Xanthophyll synthesis

The time courses of xanthophyll synthesis (especially lutein) and bulk carotene synthesis in the same developing system appear to be different [8]. Thus when either dark-grown or light-grown seedlings are incubated, in the light, with [ H]-MVA and C02 simultaneously, lutein and 6-carotene show different ratios, reflecting... 

The incorporation of label from mevalonate into ABA, a sesquiterpenoid, has been demonstrated in different parts of plants ( . . 41). This indicates that ABA can be synthesized throughout the plant. In addition to the direct incorporation of three isoprene units, derived from mevalonate, into ABA, an indirect biosynthetic pathway via carotenoids has been proposed. This idea stems from the finding that xanthophylls, in particular violaxanthin, can either photochemically or enzymatically be converted to the neutral inhibitor xanthoxin (42) (Figure 3). When labeled xanthoxin was fed in the transpiration stream to bean or tomato shoots, ca. 10% was converted to ABA over an 8-hr period (43). However, the importance of the biosynthetic route to ABA via xanthophylls and xanthoxin in normal metabolism remains to be established, and most of the evidence favors the direct synthesis route via a precursor (see 2). 

Low O2 generally delays or Inhibits the synthesis of lycopene, 11-carotene, and xanthophylls In tomato fruit (31.321. In sweet pepper, high COg delayed development of red color equally whether combined with 21% or 3% 02 (33). C2H4 Is known to accelerate the biosynthesis of carotenoids (34). 

Prior to 1995, only one locus affecting Xanthophyll biosynthesis in photosynthetic tissues of Arabidopsis had been identified, the ABA i locus, the mutation of which disrupts zeaxanthin deepoxidase, one of two xanthophyll cycle enzymes (Koomneef et al, 1982 Rock and Zeevaart, 1991 Rock et al., 1992). As a step toward advancing understanding of xanthophyll biosynthesis, incorporation, and function in plants, the author s laboratory has screened for and identified mutations defining two additional loci required for xanthophyll biosynthesis in Arabidopsis, LUTl and LUT2 LUT= LUTein deficient). Mutations at either locus result in defects in the synthesis of lutein, the most predominant xanthophyll in plants. Singly and in combination with the aba mutation, these lut mutations have allowed the genetic construction of five distinct mutant lines which differ dramatically in their carotenoid composition relative to wild-type Arabidopsis. In the remainder of this chapter I will first briefly discuss the aba mutation followed by a... 

The 3-hydroxy-K end group is characteristic of capsorubin (28), and other carotenoids isolated from paprika (Capsicum annuum). All syntheses are based on the Cio + C2o + Cio = C4o strategy, and use crocetindialdehyde (27) as central building block and the aldol condensation as the coupling reaction Scheme 6). The synthesis of optically active capsorubin (28), reported in 1973, was the first synthesis of an enantiomerically pure xanthophyll [11]. For the optically active form of the key building block, the frans-Cio-hydroxyketone 26, two approaches have been reported. In the first reaction sequence, (+)-camphor (143) was converted into camphoric acid (144) by treatment with nitric acid. Camphoric acid (144) was then esterified with dimethyl sulphate... 

The increased rate of ABA biosynthesis in dehydrated leaves can be blocked by inhibitors of transcription, such as actinomycin D and cordycepin [163-165], as well as by cycloheximide, an inhibitor of cytoplasmic protein synthesis [163,165-167]. These results indicate that nuclear gene transcription and cytosolic protein synthesis are required before an increase in ABA biosynthesis can take place. These processes probably account for the lag period prior to ABA accumulation following the onset of stress [94,163]. Water stress and cycloheximide had no effect on the conversion of XAN to ABA [54]. This indicates that the enzymes catalyzing these conversions are constitutively expressed. The most likely step stimulated by dehydration is, therefore, at the level of xanthophyll cleavage, although isomerization of xanthophylls cannot be ruled out. 

Since the stress stimulus is perceived by the plasma membrane (see above), there must be transmission of a signal from the plasma membrane to the nucleus, causing transcription and synthesis of cytoplasmic protein(s). The cleavage enzyme must then act in the plastid, the sole location of carotenoids in green plants. Since carotenoids are present in the chloroplast envelope [168], the cleavage enzyme could also be cytosolic, with the cleavage of xanthophylls taking place at the surface of the envelope. 

A new strategy for the synthesis of xanthophylls was developed in the 1970s by Roche [77]. The cyclic moieties are all derived from the common precursor 6-oxoisophorone (68), which is obtained from inexpensive a-isophorone (69) in two steps. In the liquid phase in the presence of weak acids, or in the gas phase on nickel oxide catalysts [78,79], 69 is in equilibrium with p-isophorone (70), which may be separated from the higher-boiling starting material by fractional distillation (Scheme 21). 

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