Herbicide resistance and cross-resistance: changes at three distinct sites in the herbicide-binding protein

Detection of the maximum resistance to the herbicides diuron and glyphosate, and evaluation of its phenotypic cost, in freshwater phytoplankton

One of the most important anthropogenic impacts on freshwater aquatic ecosystems close to intensive agriculture areas is the cumulative increase in herbicide concentrations. The threat is especially relevant for phytoplankton organisms because they have the same physiological targets as the plants for which herbicides have been designed. This led us to explore the evolutionary response of three phytoplanktonic species to increasing concentrations of two herbicides and its consequences in terms of growth and photosynthesis performance. Specifically, we used an experimental ratchet protocol to investigate the differential evolution and the limit of resistance of a cyanobacterium (Microcystis aeruginosa) and two chlorophyceans (Chlamydomonas reinhardtii and Dictyosphaerium chlorelloides) to two herbicides in worldwide use: glyphosate and diuron. Initially, the growth rate of M. aeruginosa and D. chlorelloides was completely inhibited when they were exposed to a dose of 0.23 ppm diuron or 40 ppm glyphosate, whereas a higher concentration of both herbicides (0.46 ppm diuron or 90 ppm glyphosate) was necessary to abolish C. reinhardtii growth. However, after running a ratchet protocol, the resistance of the three species to both herbicides increased by an adaptation process. M. aeruginosa and D. chlorelloides were able to grow at 1.84 ppm diuron and 80 ppm glyphosate and C. reinhardtii proliferated at twice these concentrations. Herbicide-resistant strains showed lower growth rates than their wild-type counterparts in the absence of herbicides, as well as changes on morphology and differences on photosynthetic pigment content. Besides, herbicide-resistant cells generally showed a lower photosynthetic performance than wild-type strains in the three species. These results indicate that the introduction of both herbicides in freshwater ecosystems could produce a diminution of primary production due to the selection of herbicide-resistant mutants, that would exhibit lower photosynthetic performance than wild-type populations.

The hydrophobicity of mutations targeting D1:Val219 modifies formate and diuron binding in the quinone-Fe-acceptor complex of Photosystem II

The D1:Val219 residue of Photosystem II in the cyanobacterium Synechocystis sp. PCC 6803 was mutated to alanine or isoleucine, creating the V219A and V219I mutants, respectively. Oxygen evolution was slowed in these mutants, while chlorophyll a fluorescence induction assays indicated slowed electron transfer. As previously observed [Erickson J.M., Rahire, M., Rochaix, J.-D. and Mets. L. (1985) Science, 228, 204-207], the V219I mutant was resistant to 3,4-dichloro-1,1-dimethyl urea (DCMU); however, the V219A strain displayed no DCMU resistance. Additionally, the V219A strain was less sensitive to the addition of formate than the control, while the V219I strain was more sensitive to formate. Both mutant strains were susceptible to photodamage and required protein synthesis for recovery. We hypothesize that the sensitivity to DCMU and the extent of bicarbonate-reversible formate-induced inhibition, as well as the capacity for recovery in cells following photodamage, are influenced by the hydrophobicity of the environment associated with the Val219 residue in D1.

Honoring Jean-David Rochaix

We honor Jean-David Rochaix, an outstanding scholar of chloroplast biogenesis and photosynthesis, who received the prestigious Lifetime Achievement Award of the International Society of Photosynthesis Research at its 17th International Photosynthesis Congress held in Maastricht, The Netherlands (August 5-12, 2016). With this award he joins other major discoverers in the field of photosynthesis: Pierre Joliot (of France, 2013); Ulrich W. Heber* (of Germany, 2010) and Kenneth Sauer (of USA, 2010); Jan M. Anderson* (of Australia, 2007); and Horst T. Witt* (of Germany, 2004). See "Appendix 1" for the list of those who have received the ISPR Communication, Innovation, Calvin-Benson, and Hill awards.

Mechanism for the SNAr reaction of atrazine with endogenous thiols: experimental and theoretical study

The mechanism for the S_NAr reaction of atrazine with endogenous thiols: a stepwise or concerted process?

