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

Discovery of N-benzyl-6-methylpicolinamide as a potential scaffold for bleaching herbicides

BACKGROUND

In pesticide research, bleaching herbicides have always been a hot topic. Our previous research showed that N-(4-fluorobenzyl)-2-methoxybenzamide is an innovative lead compound for bleaching herbicides.

RESULTS

A total of 40 derivatives of picolinamides were prepared and evaluated for their herbicidal activity by Petri dish tests and postemergence trials. The structure-activity relationship (SAR) revealed that introducing electron-withdrawing groups at the 3- or 4-positions of the benzyl significantly enhances herbicidal activity. Furthermore, ZI-04 induced similar symptoms such as bleaching effect in treated weeds and accumulation of biosynthetic precursors for carotenoids as observed with diflufenican. ZI-04 also exhibited significant cross-resistance to diflufenican and had a lower resistance risk than diflufenican.

CONCLUSION

N-benzyl-6-methylpicolinamides were discovered as a novel scaffold for bleaching herbicides. The accumulation of phytoene, phytofluene and ζ-Carotene in radish cotyledons, and cross-resistance observed with diflufenican, showed that title compounds can interfere with carotenoid biosynthesis. © 2024 Society of Chemical Industry.

An introduction to ALS-inhibiting herbicides

Herbicides that inhibit acetolactate synthase (ALS), the enzyme common to the biosynthesis of the branch-chain amino acids (valine, leucine, and isoleucine), affect many species of higher plants as well as bacteria, fungi, yeasts, and algae. The novel mechanism of action attributed to ALS inhibitors, their effect on the reproduction of some plant species, their potency at extremely low concentrations, and the rapid evolution of resistance to these herbicides in some plants and microorganisms are characteristics that set ALS inhibitors apart from their predecessors. This class of chemicals affects seedling growth. Older plants exhibit varied signs of malformation, stunting, and reduced seed production. These herbicides are so potent that they can affect plants at levels that are undetectable by any standard chemical protocol. Weeds quickly become resistant to ALS inhibitors, presumably because these herbicides have a single mode of action and because many have long residual activity. Concern now is directed towards developing the technology to detect very low concentrations of ALS inhibitors in the environment and their indirect effects on plant and animal health.

Molecular analysis of the acetolactate synthase gene of Chlamydomonas reinhardtii and development of a genetically engineered gene as a dominant selectable marker for genetic transformation

Genomic and cDNA clones of the acetolactate synthase (ALS) gene of Chlamydomonas reinhardtii have been isolated from a mutant, c85-20 (Hartnett et al., 1987), that is resistant to high concentrations of sulfometuron methyl (SMM) and related sulfonylurea herbicides. Comparison of the ALS gene sequences from the wild-type and the SMM resistant (SMMr) strains revealed two amino acid differences in the mature enzyme, a lysine to threonine change at position 257 (K257T) and a leucine to valine change at position 294 (L294V). Transformation of wild-type C. reinhardtii with the mutant ALS gene produced no transformants with ability to grow in the presence of a minimum toxic concentration of SMM (3 microm). Substitution of the ALS promoter with the promoter of the C. reinhardtii Rubisco small subunit gene (RbcS2) permitted recovery of SMMr colonies. In vitro mutagenesis of the wild-type ALS gene to produce various combinations of mutations (K257T, L294V and W580L) indicated that the K257T mutation was necessary and sufficient to confer the SMMr phenotype. Optimum transformation rates were obtained with two constructs (pJK7 and pRP-ALS) in which all introns in the coding region were present. Rates of transformation with construct pJK7 were approximately 2.5 x 10-4 transformants/cell (i.e. one transformant for each of 4000 initial cells) using electroporation and 8.5 x 10-6 transformants/cell using the glass bead vortexing method. These results suggest that pJK7 and pRP-ALS can serve as important additional dominant selectable markers for the genetic transformation of C. reinhardtii.

