Inhibitors of 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... 

Substitution of a trifluoromethyl group in the 3-position of the benzene ring and alkylation of the 4-amino group, as in norflurazon (16) (69FRP1575643), turns the pyridazin-3-ones into inhibitors of carotenoid biosynthesis. A similar substitution pattern occurs in fluridone (17) (74GEP2537753). The long-established l,2,4-triazol-3-ylamine, known as amitrole, and... 

Norfluorazon is an often-used inhibitor of carotenoid biosynthesis, interfering with the step ofphytoene desaturation (Sandmann, 1994). Norflurazon treatment of mustard seedlings showed that PS II assembly is more sensitive to reduced levels of carotenoids than PS I assembly (Markgraf and Oelmiiller, 1991). The assembly of D1 into the PS II reaction center appears to be dependent on /3-carotene. Upon treatment of Chlamydomonas reinhardtii with phytoene desaturase inhibitors, D1 degraded during photoinhibition caimot be replaced (Trebst and Depka, 1997). 

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... 

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). 

Inhibition and Regulation. There have been several reports on the effects of desaturation and cyclization " inhibitors on carotenoid biosynthesis in various micro-organisms and plant tissues. 

Some details of the stereochemistry of carotenoid cyclization have been elucidated. In the C40 series labelling with stable isotopes (deuterium) has been used for the first time in studies of carotenoid biosynthesis. A Flavobacterium species in the presence of nicotine accumulated the acyclic precursor lycopene (175). When the cells were washed free from the inhibitor and suspended in H20 cyclization of the lycopene proceeded, initiated by High-resolution n.m.r. 

A great deal of evidence for the Porter-Lincoln pathway of phytoene desaturation came with the advent of inhibitors of various steps of carotenoid biosynthesis, the discovery of carotenoid mutants in various organisms, and in vitro biosynthesis... 

Since GAs as diterpenes share many intermediates in the biosynthetic steps leading to other terpenoids, eg, cytokinins, ABA, sterols, and carotenoids, inhibitors of the mevalonate (MVA) pathway of terpene synthesis also inhibit GA synthesis (57). Biosynthesis of GAs progresses in three stages, ie, formation of / Akaurene from MVA, oxidation of /-kaurene to GA 2" hyde, and further oxidation of the GA22-aldehyde to form the different GAs more than 70 different GAs have been identified. 

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 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. 

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). 

The amount of carotenoids formed by biosynthesis was quantified by the accumulation of the colourless carotenoid phytoene in the presence of the inhibitor, norflurazon. When applied, substantial amounts of this... 

The effect of inhibitors on the biosynthesis of rubixanthin (114) and other hydroxylated carotenoids was studiedin Staphylococcus aureus. Retinol was metabolized in rat livers to give an acidic product which was allegedly different from retinoic acid. 

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