FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



the compounds of the Tangerine type or tetrahydrophytoene does not 

 occur. Instead the dominance of the factors for lycopene implies 

 lycopene is formed from these compounds by dehydrogenation." 



Three possible mechanisms of dehydrogenation of tetrahydro- 

 phytoene are considered : (a) different enzymes are necessary for each 

 dehydrogenation step thus, on the basis of the one gene-one enzyme 

 theory, this would involve a number of genes in the formation of 

 lycopene. The development of selections which contain large amounts 

 of one or more of the postulated intermediates (phytoene, phytofluene, 

 ^-carotene) is compatible with this hypothesis, (b) one gene controls 

 the production of a single enzyme which can carry out all the dehydro- 

 genations ; the formation of a strain producing predominantly one 

 intermediate (say ^-carotene) would be determined by a gene con- 

 trolling the production of a specific hydrogen acceptor (in this case, 

 one which will accept H from phytofluene), and (c) as it is known that 

 two major genes R and T are necessary for lycopene formation {see 

 p. 80) these might control the formation of two enzymes one carrying 

 out a- [3 dehydrogenation and the other y-S dehydrogenation. Absence 

 of gene T would stimulate production of colourless polyene pigments 

 characteristic of the Tangerine type, whilst in the presence of T, 

 phytoene would be converted into a compound containing 4 isolated 

 double bonds which by action if the first enzyme would be converted 

 into lycopene. Porter and Lincoln do not indicate which of these 

 alternatives is their preference. The last suggestion is not in agreement 

 with other investigators' conception of the function of the R and T 

 genes {see p. 80). 



Finally, the formation of a-, p-, y- and S- carotene from lycopene is 

 explained as follows. Inheritance studies show that a single gene 

 difference exists between selections which have principally lycopene 

 and those which have principally ^-carotene. ^ * Although dominance 

 of either is not exhibited in the first cross, it is assumed that, as the 

 genetical evidence just outlined indicates the formation of lycopene 

 from more highly saturated compounds, p-carotene is formed from 

 lycopene. y-carotene is intermediate in structure between lycopene 

 and ^-carotene, it is assumed to be an intermediate in the reaction. 

 Finally, a-carotene is assumed to be formed from ^-carotene and 

 S-carotene *' by virtue of its structure "* is assumed to be formed 

 from tetrahydrolycopene by loss of 2H. This hypothesis of Porter and 

 Lincoln is most stimulating and whatever its fate in the light of future 

 work, it will be of real value in accelerating the final elucidation of this 

 fascinating problem. 



* See page 26. 