Microalgal sensitivity varies between a diuron-resistant strain and two wild strains when exposed to diuron and irgarol, alone and in mixtures

A wild strain of Chaetoceros calcitrans and wild and diuron-resistant strains of Tetraselmis suecica, were exposed to the PSII inhibitor herbicides diuron and irgarol, individually and in mixtures. The effects of three concentrations of diuron and irgarol and four binary mixtures were evaluated on doubling time, relative reactive oxygen species and lipid content by flow cytometry, and on photosynthetic efficiency by pulse amplitude modulated fluorescence. In both wild strains, significant effects were observed for each molecule at the highest concentration tested: at irgarol 0.5 μg L(-1), C. calcitrans was shown to be more sensitive than T. suecica (+52% and +19% in doubling time, respectively), whereas at diuron 5 μg L(-1), T. suecica was more affected (+125% in doubling time) than C. calcitrans (+21%). Overall, irgarol had a higher toxicity at a lower concentration than diuron (no effect at diuron 0.5 μg L(-1)) for both wild strains. The strongest mixture (irgarol 0.5 μg L(-1) + diuron 5 μg L(-1)) increased doubling time by 356% for T. suecica, thus showing amplified effects when the two compounds were mixed. Sequencing of the diuron-resistant strain demonstrated a single mutation in the psbA gene coding sequence. Although resistance of this strain to diuron was confirmed with no effect at the highest diuron concentration, no resistance to irgarol was shown. In addition, the mutant strain exposed to the strongest mixture showed a 3.5-fold increase in doubling time compared with irgarol alone, thereby supporting the hypothesis of a biochemical interaction between these two compounds.

A new F131V mutation in Chlamydomonas phytoene desaturase locates a cluster of norflurazon resistance mutations near the FAD-binding site in 3D protein models

The green alga Chlamydomonas reinhardtii provides a tractable genetic model to study herbicide mode of action using forward genetics. The herbicide norflurazon inhibits phytoene desaturase, which is required for carotenoid synthesis. Locating amino acid substitutions in mutant phytoene desaturases conferring norflurazon resistance provides a genetic approach to map the herbicide binding site. We isolated a UV-induced mutant able to grow in very high concentrations of norflurazon (150 µM). The phytoene desaturase gene in the mutant strain contained the first resistance mutation to be localised to the dinucleotide-binding Rossmann-likedomain. A highly conserved phenylalanine amino acid at position 131 of the 564 amino acid precursor protein was changed to a valine in the mutant protein. F131, and two other amino acids whose substitution confers norflurazon resistance in homologous phytoene desaturase proteins, map to distant regions in the primary sequence of the C. reinhardtii protein (V472, L505) but in tertiary models these residues cluster together to a region close to the predicted FAD binding site. The mutant gene allowed direct 5 µM norflurazon based selection of transformants, which were tolerant to other bleaching herbicides including fluridone, flurtamone, and diflufenican but were more sensitive to beflubutamid than wild type cells. Norflurazon resistance and beflubutamid sensitivity allow either positive or negative selection against transformants expressing the mutant phytoene desaturase gene.

Towards predictive resistance models for agrochemicals by combining chemical and protein similarity via proteochemometric modelling

Resistance to pesticides is an increasing problem in agriculture. Despite practices such as phased use and cycling of ‘orthogonally resistant’ agents, resistance remains a major risk to national and global food security. To combat this problem, there is a need for both new approaches for pesticide design, as well as for novel chemical entities themselves. As summarized in this opinion article, a technique termed ‘proteochemometric modelling’ (PCM), from the field of chemoinformatics, could aid in the quantification and prediction of resistance that acts via point mutations in the target proteins of an agent. The technique combines information from both the chemical and biological domain to generate bioactivity models across large numbers of ligands as well as protein targets. PCM has previously been validated in prospective, experimental work in the medicinal chemistry area, and it draws on the growing amount of bioactivity information available in the public domain. Here, two potential applications of proteochemometric modelling to agrochemical data are described, based on previously published examples from the medicinal chemistry literature.

Identification of a new PSII target site psbA mutation leading to D1 amino acid Leu218 Val exchange in the Chenopodium album D1 protein and comparison to cross-resistance profiles of known modifications at positions 251 and 264

BACKGROUND

Resistance of Chenopodium album to triazinones and triazines can be caused by two amino acid exchanges, serine-264-glycine (Ser(264) Gly) and alanine-251-valine (Ala(251) Val), in the chloroplast D1 protein. This paper describes the identification of a biotype with a leucine-218-valine (Leu(218) Val) switch found in German sugar beet fields with unsatisfactory weed control. A greenhouse experiment has been performed to compare the resistance profile of the newly identified biotype with biotypes that carry the Ser(264) Gly and Ala(251) Val mutations.

