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.

Rational Design, Synthesis and Evaluation of Indole Nitrogen Hybrids as Photosystem II Inhibitors.

We report the synthesis of twelve indole derivatives bearing nitro or amide groups via Fischer indole methodology followed by reduction/acetylation and amidation reactions. After thorough characterization, these indoles were subjected to a number of studies in order to evaluate their bioactive potential as photosynthesis and plant growth inhibitors. Firstly, these molecular hybrids were evaluated as photosystem II (PSII) inhibitors through chlorophyll a (Chl a) fluorescence measurement. In this study, 6-chloro-8-nitro-2,3,4,9-tetrahydro-1H-carbazole (15a) and 5-chloro-2,3-dimethyl-7-nitro-1H-indole (15b) showed the best results by reducing the phenomenological parameters of reaction centers ABS/RC, TR0 /RC and ET0 /RC of PSII. Electron chain blockage by these compounds may lead to diminished ATP synthesis and CO2 fixation which interrupt the plant development. The compounds 15a and 15b both act as postemergent herbicides, reducing the dry biomass of Ipomoea grandifolia and Senna alata weeds by an average of 40% and 37%, respectively, corroborating the fluorescence results. Additionally, the molecular docking study revealed that the presence of strong electron-withdrawing groups at the indole phenyl ring is important for the ligand's interaction with the binding pocket of protein D1 on PSII. The optimization of these molecular features is the goal of our research group in further understanding and development of new potent herbicides.

Synthesis and evaluation of indole derivatives as photosynthesis and plant growth inhibitors.

Indole derivatives were synthetized based on the Fischer indole methodology using different phenyl hydrazine hydrochlorides and either cyclohexanone or 2-butanone. The pre- and post-emergent herbicidal activities were evaluated against Ipomoea grandifolia. A carbazole, 6-chloro-2,3,4,9-tetrahydro-1H-carbazole (3b), decreased the PIabs parameter by 32% and increased the cross-section related parameters, indicating the inactivation of the reaction center on photosystem II. Compound 3b acts as a post-emergent herbicide prototype since dry biomass was reduced by 50%, corroborating the fluorescence results. Comparing instead with a germination experiment, 2,3,4,9-tetrahydro-1H-carbazole (3a) was found to be the most effective agent, inhibiting seed germination by 22% and decreasing root length by 50%. The tetrahydrocarbazoles showed better results than indole derivatives potentially due to the presence of methylene groups at structures, which increase the compounds' lipophilicity and may facilitate their access to the plant. In addition, electron withdrawing groups on the aromatic ring were found to correlate with increased herbicide activity. Further optimization of this series towards the development of herbicides is ongoing.

Evaluation of Alkaloids Isolated from Ruta graveolens as Photosynthesis Inhibitors.

Eight alkaloids (1⁻8) were isolated from Ruta graveolens, and their herbicide activities were evaluated through in vitro, semivivo, and in vivo assays. The most relevant results were observed for Compounds 5 and 6⁻8 at 150 µM, which decreased dry biomass by 20% and 23%, respectively. These are significant results since they presented similar values with the positive control, commercial herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Based on the performed assays, Compound 5 (graveoline) is classified as an electron-transport inhibitor during the light phase of photosynthesis, as well as a plant-growth regulator. On the other hand, Compounds 6⁻8 inhibited electron and energy transfers, and are also plant-growth inhibitors. These phytotoxic behaviors based on acridone and quinolone alkaloids may serve as a valuable tool in the further development of a new class of herbicides since natural products represent an interesting alternative to replace commercial herbicides, potentially due their low toxicity.

Diversity of cultivable fungal endophytes in Paullinia cupana (Mart.) Ducke and bioactivity of their secondary metabolites.

Paullinia cupana is associated with a diverse community of pathogenic and endophytic microorganisms. We isolated and identified endophytic fungal communities from the roots and seeds of P. cupana genotypes susceptible and tolerant to anthracnose that grow in two sites of the Brazilian Amazonia forest. We assessed the antibacterial, antitumor and genotoxic activity in vitro of compounds isolated from the strains Trichoderma asperellum (1BDA) and Diaporthe phaseolorum (8S). In concert, we identified eight fungal species not previously reported as endophytes; some fungal species capable of inhibiting pathogen growth; and the production of antibiotics and compounds with bacteriostatic activity against Pseudomonas aeruginosa in both susceptible and multiresistant host strains. The plant genotype, geographic location and specially the organ influenced the composition of P. cupana endophytic fungal community. Together, our findings identify important functional roles of endophytic species found within the microbiome of P. cupana. This hypothesis requires experimental validation to propose management of this microbiome with the objective of promoting plant growth and protection.

