular characterization and subcellular localization of protoporphyrinogen oxidase in spinach chloroplasts

Protoporphyrinogen oxidase (Protox) is the last common enzyme in the biosynthesis of chlorophylls and heme. In plants, there are two isoenzymes of Protox, one located in plastids and other in the mitochondria. We cloned the cDNA of spinach (Spinacia oleracea) plastidal Protox and purified plastidal Protox protein from spinach chloroplasts. Sequence analysis of the cDNA indicated that the plastid Protox of spinach is composed of 562 amino acids containing the glycine-rich motif GxGxxG previously proposed to be a dinucleotide binding site of many flavin-containing proteins. The cDNA of plastidal Protox complemented a Protox mutation in Escherichia coli. N-terminal sequence analysis of the purified enzyme revealed that the plastidal Protox precursor is processed at the N-terminal site of serine-49. The predicted transit peptide (methionine-1 to cysteine-48) was sufficient for the transport of precursors into the plastid because green fluorescent protein fused with the predicted transit peptide was transported to the chloroplast. Immunocytochemical analysis using electron microscopy showed that plastidal Protox is preferentially associated with the stromal side of the thylakoid membrane, and a small portion of the enzyme is located on the stromal side of the chloroplast inner envelope membrane.

Purification and properties of protoporphyrinogen oxidase from spinach chloroplasts

Protoporphyrinogen oxidase (Protox), an enzyme that catalyzes the common step of chlorophyll and heme biosynthetic pathways, was purified from spinach chloroplasts. The molecular weight of purified protein was estimated to be approximately 60,000 by SDS-PAGE. Protox activity was stimulated by addition of FAD, suggesting that chloroplast Protox requires FAD as a cofactor. Furthermore, the Protox-inhibiting herbicide, S23142, specifically inhibited the purified Protox activity at an IC50 value of 1 nM.

Overexpression of plastidic protoporphyrinogen IX oxidase leads to resistance to the diphenyl-ether herbicide acifluorfen

The use of herbicides to control undesirable vegetation has become a universal practice. For the broad application of herbicides the risk of damage to crop plants has to be limited. We introduced a gene into the genome of tobacco (Nicotiana tabacum) plants encoding the plastid-located protoporphyrinogen oxidase of Arabidopsis, the last enzyme of the common tetrapyrrole biosynthetic pathway, under the control of the cauliflower mosaic virus 35S promoter. The transformants were screened for low protoporphyrin IX accumulation upon treatment with the diphenyl ether-type herbicide acifluorfen. Leaf disc incubation and foliar spraying with acifluorfen indicated the lower susceptibility of the transformants against the herbicide. The resistance to acifluorfen is conferred by overexpression of the plastidic isoform of protoporphyrinogen oxidase. The in vitro activity of this enzyme extracted from plastids of selected transgenic lines was at least five times higher than the control activity. Herbicide treatment that is normally inhibitory to protoporphyrinogen IX oxidase did not significantly impair the catalytic reaction in transgenic plants and, therefore, did not cause photodynamic damage in leaves. Therefore, overproduction of protoporphyrinogen oxidase neutralizes the herbicidal action, prevents the accumulation of the substrate protoporphyrinogen IX, and consequently abolishes the light-dependent phytotoxicity of acifluorfen.