RESULTS

Application rate-response curves obtained from the greenhouse experiment showed that the Leu(218) Val exchange induced significant resistance against the triazinones but not against terbuthylazine. The level of resistance against the triazinones was higher in the Ser(264) Gly and Ala(251) Val biotypes compared with the Leu(218) Val biotype. All biotypes tested were more resistant to metribuzin than to metamitron. Following terbuthylazine treatment, Ser264 Gly displayed a high level of resistance, Ala(251) Val showed moderate resistance. A PCR-RFLP assay for Ser(264) Gly has been extended to include detection of Ala251 Val and Leu(218) Val mutations.

CONCLUSION

The D1 Leu(218) Val substitution in C. album confers significant resistance to triazinones. This suggests that Leu(218) Val is involved in the binding of triazinones. First establishment of the resistance profiles of the three psbA mutations suggests that these mutations have been independently selected.

A phenotypic screening platform to identify small molecule modulators of Chlamydomonas reinhardtii growth, motility and photosynthesis

Chemical biology, the interfacial discipline of using small molecules as probes to investigate biology, is a powerful approach of developing specific, rapidly acting tools that can be applied across organisms. The single-celled alga Chlamydomonas reinhardtii is an excellent model system because of its photosynthetic ability, cilia-related motility and simple genetics. We report the results of an automated fitness screen of 5,445 small molecules and subsequent assays on motility/phototaxis and photosynthesis. Cheminformatic analysis revealed active core structures and was used to construct a naïve Bayes model that successfully predicts algal bioactive compounds.

Plastidic trnFUUC pseudogenes in North American genus Boechera (Brassicaceae): mechanistic aspects of evolution

The origin and maintenance of a plastidic tandem repeat next to the TRNF (UUC) gene were analyzed in the genus BOECHERA in a phylogenetic context and were compared to published analogous examples that emerged in parallel in the Asteraceae and Juncaceae, respectively. Although we identified some features common to these taxonomic groups with respect to structure and origin of the region, obvious differences were encountered, which argue against a specific mechanism or evolutionary principle underlying the parallel origin and maintenance of the TRNF-tandem repeats in those families. In contrast to the situation in the Asteraceae, no reciprocal recombinant repeat types have been observed in the Brassicaceae. Forty copy types, classified into three groups, were isolated from 103 chloroplast haplotypes of BOECHERA and it was demonstrated that they are composed of four subregions of various origins. We discuss various mutation mechanisms such as DNA replication slippage, and inter- and intrachromosomal recombination which were reported to mediate variation in copy numbers and other types of observed sequence length polymorphism. It is shown that the observed molecular structure of the tandem repeat region did not fully fit the particular patterns expected under a scenario of evolution including any of the known mechanisms. Nevertheless, it appeared that intermolecular unequal crossing-over is most likely the driving force in the evolution of this tandem repeat. However, it remains to be explained, why no reciprocal recombinant copy types have been observed. The reconstructed phylogenetic relationships among copies reflected different evolutionary scenarios as follows: (1) A single and ancient origin of copies pre-dates the radiation of BOECHERA. (2) Parallel expansion and shortening of the tandem repeat within different BOECHERA lineages. (3) Conservation of the first copy, as it was the only one present in all chloroplast haplotypes.

Inhibitors in the functional dissection of the photosynthetic electron transport system

The significance of inhibitors and artificial electron acceptor and donor systems as experimental tools for studying the photosynthetic system is described by reviewing early classical articles. The historical development in unravelling the role and sequence of electron carriers and energy conserving sites in the electron transport chain is acknowledged. Emphasis is given to inhibitors of the acceptor side of photosystem II and of the plastoquinol oxidation site in the cytochrome b6/f complex. Their role in regulatory processes under redox control is introduced.

A psbA mutation in Kochia scoparia (L) Schrad from railroad rights-of-way with resistance to diuron, tebuthiuron and metribuzin

