Functions of Carotenoids in Higher Plants

Energy transfer between chlorophyll and carotenoids occurs by two pathways a static route involving no intramolecular motion, and a dynamic mechanism which involves movement of the two chromophores. Sufficient carotenoid molecules are normally present to deexcite the chl and O2 present, but if this concentration is reduced, for example, by inhibition of carotenoid biosynthesis, then 2 accumulates, and rapid destruction of membranes occurs by peroxidation.  

Additional functions ascribed to carotenoids include a structural role in enabling thylakoids to form partitions and a biochemical role as a precursor of the plant hormone abscisic acid  

Since phytoene is the 15-Z isomer, whereas the fully desaturated carotenoids in plants are all-E, an isomerization of the 15-15 bond must occur in the sequence. It is thought to take place at the phytofluene stage in tomato plastids and Capsicum fruits, although it occurs at the -carotene level in a strain of Scenedesmus, The direct, in vitro conversion of 15-Z phytoene into unsaturated carotenes has been demonstrated with tomato fruit, Capsicum, and daffodil chromoplasts.  

Although oxygen functions are inserted into carotenes at early stages of the pathway (e.g., 1,2-epoxyphytoene)/ most xanthophylls are formed by oxidations at the end of the pathway. 

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DEMMIG-ADAMS B, GILMORE A M, and ADAMS w w Jr (1996) In vivo function of carotenoids in higher plants , FASEB J, 10, 403-12. 

Demming-Adams, B., Gilmore, A. M., and Adams, W. W. (1996). In vivo functions of carotenoids in higher plants. FASEB. 10,403-412. 

Demmig-Adams B, Gilmore AM and Adams III WW (1995b) Changing views of in vivo functions of carotenoids in higher plants. FASEB J 10 403 12... 

Moore TA, Gust D and Moore AL (1990) The function of carotenoid pigments in photosynthesis and their possible involvement in the evolution of higher plants. In Krinsky NI, Mathews-Roth MM and Taylor RF, (eds) Carotenoids Chemistry and Biology, pp. 223-228. Plenum Press, New York... 

The possibility that epoxide carotenoids may function in oxygen evolution and transport has been suggested. In higher plants, three xanthophylls— violaxanthin, zeaxanthin, and antheraxanthin—undergo a series of photoin-duced interconversions (violaxanthin cycle) (Fig. 15), as reviewed by Hager (1975). Evidence as to the significance of this cycle is not conclusive, however. Two Russian workers, Sapozhnikov and Saakov, support the conclusion that the violaxanthin cycle is involved in oxygen transport (for review, see Sapozhnikov, 1973). However, other workers have concluded that ca-... 

It is well known, that carotenoids serve as light-harvesting pigments and protect the photosynthetic apparatus against photooxidative damage. Within the thylakoid membranes of higher plants carotenoids show a distinct pattern in their distribution among the antenna and reaction centre complexes (1). This specific association of the different carotenoids is not well understood with respect to their function in photosynthesis. 

LHCI of PSII. LHCII of higher plants appears to be composed of several very similar proteins. Each of these proteins carries noncovalently bound chlorophyll (4 < l a 3 Chi b) and carotenoid (2 lutein or neoxanthin or 1 of each) molecules, and each possesses three thylakoid membrane-spanning a-heli-cal regions. They may be organized into trimeric functional units. LHCII proteins of the various algal families carry the chlorophyll and carotenoid species that are characteristic of them, e.g. Chi. c and fuco-xanthin in brown algae (Phaeophyceae). 

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