Plant lycopene synthesis

Neoxanthin, a precursor of the plant hormone abscisic acid, is an allenic xantho-phyll recognized as the last product of carotenoid synthesis in green plants. A cDNA for neoxanthin synthase (NSY) was isolated from tomato cv. Philippino using a molecular approach based on the mechanistic and structural similarities of NSY to two other closely related carotenogenic enzymes, lycopene cyclase (LCY) and capsanthin-capsorubin synthase (CCS) (Bouvier et al., 2000). 

The biosynthesis of carotenoids in plants has been reviewed extensively in recent years and is only briefly described here (Britton, 1988 Bartley and Scolnik, 1994 Sandmann, 1994). The committed step to carotenoid synthesis is the formation of the first compound phytoene by the head-to-head condensation of two molecules of GGDP by phytoene synthase. Phytoene is subjected to a series of four sequential desaturation reactions, by two separate enzymes to yield lycopene, which has eleven conjugated double bonds. Lycopene is then cyclized to /3-carotene by two /3-cyclizations or to a-carotene... 

Lycopene, /3-carotene, and antho-cyanins are found in the leaves of trees, but their characteristic colors are usually obscured by the green color of chlorophyll. Chlorophyll is an unstable molecule, so plants must continually synthesize it. Its synthesis requires sunlight and warm temperatures. As the weather becomes colder in the fall, plants can no longer replace chlorophyll as it degrades, so the other colors become apparent. 

The accumulation of lycopene, at the expense of jS-carotene formation, on treatment of plants with the substituted triethylamine CPTA (Figure 4.9) has been demonstrated in several cases, including citrus fruits and Aphanocapsa cells and cell extracts. In Aphanocapsa, in vivo and in vitro I50 values for lycopene cyclase were 45 and 30 pM, respectively, and a noncompetitive effect on the enzyme was shown. However, in citrus fruits a range of onium compounds related to CPTA regulate the formation of all- carotenoids, in that not only is cyclization blocked, but also the formation of lycopene is stimulated. The latter effect requires protein synthesis. Indeed, the tertiary amine MPTA [2-(4-methylphenoxy)triethyl-amine] causes gene expression and translation of [poly(A) ]RNA on SOS ribosomes. Therefore, CPTA and related compounds may have multiple effects in plants. 

Lycopene is an essential intermediate in the pathway for synthesis of the /3-ionone ring-containing carotenoids such as / -carotene in plant tissues, and... 

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