Kochia [Kochia scoparia (L) Schrad] has become resistant to many herbicides used in cropland and railroad rights-of-way in North Dakota and Minnesota. Kochia scoparia plants that had survived annual treatments with diuron and tebuthiuron were sampled along railroad rights-of-way in North Dakota and Minnesota. The samples were screened in the greenhouse for resistance to diuron, tebuthiuron, metribuzin and bromoxynil from 0.5x to 32x the recommended use rates. A resistant K scoparia accession (MN-3R) was confirmed with resistance up to 16-fold higher than recommended use rates for tebuthiuron and diuron and up to 4-fold higher for metribuzin. However, the resistant K scoparia accession was susceptible to bromoxynil even at 50% of the recommended use rate. The herbicide binding region of the psbA gene fragment of eight resistant (R) and seven susceptible (S) K scoparia accessions was PCR-amplified and sequenced for detection of mutations. The psbA gene of four R K scoparia accessions was mutated at residue 219 with substitution of isoleucine for valine (GenBank accession number AY251265). The seven S K scoparia accession sequences were wild-type at this residue (GenBank accession number AY251266). The other four R accessions sequences showed a previously known triazine R mutation with substitution of glycine for serine at residue 264. All 15 K scoparia accessions were wild-type at all other psbA residues within the region analyzed. Resistance to diuron, tebuthiuron and metribuzin among the railroad rights-of-way K scoparia is probably due to the mutation at residue 219 of the psbA gene in some plants, but due to the previously reported Ser(264)Gly substitution in other plants. Target-site resistance associated with a change of valine to isoleucine at residue 219 of the psbA target-site in weeds has previously been reported for Poa annua L selected in diuron-treated grass seed fields, and for Amaranthus powelli S Wats selected in linuron-treated carrot fields. This is the first report of the mutation in herbicide-resistant K scoparia.

Novel psbA1 gene from a naturally occurring atrazine-resistant cyanobacterial isolate

A naturally occurring atrazine-resistant cyanobacterial isolate, strain SG2, was isolated from an atrazine-containing wastewater treatment system at the Syngenta atrazine production facility in St. Gabriel, La. Strain SG2 was resistant to 1,000 microg of atrazine per ml but showed relatively low resistance to diuron [3-(3,4-dichlorophenyl)-1,1-dimethyl urea]. Analyses of 16S ribosomal DNA indicated that strain SG2 falls into the Synechocystis/Pleurocapsa/Microcystis group. Photosynthetically driven oxygen evolution in strain SG2 was only slightly inhibited (about 10%) by 2,000 microg of atrazine per ml, whereas in the control strain Synechocystis 6803, oxygen evolution was inhibited 90% by 1,000 microg of atrazine per ml. No atrazine accretion, mineralization, or metabolites were detected when strain SG2 was grown with [(14)C]atrazine. Strain SG2 contained three copies of the psbA gene, which encodes the D(1) protein of the photosystem II reaction center. Nucleotide sequence analyses indicated that the psbA2 and psbA3 genes encoded predicted proteins with the same amino acid sequence. However, the psbA1 gene product contained five extra amino acids, which were not found in PsbA proteins from five other cyanobacteria. Moreover, the PsbA1 protein from strain SG2 had an additional 13 amino acid changes compared to the PsbA2/PsbA3 proteins and contained 10 amino acid alterations compared to conserved residues found in other cyanobacteria. Reverse transcriptase PCR analysis indicated that the psbA1 gene and the psbA2/psbA3 gene(s) were expressed in photosynthetically grown cells in the presence of atrazine. These results suggest that strong selection pressure conferred by the continual input of atrazine has contributed to the evolution of a herbicide-resistant, yet photosynthetically efficient, psbA gene in a cyanobacterium.

A simple procedure for polymerase chain reaction of the PSBA gene in algae: application to the screening of mutations conferring atrazine resistance and discrimination of natural populations of Porphyra linearis

A simplified procedure is described for polymerase chain reaction (PCR) of a partial sequence (bp 601-893) of the plastid gene psbA in the rhodophyte Porphyra linearis and the diatoms Haslea ostreria and Skeletonema costatum. This procedure involves the use of all tissues of P. linearis and live cell suspensions of H. ostreria or S. costatum, as DNA templates, without any further purification of DNA. As in the case of PCR with DNA extracts, a single major band of the expected size (292 bp) was obtained after PCR for the three species. Sequences of the amplified fragments were aligned, confirming that the amplified products were part of the psbA gene. The method was then used to screen mutations in partial psbA genes of 23 samples of P. linearis collected at four different stations along the mid-Atlantic coast of France. An alignment was obtained indicating the existence of mutations, though not in codons known for herbicide resistance. All mutations found were silent. However, genetic polymorphism discriminated between samples collected from two stations. The method employed allows rapid amplification of the herbicide target gene and simplifies the procedure for screening mutations or populations in algae. Its application to other genes and species is considered.

