Herbicides carotenoid biosynthesis

Bleaching Herbicides. Membrane-based modes of herbicidal action relevant to photosynthesis (37) include those of inhibitors of carotenoid biosynthesis, eg, norflura2on, diftmon, y -phenoxyben2amines inhibitors of chlorophyll biosynthesis, eg, oxadia2on, DTP or... 

The inhibition of carotenoid biosynthesis removes these protective pigments from the chloroplasts and leads to damaging effects within them. Herbicides that have been shown to interfere with carotenoid... 

Figure 2.6 Herbicides that interfere with carotenoid biosynthesis...

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... 

The pyridazin-3-ones are interesting because they include herbicides having two different modes of action, distinguished only by small changes in substitution pattern. Thus pyrazon (8) (61GEP1105232) is a photosynthesis inhibitor, while other discussed later are carotenoid biosynthesis inhibitors. The pyridazin-3-one ring is constructed by condensation of phenyl-hydrazine with 3,4-dichloro-2,5-dihydro-5-hydroxyfuran-2-one (9), in turn produced by chlorination of furan-2-carbaldehyde. Amination of (10) then occurs exclusively at the 5-position to give pyrazon (Scheme 4). 

A number of carotenoid biosynthesis inhibitors that came later include flurtam-one [36,37], introduced in 1997, which is used as a pre-emergence and preplant incorporated herbicide. Flurtamone can be prepared in several steps from... 

Two phytoene desaturase herbicides have been introduced since 2000 picolina-fen (Pico ) [182], introduced in 2001 by BASF, and beflubutamid [183], introduced in 2003 by Ube Industries. The primary mode of action of picolinafen and beflutamid is interference of carotenoid biosynthesis at the phytoene desaturation level, causing bleaching of the plant affected. As in previously developed phytoene desaturase herbicides, a meta-substituted trifluoromethylphenyl group is key for activity in this class of herbicides, pointing to the need for a lipophilic and electron-withdrawing group at this position of the molecule. 

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). 

Herbicides have several different mechanisms of action. Carotenoid biosynthesis inhibitors (Norflurazon, Fluridone, Flurochoridone, Diflufenican) block the formation of antioxidants which protect the photosynthetic apparatus in plants [3]. Plants treated with this class of substance are bleached and become unable to photo-synthesize [4]. 

Scheme 4.45 Different types of fluorine-containing herbicide. From top to bottom carotenoid biosynthesis inhibitors, aryloxyphenoxypropionates, pyridyloxyacetic acids, sulfonylureas, trifluoromethane sulfonanilides, benzylamine derivatives [3 — 14].

Pyridazine herbicides reduce or inhibit photosynthesis and carotenoid biosynthesis, - and they interfere with the formation of polar... 

Primary herbicidal effects are followed by secondary ones that show up before death of the plant cell. The 70-S ribosomes of wheat chloroplasts are decreased by bleaching pyridazinones in the light, but not in the dark ( 9) A prominent mode of action is observed upon the composition of fatty acids by, e.g., BAS 13338 (SAN 9785) (24, 5), which does not substcuatially interfere with carotenoid biosynthesis. Good direct inhibition of photosynthetic electron transport (I50 3 x 10 7m) is observed with the phenylpyridazinone BAS 100822 electron transport inhibition of other phenyl-pyridazinones is less than with BAS 100822 (28). 

L.) ( ) I corn, and soybeans (Glycine max L. Merr.) (4), support the contention that a carotenoid is involved in activating the DPE molecule. Yellow and green mutants were equally susceptible to herbicidal injury however, the albino mutants were resistant. Cucumber seedlings pretreated with fluridone, a known carotenoid biosynthesis inhibitor ( ), were also resistant By quantitatively examining herbicidal activity differences in tissues treated with compounds (e.g., CPTA [2-(4-chlorophenylthio)-triethylamine hydrochloride], onium compounds, etc.) capable of regulating the relative amounts of carotenoids present 33 pigment(s) involved in... 

Biosynthesis and Metabolism.—Reviews. A review has been published which discusses carotenoid biosynthesis as a possible target for herbicide activity. Another article reviews the photocontrol of carotenoid biosynthesis,and a yearly literature survey on light-mediated biosynthesis in plants includes carotenoids. 

Carotenoid Biosynthesis and Phytotoxic Effects of Bleaching Herbicides 4.1.2.1 Targets for Bleaching Herbicides... 

Table 4.1.1 presents IC50S of commercial herbicides for inhibition of carotenoid biosynthesis obtained in vivo according to Ref [26). The assay is easier to run than the early radioactive approach with unicellular cyanobacteria [35], giving in three cases differing results. This may be caused by differences in target site sensitivity, uptake and translocation effects or metabolism of the herbicides in the treated bacteria cells of the early test assays. Table 4.1.1 also presents physical and acute oral toxicity data [36, 37). 

Table 4.1.1 IC50 values and physicochemical and oral toxicity data for commercial herbicides for carotenoid biosynthesis inhibition.

Most commercial so-called bleaching herbicides inhibit the synthesis of carotenoids by interfering with carotenoid biosynthesis at the level of phytoene desatur-ase [170, 171, 172]. Errzyme kinetics with several inhibitors have revealed a reversible binding to the en2yme and non-competitive inhibition [173],... 

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