Evaluation of antidesmone alkaloid as a photosynthesis inhibitor.

Antidesmone, isolated from Waltheria brachypetala Turcz., owns special structural features as two α,ß-unsaturated carbonyl groups and a side alkyl chain that can compete with the quinones involved in the pool of plastoquinones at photosystem II (PSII). In this work, we showed that the alkaloid is an inhibitor of Hill reaction and its target was located at the acceptor side of PSII. Studies of chlorophyll (Chl) a fluorescence showed a J-band that indicates direct action of antidesmone in accumulation of QA- (reduced plastoquinone A) due to the electron transport blocked at the QB (plastoquinone B) level similar to DCMU. In vivo assays indicated that antidesmone is a selective post-emergent herbicide probe at 300µM by reducing the biomass production of Physalis ixacarpa plants. Furthermore, antidesmone also behaves as pre-emergent herbicide due to inhibit Physalis ixacarpa plant growth about 60%. Antidesmone, a natural product containing a 4(1H)-pyridones scaffold, will serve as a valuable tool in further development of a new class of herbicides.

Furoquinoline alkaloids isolated from Balfourodendron riedelianum as photosynthetic inhibitors in spinach chloroplasts.

In the search for natural inhibitors of plant growth, we investigate the mechanism of action of the natural furoquinoline alkaloids isolated from Balfourodendron riedelianum (Rutaceae): evolitrine (1), kokusaginine (2), γ-fagarine (3), skimmianine (4) and maculosidine (5) on the photosynthesis light reactions. Their effect on the electron transport chain on thylakoids was analyzed. Alkaloids 1, 2, 4 and 5 inhibited ATP synthesis, basal, phosphorylating and uncoupled electron transport acting as Hill reaction inhibitors on spinach chloroplasts. Alkaloid 3 was not active. The inhibition and interaction site of alkaloids 1, 2, 4 and 5 on the non-cyclic electron transport chain was studied by polarography and fluorescence of the chlorophyll a (Chl a). The results indicate that the target for 1 was localized on the donor and acceptor side of PS II. In addition alkaloids 2 and 5 affect the PS I electron acceptors on leaf discs.

Chemical profile of rums as a function of their origin. The use of chemometric techniques for their identification.

To identify chemical descriptors to distinguish Cuban from non-Cuban rums, analyses of 44 samples of rum from 15 different countries are described. To provide the chemical descriptors, analyses of the the mineral fraction, phenolic compounds, caramel, alcohols, acetic acid, ethyl acetate, ketones, and aldehydes were carried out. The analytical data were treated through the following chemometric methods: principal component analysis (PCA), partial least square-discriminate analysis (PLS-DA), and linear discriminate analysis (LDA). These analyses indicated 23 analytes as relevant chemical descriptors for the separation of rums into two distinct groups. The possibility of clustering the rum samples investigated through PCA analysis led to an accumulative percentage of 70.4% in the first three principal components, and isoamyl alcohol, n-propyl alcohol, copper, iron, 2-furfuraldehyde (furfuraldehyde), phenylmethanal (benzaldehyde), epicatechin, and vanillin were used as chemical descriptors. By applying the PLS-DA technique to the whole set of analytical data, the following analytes have been selected as descriptors: acetone, sec-butyl alcohol, isobutyl alcohol, ethyl acetate, methanol, isoamyl alcohol, magnesium, sodium, lead, iron, manganese, copper, zinc, 4-hydroxy-3,5-dimethoxybenzaldehyde (syringaldehyde), methaldehyde (formaldehyde), 5-hydroxymethyl-2-furfuraldehyde (5-HMF), acetaldehyde, 2-furfuraldehyde, 2-butenal (crotonaldehyde), n-pentanal (valeraldehyde), iso-pentanal (isovaleraldehyde), benzaldehyde, 2,3-butanodione monoxime, acetylacetone, epicatechin, and vanillin. By applying the LDA technique, a model was developed, and the following analytes were selected as descriptors: ethyl acetate, sec-butyl alcohol, n-propyl alcohol, n-butyl alcohol, isoamyl alcohol, isobutyl alcohol, caramel, catechin, vanillin, epicatechin, manganese, acetaldehyde, 4-hydroxy-3-methoxybenzoic acid, 2-butenal, 4-hydroxy-3,5-dimethoxybenzoic acid, cyclopentanone, acetone, lead, zinc, calcium, barium, strontium, and sodium. This model allowed the discrimination of Cuban rums from the others with 88.2% accuracy.