Bacterial gene insertion could improve pro-vitamin A activity of common staple

In a drive for nutritional improvement of foods, scientists at the Scottish Crop Research Institute, Invergowrie, UK, have developed a transgenic potato plant with carotenoid enriched tubers.

These isoprenoid pigments, found in photosynthetic tissues, and many flowers and fruits, are generally lacking in the seeds and storage organs that form common staples. But they are dietary essentials. Pro-vitamin A activity of ?-carotene is well established, and the nutrient’s deficiency remains widespread in the developing world. Now, evidence of a role for other carotenoids in protection against specific diseases is emerging. Zeaxanthin and lutein, for example, accumulate in the retina to reduce risk of macula degeneration, a common cause of blindness in the elderly.

The enzyme phytoene synthase provides the gateway to biosynthesis through conversion of the C20 isoprenoid, geranylgeranyl pyrophosphate to the C40 proto-carotenoid, phytoene. In growing tubers of the transgenic plants developed by Mark Taylor and colleagues, a phytoene synthase gene from the bacterium Erwinia uredovora raised total carotenoids up to sixfold. The carotenoid profile was radically altered, with ?-carotene enhanced from zero to a nutritionally significant 30 per cent of the total. Lutein levels were similarly raised.

Evidence for regulation of biosynthesis was revealed in measurements of expression levels of major carotenoid pathway genes - phytoene desaturase, involved in conversion of phytoene to ?-carotene, being reduced. ’Perhaps the plant, in response to greater flux in carotenoid pathways, is limiting biosynthesis through regulation of this gene,’ suggests Taylor, ’so a microbial phytoene desaturase gene, free of such regulation, alongside phytoene synthase may raise ?-carotene levels even higher.’

Carotenoid enhancement could have an impact on health, but Taylor is guarded about the practical implications. ’Development and exploitation of the technology ultimately depends on consumer acceptance,’ he said.

But there may be opportunities for improvement of the common potato, Solanum tuberosum, by conventional breeding - perhaps through introduction of novel genes from a South American relative, S. phureja, naturally rich in zeaxanthin. ’Advances in basic understanding of carotenoid biosynthesis regulation, achieved from transgenic studies, are essential if such goals are to be realised,’ says Taylor.

Russ Clare

L Ducreux et alJ. Exp. Botany, 2005, 56, 81