Modeling the quinone-B binding site of the photosystem-II reaction center using notions of complementarity and contact-surface between atoms

Functional identity and significant similarities in cofactors and sequence exist between the L and M reaction center proteins of the photosynthetic bacteria and the D1 and D2 photosystem-II reaction center proteins of cyanobacteria, algae, and plants. A model of the quinone (QB) binding site of the D1 protein is presented based upon the resolved structure of the QB binding pocket of the L subunit, and introducing novel quantitative notions of complementarity and contact surface between atoms. This model, built without using traditional methods of molecular mechanics and restricted to residues in direct contact with QB, accounts for the experimentally derived functional state of mutants of the D1 protein in the region of QB. It predicts the binding of both the classical and phenol-type PSII herbicides and rationalizes the relative levels of tolerance of mutant phenotypes.

A robust protocol for site-directed mutagenesis of the D1 protein inChlamydomonas reinhardtii: A PCR-splicedpsbA gene in a plasmid conferring spectinomycin resistance was introduced into apsbA deletion strain

In this paper, we describe a protocol to obtain a site-directed mutants in thepsbA gene ofChlamydomonas reinhardtii, which overcomes several drawbacks of previous protocols, and makes it possible to generate a mutant within a month. Since the large size of the gene, and the presence of four large introns has made molecular genetics of thepsbA gene rather unwieldy, we have spliced all of the exons of thepsbA gene by PCR to facilitate genetic manipulation and sequencing of the gene. The resultant construct (plasmid pBA153, with several unique restriction sites introduced at exon boundaries) carried 1.2 and 1.8 kb intact sequences from the 5'- and 3'-flanking regions, respectively. The plasmid was used to transform a D1-deletion mutant and was found to complement the deletion and restore photosynthetic activity. In addition, a bacterialaadA gene conferring spectinomycin resistance (spe (r)) was inserted downstream of the intron-freepsbA gene, to give construct pBA155. This allowed selection of mutant strains deficient in photosynthesis by using spectinomycin resistance, and eliminated the possibility of selection for revertant strains which is a consequence of having to use photosynthetic activity as a selection pressure. Finally, pBA155 was used to construct pBA157, in which additional restriction sites were inserted to facilitate cassette mutagenesis for generation of mutations in spans thought to be involved in donor-side interactions. AllpsbA deletion strains transformed with intron-freepsbA-aadA constructs encoding the wild-type D1 sequence, and screened on spectinomycin plates for thespe (r) phenotype, were able to grow photosynthetically, and all showed identical kinetics for electron transfer from primary (QA) to secondary quinone (QB) in Photosystem II, as assayed by the decay of the high fluorescence yield on oxidation of the reduced primary acceptor (QA (-)).

Metribuzin resistance in photoautotrophic Chenopodium rubrum cell cultures. Characterization of double and triple mutations in the psbA gene

Sequence analysis of eight metribuzin-resistant mutants of photoautotrophic Chenopodium rubrum cell cultures revealed new mutations in the psbA gene coding for the 32 kDa herbicide binding protein. Mutants were found to possess either two or three changes in the amino acid sequence of the D1-protein between positions 219 and 272.

Nonrandom distribution of chloroplast recombination events in Chlamydomonas reinhardtii: evidence for a hotspot and an adjacent cold region

Intermolecular recombination of Chlamydomonas chloroplast genes has been analyzed in sexual crosses and following biolistic transformation. The pattern and position of specific exchange events within 15 kb of the 22-kb inverted repeat have been mapped with respect to known restriction fragment length polymorphism markers that distinguish the chloroplast genomes of the interfertile species Chlamydomonas reinhardtii and Chlamydomonas smithii. Recombinant progeny were selected from two- and three-factor crosses involving point mutations conferring herbicide (dr) and antibiotic resistance (er and spr) in the psbA, 23S and 16S ribosomal RNA genes, respectively. Exchange events were not randomly distributed over the 15-kb region, but were found to occur preferentially in a 0.7-kb sequence spanning the 3' end of the psbA gene and were much less common in an adjacent region of ca. 2.0 kb. These findings are corroborated by data showing that the dr mutation is unlinked genetically (3% recombination/kb) to the er and spr rRNA mutations, which are themselves linked and show ca. 1% recombination/kb. This discrepancy is significant since the dr-er and er-spr intervals are about the same length (ca. 7 kb). During chloroplast transformation, the 0.7-kb recombination hotspot also functions as a preferential site for exchange events leading to the integration of donor psbA gene sequences. The 0.7-kb hotspot region contains four classes of 18-37-bp direct repeats also found in other intergenic regions, but no open reading frame. Using deletion constructs in a chloroplast transformation assay, the hotspot was localized to a 500-bp region that lacks most of these repeats, which suggests that the repeats themselves are not responsible for the increased recombination frequency. Within this region, a 400-bp sequence is highly conserved between the chloroplast genomes of C. reinhardtii and C. smithii and includes several structural motifs characteristic of recombination hotspots in other systems.