An ISFET-algal (Chlamydomonas) hybrid provides a system for eco-toxicological tests

A cellular sensoring system was designed in which metabolism-dedicated pH-ISFETs and the unicellular green alga Chlamydomonas reinhardtii as a biological component, were combined. The system permits on-line detection of pH changes caused by the metabolic and photosynthetic activities of the cells. Photosynthetic activity results in a basification of the medium caused by uptake of CO2. In darkness, an acidification of the medium, resulting from the production of CO2 by degradation of starch was observed. Both, acidification and basification, are sensitive indicators for the physiological activity of the alga. Experiments using inhibitors of energy metabolism or photosynthesis illustrate the utility of this system for an on-line monitoring of substances of eco-toxicological importance.

Costs of resistance: a test using transgenic Arabidopsis thaliana

Evolutionary biologists have long attributed polymorphisms in resistance status to fitness costs of resistance traits. Nevertheless, pleiotropic fitness costs of resistance have been notoriously difficult to detect. We have transformed Arabidopsis thaliana with a mutant acetolactate synthase gene that confers resistance to the herbicide, chlorsulfuron. Our experiment revealed a 34% reduction in the lifetime seed production of transgenic, herbicide resistant Arabidopsis thaliana relative to their susceptible null segregants. Our experimental design allows us to conclude that this fitness cost of resistance is caused by the pleiotropic effect of the introduced acetolactate synthase gene rather than other potential costs associated with the plasmid or mutational changes induced by plant transformation. In addition, we can attribute the cost of resistance to the presence of the resistance gene rather than an increase in gene dosages. The implications of these results for the risk assessment of transgenic crops are discussed.

Multiple resistance to sulfonylureas and imidazolinones conferred by an acetohydroxyacid synthase gene with separate mutations for selective resistance

The acetohydroxyacid synthase (AHAS) gene from the Arabidopsis thaliana mutant line GH90 carrying the imidazolinone resistance allele imr1 was cloned. Expression of the AHAS gene under the control of the CaMV 35S promoter in transgenic tobacco resulted in selective imidazolinone resistance, confirming that the single base-pair change found near the 3' end of the coding region of this gene is responsible for imidazolinone resistance. A chimeric AHAS gene containing both the imr1 mutation and the csr1 mutation, responsible for selective resistance to sulfonylurea herbicides, was constructed. It conferred on transgenic tobacco plants resistance to both sulfonylurea and imidazolinone herbicides. The data illustrate that a multiple-resistance phenotype can be achieved in an AHAS gene through combinations of separate mutations, each of which individually confers resistance to only one class of herbicides.

Effect of O2 and CO 2 on net CO 2 exchange in a high-CO 2-requiring mutant of Chlamydomonas reinhardtii during dark-light-dark transitions

Net CO2 exchange was monitored through a dark-light-dark transition, under 2% and 21% O2 in the presence and absence of CO2, in Chlamydomonas reinhardtii wild type and the high-CO2-requiring mutant ca-1-12-1C. Upon illumination at 350 μl/l CO2, ca-1-12-1C cell exhibited a large decrease in net CO2 uptake following an initial surge of CO2 uptake. Net CO2 uptake subsequently attained a steady-state rate substantially lower than the maximum. A large, O2-enchanced post-illumination burst of CO2 efflux was observed after a 10-min illumination period, corresponding to a minimum in the net CO2 uptake rate. A smaller, but O2-insensitive post-illumination burst was observed following a 30-min illumination period, when net CO2 uptake was at a steady-state rate. These post-illumination bursts appeared to reflect the release of an intracellular pool of inorganic carbon, which was much larger following the initial surge of net CO2 uptake than during the subsequent steady-state CO2 uptake period.With the mutant in CO2-free gas, O2-stimulated, net CO2 efflux was observed in the light, and a small, O2-dependent post-illumination burst was observed. With wild-type cells no CO2 efflux was observed in the light in CO2-free gas under either 2% or 21% O2, but a small, O2-dependent post-illumination burst was observed. These results were interpreted as indicating that photorespiratory rates were similar in the mutant and wild-type cells in the absence of CO2, but that the wild-type cells were better able to scavenge the photorespiratory CO2.