Inhibition phytoene desaturase

A second class of herbicides primarily affects ( -carotene desaturase. These herbicides are apparent feedback inhibitors of PD as well. This class of compounds includes dihydropyrones like LS 80707 [90936-96-2] (56) and 6-methylpyridines (57,58). The third class consists of the ben2oylcyclohexane-diones, eg, 2-(4-chloro-2-nitroben2oyl)-5,5-dimethyl-cyclohexane-I,3-dione. This class of atypical bleaching herbicides induces phytoene accumulation when appHed either pre- or post-emergence. However, it does not inhibit phytoene desaturase activity in vitro (59). Amitrole also has been considered a bleaching herbicide, though its main mode of action is inhibition of amino acid synthesis. 

The molecular target site of triketone herbicides is the enzyme -hydroxyphenylpyruvate dioxygenase (HPPD). Inhibition of this enzyme disrupts the biosynthesis of carotenoids and causes a bleaching (loss of chlorophyll) effect on the foliage similar to that observed with inhibitors ofphytoene desaturase (e.g. norflurazon). However, the mechanism of action of HPPD inhibitors is different. Inhibtion of HPPD stops the synthesis of homogen tisate (HGA), which is a key precursor of the 8 different tocochromanols (tocopherols and tocotrienols) and prenyl quinones. In the absence of prenylquinone plastoquinone, phytoene desaturase activity is interrupted. The bleaching of the green tissues ensues as if these compounds inhibited phytoene desaturase. 

If our hypothesis is correct, this hypothetical binding site should also accommodate fluridone, norflurazon and difunone and some possible binding orientations of these molecules are compared with furanone 13 in Figures 7-9. Note that we have attempted to depict the molecules in such a way that key structural features, e.g., the CF3 phenyl and vinylogous amide subunits, occupy the same positions as nearly as possible. Finally, it should be emphasized that considerable further work is required to demonstrate that the furanones actually inhibit phytoene desaturase and to further probe the possibility of a common binding site for the proven inhibitors including those such as fluorochloridone (10) and the m-phenoxybenzamides (4), which do not incorporate the vinylogous amide substructure. 

Beflubutamid is a newly described carotenoid synthesis inhibitor, inhibiting phytoene desaturase. It has a very low toxicity to animals but is very toxic to algae and plants. It is not mutagenic or teratogenic in standard tests. 

Substituted pyridazinones can be categorized into five groups by cluster analysis, of which one is of analogues which inhibit phytoene desaturase. A comparison of seven different phenylpyridazinones found that an m-trifluoro substituent at the phenyl ring and a methyl-... 

The carotenoid pathway may also be regulated by feedback inhibition from the end products. Inhibition of lycopene cyclisation in leaves of tomato causes increase in the expression of Pds and Psy-1 (Giuliano et al, 1993 Corona et al, 1996). This hypothesis is supported by other studies using carotenoid biosynthesis inhibitors where treated photosynthetic tissues accumulated higher concentrations of carotenoids than untreated tissues (reviewed by Bramley, 1993). The mechanism of this regulation is unknown. A contrary view, however, comes from studies on the phytoene-accumulating immutans mutant of Arabidopsis, where there is no feedback inhibition of phytoene desaturase gene expression (Wetzel and Rodermel, 1998). 

The enzyme p-hydroxyphenylpyruvate dioxygenase is involved in the conversion of p-hydroxyphenylpyruvate into homogentisate, a key step in plastoquinone biosynthesis. Inhibition of this enzyme has an indirect effect on carotenoid biosynthesis as plastoquinone is a co-factor of the enzyme phytoene desaturase. The new maize herbicide isoxaflutole and the triketone herbicides such as sulcotrione (Figure 2.7), inhibit p-hydroxyphenylpyruvate dioxygenase and this leads to the onset of bleaching in susceptible weeds and ultimately plant death.4... 

F, Bleaching inhibition of carotenoid biosynthesis at the phytoene desaturase step (PDS) Pyridazinones Nicotinanilides Others 12... 

Phytoene desaturase bleaching herbicides. Bleaching herbicides inhibit the synthesis of carotenoids in plants [30,31], A number of phytoene desaturase herbicides such as norflurazon (Solicam , Zorial ), flurochloridone (Racer ), fluri-done (Sonar ), and diflufenican (Fenican , Legacy ) have been commercialized. 

