Tailoring Natural Abenquines To Inhibit the Photosynthetic Electron Transport through Interaction with the D1 Protein in Photosystem II

Natural Rubrolides and Their Synthetic Congeners as Inhibitors of the Photosynthetic Electron Transport Chain

Rubrolides are a family of naturally occurring 5-benzylidenebutenolides, which generally contain brominated phenol groups, and nearly half of them also present a chlorine attached to the butenolide core. Seven natural rubrolides were previously synthesized. When these compounds were tested against the model plant Raphanus sativus, six were found to exert a slight inhibition on plant growth. Aiming to exploit their scaffold as a model for the synthesis of new compounds targeting photosynthesis, nine new rubrolide analogues were prepared. The synthesis was accomplished in 2-4 steps with a 10-39% overall yield from 3,4-dichlorofuran-2(5H)-one. All compounds were evaluated for their ability to inhibit the whole Hill reaction or excluding photosystem I (PSI). Several natural rubrolides and their analogues displayed good inhibitory potential (IC50 = 2-8 μM). Molecular docking studies on the photosystem II-light harvesting complex II (PSII-LHCII supercomplex) binding site were also performed. Overall, data support the use of rubrolides as a model for the development of new active principles targeting the photosynthetic electron transport chain to be used as herbicides.

Synthesis and Evaluation of New Phytotoxic Fluorinated Chalcones as Photosystem II and Seedling Growth Inhibitors

The development of novel phytotoxic compounds has been an important aim of weed control research. In this study, we synthesized fluorinated chalcone derivatives featuring both electron-donating and electron-withdrawing groups. These compounds were evaluated both as inhibitors of the photosystem II (PSII) electron chain as well as inhibitors of the germination and seedling growth of Amaranthus plants. Chlorophyll a (Chl a) fluorescence assay was employed to evaluate their effects on PSII, while germination experiments were conducted to assess their impact on germination and seedling development. The results revealed promising herbicidal activity for (E)-3-(4-bromophenyl)-1-(4-fluorophenyl)prop-2-en-1-one (7 a) and (E)-1-(4-fluorophenyl)-3-phenylprop-2-en-1-one (7 e). Compounds 7 a and 7 e exhibited a reduction in Chl a parameters associated with performance indexes and electron transport per reaction center. This reduction suggests a decrease in PSII activity, attributed to the blockage of electron flow at the quinone pool. Molecular docking analyses of chalcone derivatives with the D1 protein of PSII revealed a stable binding conformation, wherein the carbonyl and fluorine groups interacted with Phe265 and His215 residues, respectively. Additionally, at a concentration of 100 μM, compound 7 e demonstrated pre- and post-emergent herbicidal activity, resulting in a reduction of the seed germination index, radicle and hypocotyl lengths of Amaranthus weeds.

Potential Candidate Molecule of Photosystem II Inhibitor Herbicide-Brassicanate A Sulfoxide

Brassicanate A sulfoxide, a secondary metabolite of broccoli, exhibited the inhibition of weed growth, but its mechanism of action on weeds remains unclear. To elucidate the mechanism by which brassicanate A sulfoxide suppresses weeds, this study explores the interaction between brassicanate A sulfoxide and the photosystem II D1 protein through molecular docking and molecular dynamics simulations. This research demonstrates that brassicanate A sulfoxide interacts with the photosystem II D1 protein by forming hydrogen bonds with Phe-261 and His-214. The successful expression of the photosystem II D1 protein in an insect cell/baculovirus system validated the molecular docking and dynamics simulations. Biolayer interferometry experiments elucidated that the affinity constant of brassicanate A sulfoxide with photosystem II was 2.69 × 10-3 M, suggesting that brassicanate A sulfoxide can stably bind to the photosystem II D1 protein. The findings of this study contribute to the understanding of the mode of action of brassicanate A sulfoxide and also aid in the development of natural-product-based photosynthesis-inhibiting herbicides.

Mode-of-action of the natural herbicide radulanin A as an inhibitor of photosystem II

BACKGROUND

Radulanin A is a natural 2,5-dihydrobenzoxepin synthesized by several liverworts of the Radula genus. Breakthroughs in the total synthesis of radulanin A paved the way for the discovery of its phytotoxic activity. Nevertheless, its mode-of-action (MoA) has remained unknown so far and thus was investigated, in Arabidopsis thaliana.

RESULTS

Radulanin A phytotoxicity was associated with cell death and partially depended on light exposure. Photosynthesis measurements based on chlorophyll-a fluorescence evidenced that radulanin A and a Radula chromene inhibited photosynthetic electron transport with IC50 of 95 and 100 μm, respectively. We established a strong correlation between inhibition of photosynthesis and phytotoxicity for a range of radulanin A analogs. Based on these data, we also determined that radulanin A phytotoxicity was abolished when the hydroxyl group was modified, and was modulated by the presence of the heterocycle and its aliphatic chain. Thermoluminescence studies highlighted that radulanin A targeted the QB site of the Photosystem II (PSII) with a similar MoA as 3-(3,4-dichloropheny)-1,1-dimethylurea (DCMU).

