Inhibition carotenoid biosynthesis

This spectacular bleaching effect is discussed in other chapters of this work, such as the N-benzylideneamino heterocycles of the Shell workers, the nicotinamides of Stauffer, the pyridazines of American Cyanamid and the furanones of Chevron. Various aspects of carotenoid biosynthesis inhibition have been presented in other places (27,31). however, we see here some of the exciting new chemistry associated with these powerful compounds. 

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

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). 

BIRD C R, RAY J A, FLETCHER J D, BONIWELL J M, BIRD A S, TEULIERES C, BLAIN I, BRAMLEY P M and SCHUCH w (1991) Using antisense RNA to study gene function inhibition of carotenoid biosynthesis in transgenic tomatoes , BioTechnology, 9, 635-9. 

BOUVIER F, d harlingue A, BACKHAUs R A, KUMAGAi H and CAMARA B (2000) Identification of neoxanthin synthase as a carotenoid cyclase pmalog , FEBSLetters, 267, 6346-52. BRAMLEY P M (1993) Inhibition of carotenoid biosynthesis , in Yoimg A J and Britton G, Carotenoids in Photosynthesis, London, Chapman and HaU, 127-59. 

Regulation. Inhibition continues to be useful for the elucidation of some details of carotenoid biosynthesis some examples have been reported above. 4-[ -... 

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... 

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... 

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). 

Because bleaching can also be caused by inhibition of carotenoid biosynthesis, we tested our compounds for effects on carotenoid content. There was a dose-dependent increase in carotenoid content using the series 1 emodin analogues (Fig. 1.8). This was seen in both monocots and dicots and was not observed with the series 2 analogues. 

Although chloramphenicol inhibits carotenoid synthesis, it also inactivates photo-chemically responsive membranes in which carotenoids are located, and hence previous conclusions that the enzymes involved in carotenoid biosynthesis are themselves synthesized in the cytoplasm are not strictly valid. However, the demonstration367 that dark-grown Euglena gracilis, which has carotenoids localized in the proplastids, can synthesize carotenoids in the presence of chloramphenicol indicates that such synthesis is not dependent on protein synthesis in the proplastid. 

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. 

Aclonifen or 2-chloro-6-nitro-3- phenoxybenzenamine Diphenyl ether Inhibits carotenoid biosynthesis Preemergence control of weeds in winter wheat, potatoes, etc. 11-13... 

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

Even though a clear inhibition of sterol accumulation in cotyledons by mevinolin can be achieved at higher concentrations as compared to the roots, there was no inhibition of chlorophyll and carotenoid biosynthesis under these conditions (( ) and Table... 

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). 

Inhibition and Regulation. Evidence has been obtained for the existence of two /S-carotene pools and two biosynthetic sites in the chloroplast. Light-induced carotene synthesis has been studied in mutants of Phycomyces blakesleeanus with abnormal phototropism. The use of substances which affect carotenoid biosynthesis continues. Nicotine has been used in studies of C50 carotenoid formation in Halobacteria and of arylcarotene synthesis in Chlorobium. ... 

KUMAGAI, M. H., DONSON, J., DELLA-CIOPPA, G., HARVEY, D., HANLEY, K., GRILL, L. K., Cytoplasmic inhibition of carotenoid biosynthesis with virus-derived RNA, Proc. Natl. Acad. Sci. USA 1995, 92,1679-83. 

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