Carotenoid biosynthesis content

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. 

The aba mutant of Arabidopsis is impaired in epoxy carotenoid biosynthesis and thus, makes zeaxanthin but not violaxanthin, antheraxanthin, and neoxanthin (Rock and Zeevaart, 1991 Rock et al, 1992). The pigment stoichiometries indicate a 1 1 replacement of neoxanthin and violaxanthin by zeaxanthin. The lutein content decreases compared to wildtype Arabidopsis, indicating a reduced Chi a/b antenna. Also comparing the composition of the antenna changes, there is less ofthe major LHCII and more of the minor Chi a/b complexes (Hurry et al, 1997). 

FIGURE 17.4 Growth change of salt-treated romaine lettuce (top), carotenoid biosynthesis pathway (left), and the contents of lutein and (3-carotene in salt-treated romaine lettuce (right). (From [43].)... 

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... 

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