The structural modification of natural pyrethrins has led to a number of synthetic pyrethroid insecticides, and each compound has its own characteristics. At present, pyrethroid insecticides are applied not only for household use, the original use for pyrethrins, but also for a wide range of uses such as crop protection, pharmaceuticals, and veterinary applications. Quoting primary sources, this review describes the historical view of structural modifications of pyrethroids, with a focus on structural similarities, and their use.
Seven algal species were used to conduct toxicity assays with 12 herbicides to determine differences in species sensitivity. A fluorescence microplate toxicity assay was used as an efficient and economical high-throughput assay. The obtained toxicity data were standardized based on the species sensitivity distribution concept. The most-sensitive individual species differed among herbicides: Desmodesmus subspicatus was most sensitive to chloronitrofen and pendimethalin; Achnanthidium minutissimum was most sensitive to chlorpropham; Nitzschia palea was most sensitive to diquat, glyphosate, and dichlobenil; Navicula pelliculosa was most sensitive to trifluralin; and Pseudanabaena foetida was most sensitive to glufosinate, asulam, and 2,4-D. Surprisingly, Raphidocelis (formerly Pseudokirchneriella) subcapitata, a standard green alga, was not the most sensitive to any of the herbicides. The results clearly showed that a single algal species cannot represent the algal assemblage in terms of sensitivity. Therefore, multispecies algal toxicity data sets are essential for assessing the ecological effect of herbicides.
Twenty-seven analogues of pyrazole derivatives were synthesized and subjected to structure–activity relationship studies on inducing the triple response in Arabidopsis seedlings. We found that 3,4-Dichloro-N-methyl-N-[(1-allyl-3,5-dimethyl-1H-pyrazol-4-yl)methyl]benzenesulfonamide (C26) exhibits potent activity on inducing the triple response in Arabidopsis seedlings. C26 (10 µM) induced an exaggerated apical hook in Arabidopsis seedlings. The curvature of the hook of the Arabidopsis seedlings was found to be 300±23 degrees, while ethephon (10 µM), a prodrug of ethylene, and a non-chemically treated control were found to be 128±19 and 58±16 degrees, respectively. C26 also exhibited potent activity on reducing stem elongation. The hypocotyl length of Arabidopsis seedlings treated with C26 (10 µM) was found to be 0.25±0.02 cm, while those of ethephon-treated (10 µM) and treated controls were found to be 0.69±0.06 and 1.15±0.01 cm, respectively. C26 displayed potency inhibiting the root growth of Arabidopsis seedlings similar to that of ethephon.
A series of penta-1,4-diene-3-one oxime ether derivatives were synthesized, and their antiviral and antifungal activities were evaluated. Bioactivity evaluations showed that most target compounds had significant antiviral effects against tobacco mosaic virus (TMV). Among them, (1E,3Z,4E)-1-(4-(benzyloxy)phenyl)-5-(furan-2-yl)penta-1,4-dien-3-one O-(3-fluorobenzyl) oxime (5e) was found to have good curative activity against TMV, with an inhibition rate of 64.6%, which was better than that of ribavirin (45.2%). (1E,3Z,4E)-1-(4-(benzyloxy) phenyl)-5-(furan-2-yl)penta-1,4-dien-3-one O-((6-chloropyridin-3-yl)methyl) oxime (5d) had a remarkable protective effect against TMV, with an inhibitory rate of 66.9%, which was better than that of ribavirin (61.8%). The inhibitory rate of (1E,3Z,4E)-1-(2-(benzyloxy)phenyl)-5-(furan-2-yl)penta-1,4-dien-3-one O-(4-chlorobenzyl) oxime(5m) in inactivation activity against TMV was 87.0%, which was better than that of ribavirin (77.9%). Further molecular docking studies indicated that compound 5m shows strong binding affinities toward the coat protein of tobacco mosaic virus. This result indicates that penta-1,4-diene-3-one oxime ether derivatives can play a significant role in discovering new antiviral agents.
