Colour inheritance in cactus pear (Opuntia ficus-indica) fruits

The pigments of Opuntia ficus-indica fruits, which are derived from the betalain rather than anthocyanin pathway, have an extraordinary range in colour from lime green, orange, red to purple. This is a result from varying concentrations and proportions of about half a dozen betaxanthins and betacyanins. The yellow-orange betaxanthins are derived from spontaneous condensation of betalamic acid with amines or amino acids. The reddish-purple betacyanins are enzymatically formed from betalamic acid and cyclo-dihydroxyphenylalanine (DOPA) yielding betanidin and further glycosylated on either of the two hydroxyls of the cyclo-DOPA moiety. In the present work, degenerated primers were used to obtain partial genomic sequences of two major genes in the biosynthetic pathway for betalains, that is the 4,5-extradiol dioxygenase which forms the betalamic acid responsible for the yellow colour and a putative 5-O-glucosyltransferase which glycosylates betanidin in Dorotheanthus bellidiformis and may be responsible for the red colour. Differences in the genomic DNA between coloured versus non-coloured varieties were not found. Regulatory mechanisms seem to independently control pigmentation of O. ficus-indica fruit tissues for inner core, peel and epidermis. Core pigmentation occurs first and well before fruit maturity and peel pigmentation. Peel pigmentation is fully developed at maturity, presumably related to maximum soluble solids. Epidermal pigmentation appears to be independent of core and peel pigmentation, perhaps because of light stimulation. Similar control mechanisms exist through transcription factors for the major enzyme regulating anthocyanin production in grapes.