Mutant carotenoid biosynthesis pathway

Scheme 92.1 Carotenoid biosynthesis pathway in tomato (Adapted from [18], [25], [26] and [27]). Mutants for each enzyme are identified in square brackets. Genes involved in external regulation represented in the squares...

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). 

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... 

Pathways. Studies of carotenoid transformations that take place when a mutant strain, PGl, of the green alga Scenedesmus obliquus is transferred from dark to light conditions have indicated that the transformations 15-cw-phytoene (180) 15-c/5-phytofluene (181) - 15-cis- -carotene (182) -> trans-C-caro-tene (183) (Scheme 7) take place in the biosynthesis of the normal cyclic carotenoids. The results were also in agreement with the formation of the xanthophylls lutein (16) and zeaxanthin (174) from the corresponding carotenes. 

Defective ZE mutants may result in an arrest of the ABA biosynthetic pathway that starts from the neoxanthin produced with zeaxanthin [39]. Consequently these mutants may result in wilty phenotypes. In the case of the mutant high pigment-3 (hp-3), coding a defective ZE, it seems that ABA deficiency would increase plastid division, enabling higher biosynthesis and accumulation of carotenoids [40]. 

Violaxanthin -> a//-/ran -neoxanthin 9 -c -neoxanthin xanthoxin (C,) are intermediates in the carotenoid pathway leading to the phytohormone abscisic acid (Cjj). Mutants of Nicotiana plumbaginifolia Viviani and Solanum lycopersicum L. sub nom. Lycopersicon esculenlum Mill, were involved in the elucidation of this biosynthesis (Dewick 1999 Oritani and Kiyota 2003 and references therein). 

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