Inhibitory herbicides

Future research may show that none of the hypotheses discussed accounts for the action of herbicides that interfere with the photochemistry of isolated chloroplasts. No single hypothesis may explain adequately the action of all inhibitory herbicides under all conditions. With a given herbicide, at one concentration, when applied to a certain species or variety of plant, and under particular environmental conditions, one hypothesis may account for the observed phytotoxicity. However, under other conditions or situations, another hypothesis may be more applicable (2). 

Yoneyama K, Nakajima Y, Maejima N et al. Simple and rapid screening method for Photosystem II inhibitory herbicides using photoautotrophically cultured plant cells with chlorophyll fluorescence monitoring. Biosci Biotech Biochem 1993 57(8) 1389-1390. 

Mechanism 1. Inhibitory herbicides displace plastoquinone B from its proteinaceous binding site on the reducing site of PS II. 

Cell Division Inhibitors. The most common mode of action of soil-appHed herbicides is growth inhibition, primarily through dkect or indkect interference with cell division (163). Such growth inhibitory activity is the basis for most pre- or post-emergent herbicides intended to control germinating weed seeds. In germinating seeds, cell division occurs in the meristems of the root and the shoot. Meristematic cells go through a cycle... 

The growth inhibitory mechanism of the thiocarbamate herbicides, eg, EPTC, butylate, cycloate, diaHate, and triaHate, is not well defined. Cell elongation, rather than cell division, appears to be inhibited (183), although mitotic entry may be inhibited by diaHate (184). Thiocarbamates have a greater effect on shoot than toot tissue (163,184). The weU-documented inhibition of Hpid synthesis by thiocarbamates certainly contributes to the observed inhibitions of cell division and elongation. These compounds may also inhibit gibbereUic acid synthesis (185). 

Baldwin, R.C., Pasi, A., MacGregor, T.J. and Hine, C.H. (1975). The rates of radical formation fixjm the dipyridylium herbicides paraquat, diquat and morfemquat in homc nates of rat lung, kidney and liver an inhibitory effect of carbon monoxide. Tox. Appl. Pharmacol. 32, 298-304. 

Nitisinone (59 Orfadin Swedish Orphan, 2002) is a derivative of lepto-spermone, an important new class of herbicides from the bottlebmsh plant (Callistemon citrinus), and exerts an inhibitory effect for /7-hydroxyphenyl-pyruvate dioxygenase (HPPD) involved in plastoquinone synthesis the... 

Leptospermone (34), a representative of an important new class of herbicides from the bottlebrush plant, Callistemon citrinus (Curtis) Skeels, has been found to have an inhibitory effect on the enzyme, -hydroxyphenylpyruvate dioxygenase (HPPD), involved in the synthesis of plastoquinone in plants. Nitisinone (35), a synthetic derivative of (34), has recently been introduced to the market for the treatment of hereditary tyrosinemia type 1 (HT-1), a severe genetic disease caused by a deficiency of fumaryl acetoacetate hydrolase (FAH). ... 

Table 1. Bioactive fungal products from endophytic fungi in recent literature reviews. The compounds include antifungal, antibacterial, antimycobacterial, antiviral, nematicidal, cytotoxic, antineoplastic, antioxidant, antiinsectant, antifeedant, antidiabetic, herbicidal, algicidal, specific en mie inhibitory, and immunomodulatory activities. 

Similar to pyrazolo[l,5-a]pyrimidines certain pyrazolo[3,4-4]pyrimidines exhibit phosphodiesterase inhibitory action (68MI1). Herbicidal activity of pyrazolo[3,4-4]pyrimidines has also been observed (79MIP354186). 

Chloroacetamide derivatives such as N,N-diallyl (5) and N-alkoxymethyl-N-2,6-diethylphenyl (<5) analogs are widely used as upland field herbicides. By modification of chloroacetamide structure, Fujinami et al. found that the N-chloroacetyl-N-phenylglycine esters (7) show varying degrees of inhibitory activity, in particular, against shoot growth of annual grasses 17). They developed Eq. 15 and 16 for the herbicidal activities of various esters where the aromatic substituents are fixed... 

Mode of Action of the Herbicide DCMU When chloroplasts are treated with 3-(3,4-dichlorophenyl)-l,l-dimethylurea (DCMU, or diuron), a potent herbicide, 02 evolution and photophosphorylation cease. Oxygen evolution, but not photophosphorylation, can be restored by addition of an external electron acceptor, or Hill reagent. How does DCMU act as a weed killer Suggest a location for the inhibitory action of this herbicide in the scheme shown in Figure 19-49. Explain. 

Triazines inhibit photosynthesis in all organisms with oxygen-evolving photosystems. They block photosynthetic electron transport by displacing plastoquinone from a specific-binding site on the D1 protein subunit of photosystem II (PS II). This mode of action is shared with several structurally different groups of other herbicides. The elucidation of the mechanism of the inhibitory action is followed in this review. 

Photosynthesis in all photosynthetic organisms is blocked by triazines, as well as by other PS II herbicides, when isolated thylakoid systems are tested. However, in intact plants, they express either different inhibitory potency or no inhibition. This shows that the specificity of these photosynthesis herbicides to certain weeds is not related to a difference in the chemistry of their primary target, but rather is attributed to degradative mechanisms, translocation, and translocation mechanisms. 

Sandman, G., P.M. Brantley, and P. Boger. (1980). The inhibitory mode of action of pyridazinone herbicide norflurazon on cell-free carotenogenic enzyme system. Pestic. Biochem.Physiol., 14 185-191. 

---