Analysis of a herbicide resistant mutant obtained by transformation of the Chlamydomonas chloroplast

A herbicide resistant Chlamydomonas double mutant (I219A264) has been obtained by transforming the psbA deletion mutant FuD7 with a cloned psbA gene fragment containing mutations in codons 219 and 264. Copies from both the recipient (FuD7) genome and the genome carrying the mutated psbA gene persist in the transformant. This stable heteroplasmic state appears to be required for photoautotrophic growth. Comparison of resistance profiles for classical and phenol-type inhibitors of the double mutant and the corresponding single mutants demonstrates independent, additive contributions of both amino acids to herbicide binding. The approach chosen here to modify the psbA gene should be useful in those cases where consequences of psbA gene manipulations are not predictable with respect to inhibitor resistance.

Photosynthetic electron transport in genetically altered photosystem II reaction centers of chloroplasts

Using a cotransformation system to identify chloroplast transformants in Chlamydomonas reinhardtii, we converted histidine-195 of the photosystem II reaction center D1 protein to a tyrosine residue. The mutants were characterized by a reduced quantum efficiency for photosynthetic oxygen evolution, which varied in a pH-dependent manner, a reduced capacity to oxidize artificial donors to photosystem II, and P680+ reduction kinetics (microsecond) that were essentially similar to wild type. In addition, a dark-stable radical was detected by ESR in mutant photosystem II particles but not in wild-type particles. This radical was similar in g value and lineshape to chlorophyll or carotenoid cations but could have arisen from a tyrosine-195 cation. The ability of the photosystem II trap (P680+) to oxidize tyrosine residues suggests that the mutant tyrosine residue could be used as a redox-sensitive probe to investigate the environment around the photosystem II trap.

Interaction of the herbicide atrazine with model membranes. I: Physico-chemical studies on dipalmitoyl phosphatidylcholine liposomes

Atrazine (2-chloro-4 ethylamino-6-(isopropylamino)-s-triazine) is one of the most widely used herbicides. Fourier transform infrared spectroscopy, differential scanning calorimetry and fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and of its derivative 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) were used to study the interaction of atrazine with dipalmitoyl phosphatidylcholine liposomes used as a model for biological membranes. The results show that atrazine does not perturb the hydrophobic core of the lipid bilayer and suggest that the herbicide localizes near the glycerol backbone of the lipid.

Isolation and RNA sequence analysis of cytochrome b mutants resistant to funiculosin, a center i inhibitor of the mitochondrial ubiquinol-cytochrome c reductase in Saccharomyces cerevisiae

Funiculosin is a well-known inhibitor of the mitochondrial respiratory chain, probably acting at the ubiquinone reducing site or center i of QH2-cytochrome c reductase. We report here the isolation, mapping and RNA sequence analysis of yeast apo-cytochrome b mutants resistant to this inhibitor. Funiculosin-resistance was found to be conferred, in 4 independent isolates, upon replacement of a leucine residue by phenylalanine in position 198 of the cytochrome b polypeptide chain.

Characterization of Naturally Occurring Atrazine-Resistant Isolates of the Purple Non-Sulfur Bacteria

Six isolates of the purple non-sulfur bacteria, which upon primary isolation were naturally resistant to the herbicide atrazine, were characterized with respect to their taxonomic identity and the mechanism of their resistance. On the basis of electron microscopy, photopigment analysis, and other criteria, they were identified as strains of Rhodopseudomonas acidophila, Rhodopseudomonas palustris , or Rhodocyclus gelatinosus . These isolates exhibited degrees of atrazine resistance which ranged from 1.5 to about 4 times greater than that of cognate reference strains (American Type Culture Collection) tested. Furthermore, all of the reference strains tested were more intrinsically resistant to atrazine than was Rhodobacter sphaeroides . No unique plasmids which might encode for herbicide degradation or inactivation were found in these isolates. Resistance to the herbicide in these isolates was not the result of diminished binding of the herbicide to the L subunit of the bacterial reaction center. Differences in herbicide resistance among the various species of this group may be the result of compositional and chemical differences in the individual reaction centers. However, the increase in atrazine resistance for the isolates characterized in this study probably occurs by undefined mechanisms and not necessarily by changes in the binding of the herbicide to the L subunit of the photosynthetic reaction center.