Inhibitors of carotenoid synthesis also lead to chlorophyll destruction by destabilizing the photosynthetic apparatus. Total carotenoid content decreased with increased (-)-usnic concentration (Fig. 1.4). Carotenoid biosynthesis can be interrupted by inhibiting the enzyme phytoene desaturase that converts phytoene to carotenes or by inhibiting the enzyme HPPD responsible for plastoquinone (required for phytoene desaturase activity) synthesis.14 Usnic acid possesses some of the structural features of the triketone HPPD inhibitors, such as sulcotrione (Fig. 1.1C).8 (-)-Usnic acid had a strong inhibitory activity on HPPD, with an apparent IC50 of 70 nM, surpassing the activity obtained with the commercial herbicide sulcotrione (Fig. 1.5). 

Sandman et. l. (8) have found the N-phenyl-2-phenoxynicotin-amides to be powerful inhibitors of phytoene desaturase. Several bleaching herbicides that inhibit the phytoene to phytofluene transformation have the same 3-trifluoromethylphenyl group (8, 9). This group includes norflurazon, metflurazon, fluridone, 7luometuron and fluorochloridone. In the nicotinamide series the same 3-trifluoromethylphenyl group gives optimum herbicide activity. 

A small amount of lycopene is found in Rpi. globiformis (Table 3 S. Takaichi, unpublished) and in the diphenylamine-inhibited cultures of Chr. okenii (Schmidt et al, 1963). However in diphenylamine-inhibited cultures of Rpi. globiformis, lycopene was not reported (Schmidt and Liaaen-Jensen, 1973). Although small amounts of the intermediates of phytoene desaturase are usually found in photosynthetic bacteria, if the activity of the C-1,2 hydratase is high, lycopene as the substrate may not be found. Therefore, the carotenogenesis pathways of okenone and / .g.-keto carotenoids could possibly start from lycopene rather than from neurosporene (Fig. 7). Nothing is known about how the keto group is introduced under the anaerobic conditions. 

The mode of action of file phenylthiazolines has been investigated by looking for file accumulation of metabolic intermediates in chlorotic cress leaves. Treated plants showed an increase in levels of a compound that can not be observed in controls, identified by UV spectroscopy and HPLC cochromatography with an authentic phytoene. The latter has been produced by the application of file known carotenoid synthesis inhibitors such as flurtamon and identified independently by UV and mass spectroscopy. From these results, it can be assumed that chlorosis is caused by inhibition of carotenoid synthesis at the phytoene desaturase step. 

In order to exclude the possibility that inhibition of phytoene desaturation is caused by indirect or regulatory mechanisms, direct interaction of substituted phenylthiazolines with phytoene desaturase was demonstrated by in vitro studies. As a result, with increasing concentrations, more phytoene is retained which means that less phytoene was desaturated. Therefore, less 3-carotene is formed as the end product of this in vitro biosynthetic chain. Interaction of phenylthiazolines is very similar to inhibition of this enzyme by flurtamone. For the latter herbicide, it has been shown to be a reversible non-competitive inhibitor of phytoene desaturase (P). 

QSAR data on substituted 2-phenylpyridazinones have shown that the electronic properties of substituents strongly influence activity but can be countered by lipophilicity. Data from whole cells and cell extracts of Aphanocapsa correlated well, with the in vitro results showing a linear relationship between lipophilicity and inhibition of phytoene desaturase in accordance with the QSAR equation for intact cells. ... 

The essential structural units of difunone (Figure 4.7) are the phenyl ring at C-3, a cyano group at C-4, and an oxo function at position 5. Inhibition of phytoene desaturase increases with the insertion of additional... 

Sandmann and Boger have suggested that the phenylpyridazinones have two modes of action on phytoene desaturase." Firstly, the compound mimics a carotene molecule and acts as an allosteric regulator. Secondly, an irreversible binding of the —CF3 derivative occurs at the active site. Phenylpyridazinones such as Sandoz 9785 inhibit fatty acid desaturation in plants, which may reflect the potential of these compounds to inhibit a range of desaturases (see also Chapter 3). 

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