Undesirable side effects on ecosystems and strong selection for weed resistance demand an increase in the efficacy and a reduction in the dosage of glyphosate herbicide used. The synergistic effect of tank-mixed adjuvant KAO® A-134 (A-134) on the post-emergence activity of the commercial glyphosate formulation Roundup® (RDP) against crabgrass (Digitaria sanguinalis) was detected. Field study also showed that A-134 can increase the herbicidal effect of RDP. Meanwhile, A-134 concentration-dependently decreased the surface tension and increased the spreading area of RDP, causing faster penetration and improved uptake of glyphosate into crabgrass. Moreover, the tank mix with A-134 also increased the adhesion of spray droplets of glyphosate isopropylamine salt (GP) to the leaf surface after rainfall treatment, thus maintaining its herbicidal effect. Data suggested the necessity of using these synergistic properties of A-134 to reduce environmental exposure and glyphosate resistance selection.
Pyripyropene A (PP-A), a secondary metabolite produced by filamentous fungi, shows insecticidal activity against agricultural insect pests. Synthesized PP derivatives also show a narrow insecticidal spectrum but high insecticidal activities against such sucking pests. PP-A has a low eco-toxicological impact and satisfies a prerequisite for next-generation insecticides. We investigated the effects of conversion of the 3-pyridyl and α-pyrone rings to other rings, as well as the effects of esterification, dehydration, and oxidization at the C-13 position in natural PP analogues, on the insecticidal activity and spectrum. The conversions of the 3-pyridyl and α-pyrone rings markedly reduced the insecticidal activity with a minimal impact on the spectrum, indicative of an important role for these rings in insecticidal activity. Some derivatives with modified structures at the C-13 position showed a higher inhibitory effect on the motility of canine heartworms and mosquito vectors than did PP-A, suggesting their utility as filaria control drugs.
We have evaluated the toxicity of flutianil, which was developed by OAT Agrio. Flutianil shows low toxicity, no carcinogenicity, no reproductive toxicity, and no genotoxicity. Based on these results, the ADI of flutianil has been set at 2.4 mg/kg bw/day with a safety factor of 100.
In recent years, it has become clear that the crosstalk of various plant hormones controls plant growth and disease resistance. Plant hormone signals may also be involved in the actions of a variety of pesticides and disease control techniques used for crop protection. From this point of view, we have focused on plant hormones to analyze the mode of action of pesticides that function in plants. Disease resistance inducers are pesticides that induce systemic acquired resistance (SAR) by activating the salicylic acid (SA)-mediated signaling pathway. However, when under unfavorable climate conditions, such as cold and cloudy weather, the resistance inducers are not sufficiently effective. Since the environmental stress response mediated by abscisic acid (ABA) may affect disease resistance, extensive studies of tobacco and tomato plants were performed, which clarified that SAR induction was suppressed by ABA. On the other hand, it was shown that transient high temperature treatment enhanced disease resistance via SA biosynthesis. These results suggest that changes in temperature due to climate change have an impact on disease resistance. The mode of action of a plant-growth regulator was analyzed by focusing on plant hormones. Isoprothiolane (IPT), an active ingredient of Fuji-one, is used as a plant-growth regulator and a fungicide. In Arabidopsis thaliana, we demonstrated that jasmonic acid and ethylene are required for the root elongation-promoting effect of IPT. As shown above, mode-of-action studies on pesticides in relation to plant hormones will lead to the development of new techniques for the better cultivation and protection of crops.
Fusarium diseases are significant hindrances to food plant production and are very difficult to control, especially soilborne diseases caused by F. oxysporum. First I outline the Fusarium diseases and introduce examples of the recent outbreak of Fusarium diseases in Japan. Then I summarize my studies on (1) the control of Fusarium diseases by biological agents and by inducing resistance to diseases in plants, (2) the specific detection of forms and races in F. oxysporum using immunological measures and molecular measures based on phylogeny and pathogenicity-determining genes, and (3) molecular and genetic studies on Fusarium diseases, including evolutionary, genetic, and genomic analyses of the emergence and divergence of forms and races in F. oxysporum.
Fenoxasulfone is a novel rice herbicide that was discovered and developed by Kumiai Chemical Industry Co., Ltd. It displays excellent herbicidal activity against Echinochloa spp. and other annual weeds at 150–200 g a.i./ha with long residual activity and has a favorable toxicological, ecotoxicological, and environmental profile. Fenoxasulfone’s mode of action was investigated, and it has been shown to inhibit the biosynthesis of very-long-chain fatty acids in plants. Fenoxasulfone was registered in Japan in 2014, and various products containing fenoxasulfone have been launched. With its high efficacy and long residual activity, we believe that fenoxasulfone will contribute to efficient food production in the future.