Evolving strategies for the genetic engineering of herbicide resistance in plants

Developments in plant genetic engineering technology will shortly permit the commercial introduction of transgenic crop varieties resistant to a number of non-selective herbicides. High levels of tolerance have been achieved both by overexpression of a target protein and by modification of that target to an insensitive form. However the results of preliminary trials suggest that in some instances the yield penalty for such genetic alterations will be prohibitive. An alternative strategy, based on the transfer and expression of a gene encoding a herbicide-detoxifying enzyme, appears to offer high resistance levels at low metabolic cost and is expected to assume increasing importance, although it may not prove suitable for all herbicides.

Recombination of Chlamydomonas chloroplast DNA occurs more frequently in the large inverted repeat sequence than in the single-copy regions

It is well documented that chloroplast DNA (cpDNA) recombination occurs at a relatively high frequency during sexual reproduction of unicellular green algae from the Chlamydomonas genus. Like the cpDNAs of most land plants, those of Chlamydomonas species are divided into two single-copy regions by a large inverted repeat sequence, part of which encodes the chloroplast rRNA genes. In the present study, we scored the inheritance of polymorphic loci spanning the entire chloroplast genome in hybrids recovered from reciprocal interspecific and F1 crosses between Chlamydomonas eugametes and C. moewusii, and from these data, estimated the density of recombination junctions within each region of recombinant cpDNAs. Our results indicate that recombination junctions occur at highly variable frequencies across the three main domains of the chloroplast genome. The large inverted repeat sequence was found to exhibit at least a five-fold higher density of recombination junctions compared to one of the singlecopy regions, whereas junctions in the latter region were five-fold more abundant relative to those in the other single-copy region. This marked difference in the densities of recombination junctions implies that the extent of genetic linkage between two given chloroplast loci will depend not only on their physical distance, but also on their locations within the genome.

Mutation in phenol-type herbicide resistance maps within the psbA gene in Synechocystis 6714

A Synechocytis 6714 mutant resistant to the phenol-type herbicide ioxynil was isolated and characterized. Sensitivity to DCMU and atrazine was tf measured in whole cells and isolated thylakoids. The mutant presents the same sensitivity to atrazine as the wild type and a slightly increased sensitivity to DCMU. A point mutation has been found at codon 266 in the psbAI coding locus (AAC to ACC) resulting in an amino acid change from asparagine to threonine in the D1 protein.

Urinary atrazine metabolites as indicators for rat and human exposure to atrazine

Rats were given atrazine (2-chloro-4-ethylamino-6-(isopropylamino)-s-triazine) in drinking water for 1 or 3 weeks at 0.1 (0.45 mM), 0.2 (0.9 mM) or 0.5 g/l (2.3 mM) concentrations of the commercial agent. They excreted at both time points as the principal metabolite 2-chloro-4-ethylamino-6-amino-s-trazine in a dose-dependent fashion. The same urine test was applied on 6 railway men engaged in the weeding operation of railway lines with known atrazine exposure measured with hygienic techniques in their breathing zone. The spectrum of their atrazine metabolites was comprised of fully N-dealkylated atrazine and 2-chloro-4-ethylamino-6-amino-s-triazine. The sum of the two urinary metabolites reflected quantitatively the exposure.

Variation in copy number of a 24-base pair tandem repeat in the chloroplast DNA of Oenothera hookeri strain Johansen

A highly variable region of chloroplast DNA has been analyzed from three isolates of Oenothera hookeri strain Johansen. The variability results from the presence of two, four or seven copies of a discrete 24-base pair tandem repeat in a segment of the chloroplast DNA within the inverted repeat. Alignment of this DNA region with the published tobacco cpDNA sequence shows that in Oenothera, the repeats are insertions within a large unidentified reading frame, with each repeat unit specifying an eight amino acid in-frame addition. A model to explain the frequent alterations in the copy number of this 24-bp unit is proposed: imprecise alignment and recombination between the two large inverted repeats followed by copy correction could result in an amplification or deletion of the 24-bp segments.

Imazaquin and chlorsulfuron resistance and cross resistance in mutants of Chlamydomonas reinhardtii

Chlorsulfuron and/or imazaquin resistant mutants of Chlamydomonas reinhardtii strain CW15 have been obtained and shown to have actolactate synthase (ALS) with altered sensitivity to one or both of these herbicides. Herbicide resistance in the three mutants described is allelic, and resistance appears to result from a dominant or semidominant mutation in a single, nuclear gene. Imazaquin and chlorsulfuron resistant ALS from imazaquin and chlorsulfuron resistant mutants, together with single-gene Mendelian inheritance of these phenotypes, suggests that ALS is the sole site of action of the two herbicides in Chlamydomonas. A high degree of cross resistance between the two herbicides was found in only one mutant. This mutant (IMR-13) was selected for resistance to imazaquin and has a high level of in vitro resistance to both imazaquin (270-fold increased I50) and chlorsulfuron (900-fold increased I50). In another mutant selected for resistance to imazaquin (IMR-2), hyper-sensitivity to chlorsulfuron was found. A mutant selected for resistance to chlorsulfuron (CSR-5), had a substantial degree of resistance of chlorsulfuron (80-fold increased I50), but not to imazaquin (7-fold increased I50).