CONCLUSION

We establish that radulanin A targets PSII, expanding QB sites inhibitors to bibenzyl compounds. The identification of an easy-to-synthesize analog of radulanin A with similar MoA and efficiency might be useful for future herbicide development. © 2023 Society of Chemical Industry.

Binding Properties of Photosynthetic Herbicides: Photosynthetic Activity and Molecular Docking Approach towards 1,4-Dihydropyridines Derivatives

In the current work, we describe the synthesis of 1,4-dihydropyridine (1,4-DHP) derivatives via Hantzsch multicomponent reaction and their evaluation as photosystem II (PSII) inhibitors through chlorophyll a fluorescence bioassay. Among all the compounds tested, 1,1'-(2,4,6-trimethyl-1,4-dihydropyridine-3,5-diyl)bis(ethan-1-one) (4b) showed best results, reducing the parameters performance index on absorption basis (PI_abs ) and electron transport per reaction center by 61 % and 49 %, respectively, as compared to the control. These results indicate the inhibitory activity of PSII over the electron transport chain. Additionally, a molecular docking approach using the protein D1 (PDB code 4V82) was performed in order to assess the structure-activity relationship among the 1,4-DHP derivatives over the PSII, which revealed that both, size of the group at position 4 and the carbonyl groups at the dihydropyridine ring are important for the ligand's interaction, particularly for the hydrogen-bonding interaction with the residues His215, Ser264, and Phe265. Thus, the optimization of these molecular features is the aim of our research group to extend the knowledge of PSII electron chain inhibitors and the establishment of new potent bioactive molecular scaffolds.

Evaluation of Chalcone Derivatives as Photosynthesis and Plant Growth Inhibitors

We report the evaluation of chalcone derivatives as photosystem II (PSII) and plant growth inhibitors. Chalcone derivatives were evaluated as PSII inhibitors through Chl a fluorescence measurement. (E)-Chalcone (6a) and (E)-3-(4-bromophenyl)-1-(4-fluorophenyl)prop-2-en-1-one (6j) showed the best results, reducing the performance index on absorption basis parameter (PI_abs ) by 70 %. Additionally, the decrease of TR₀ /RC and ET₀ /RC parameters indicates that the chalcone derivatives limited the number of active PSII reaction centers and the amount of trapped energy within them. Compounds 6a and 6j both act as post-emergent herbicides at 50 μM, reducing the root biomass of the Ipomoea grandifolia weed by 72 % and 83 %, respectively, corroborating the fluorescence results. The selectivity against weeds as compared to valuable crops by compounds 6a and 6j were evaluated employing Zea mays and Phaseolus vulgaris plants. In these, our newly synthesized compounds showed no effects on biomass accumulation of roots and aerial parts when compared to the control, providing valuable evidence for the role of these compounds as selective inhibitors of the growth of undesired weeds.

Herbicidal Secondary Metabolites from Actinomycetes: Structure Diversity, Modes of Action, and Their Roles in the Development of Herbicides

Weeds had caused significant loss for crop production in the process of agriculture. Herbicides have played an important role in securing crop production. However, the high reliance on herbicides has led to environmental issues as well as the evolution of herbicide resistance. Thus, there is an urgent need for new herbicides with safer toxicological profiles and novel modes of action. Actinomycetes produce very diverse bioactive compounds, of which some show potent biopesticidal activity. The herbicidal secondary metabolites from actinomycetes can be classified into several groups, such as amino acids, peptides, nucleosides, macrolides, lactones, amide, amines, etc., some of which have been successfully developed as commercial herbicides. The structure diversity and evolved biological activity of secondary metabolites from actinomycetes can offer opportunities for the development of both directly used bioherbicides and synthetic herbicides with new target sites, and thus, this review focuses on the structure, herbicidal activity, and modes of action of secondary metabolites from actinomycetes.

NADP-Malate Dehydrogenase of Sweet Sorghum Improves Salt Tolerance of Arabidopsis thaliana

Sweet sorghum is a C₄ crop that shows high salt tolerance and high yield. NADP-malate dehydrogenase ( NADP-ME) is a crucial enzyme of the C₄ pathway. The regulatory mechanism of NADP-ME remains unclear. In this study, we isolated SbNADP-ME from sweet sorghum. The open reading frame of SbNADP-ME is 1911 bp and 637 amino acid residues. Quantitative real-time PCR analysis showed that SbNADP-ME transcription in sweet sorghum was enhanced by salt stress. The SbNADP-ME transcript level was highest under exposure to 150 mM NaCl. Arabidopsis overexpressing SbNADP-ME showed increased germination rate and root length under NaCl treatments. At the seedling stage, physiological photosynthesis parameters, chlorophyll content, PSII photochemical efficiency, and PSI oxidoreductive activity in the wild type decreased more severely than in the overexpression lines but less than in T-DNA insertion mutants under salt stress. Overexpression of SbNADP-ME in Arabidopsis may also increase osmotic adjustment and scavenging activity on DPPH and decrease membrane peroxidation. These results suggest that SbNADP-ME overexpression in Arabidopsis increases salt tolerance and alleviates PSII and PSI photoinhibition under salt stress by improving photosynthetic capacity.