Molecular genetics of herbicide resistance in cyanobacteria

Cyanobacteria offer an excellent model system for studies of herbicide resistance in higher plants. Mutants resistant to classical and non-classical herbicides have been isolated, and in some cases the amino acid alteration(s) are known. Mutations in plants, algae, photosyntehtic bacteria, and cyanobacteria are compared. Data concerning the question of dominance or recessiveness of herbicide resistance in cyanobacteria are also discussed.

Reaction centers from three herbicide-resistant mutants of Rhodobacter sphaeroides 2.4.1: sequence analysis and preliminary characterization

Many herbicides that inhibit photosynthesis in plants also inhibit photosynthesis in bacteria. We have isolated three mutants of the photosynthetic bacterium Rhodobacter sphaeroides that were selected for increased resistance to the herbicide terbutryne. All three mutants also showed increased resistance to the known electron transfer inhibitor o-phenanthroline. The primary structures of the mutants were determined by recombinant DNA techniques. All mutations were located on the gene coding for the L-subunit resulting in these changes Ile(229) → Met, Ser(223) → Pro and Tyr(222) → Gly. The mutations of Ser(223) is analogous to the mutation of Ser(264) in the D1 subunit of photosystem II in green plants, strengthening the functional analogy between D1 and the bacterial L-subunit. The changed amino acids of the mutant strains form part of the binding pocket for the secondary quinone, Q b . This is consistent with the idea that the herbicides are competitive inhibitors for the Q b binding site. The reaction centers of the mutants were characterized with respect to electron transfer rates, inhibition constants of terbutryne and o-phenanthroline, and binding constants of the quinone UQ0 and the inhibitors. By correlating these results with the three-dimensional structure obtained from x-ray analysis by Allen et al. (1987a, 1987b), the likely positions of o-phenanthroline and terbutryne were deduced. These correspond to the positions deduced by Michel et al. (1986a) for Rhodopseudomonas viridis.

Plants: novel developmental processes

Plants represent a diverse group of organisms that have unique reproductive, developmental, and physiological processes. Although morphologically simple, plants have molecular genetic processes that are equivalent in complexity to those found in animals. Sophisticated gene transfer procedures, transposon mutagenesis in homologous and heterologous plants, and development of model organisms such as Arabidopsis permit almost any gene that is associated with an observable phenotype to be isolated and studied. These advances, coupled with general advances in molecular biology, now make it possible to dissect the molecular and cellular events responsible for controlling plant-specific processes.

Genetic analysis of two new mutations resulting in herbicide resistance in the cyanobacterium Synechcoccus sp. PCC 7002

Two herbicide-resistant strains of the cyanobacterium Synechococcus sp. PCC 7002 are compared to the wild-type with respect to the DNA changes which result in herbicide resstance. The mutations have previously been mapped to a region of the cyanobacterial genome which encodes oneof three copies of psbA, the gene which encodes the 32 kDa Qb-binding protein also known as D1 (Buzby et al. 1987). The DNA sequence of the wild-type gene was first determined and used as a comparison to that of the mutant alleles. A point mutation at codon 211 in the psbA1 coding locus (TTC) to TCC) results in an amino acid change from phenylalanine to serine in the D1 protein. This mutation confers resistance to atrazine and diuron at seven times and at two times the minimal inhibitory concentration (MIC) for the wild-type, respectively. A mutation at codon 211 resulting in herbicide resistance has not previously been described in the literature. A second point mutation at codon 219 in the psbA1 coding locus (GTA to ATA) results in an amino acid change from valine to isoleucine in the D1 protein. This mutation confers resistance to diuron and atrazine at ten times and at two times the MIC for the wild-type, respectively. An identical codon change conferring similar herbicide resistance patterns has previously been described in Chlamydomonas reinhardtii. The atrazine-resistance phenotype in Synechococcus sp. PCC 7002 was shown to be dominant by plasmid segregation analysis.