In this article we will discuss about the metabolic engineering of golden rice.
Golden rice was contributed by two German research team headed by Potrykus of Swiss Institute of Technology in Zurich. Peter Beyer of the University of Freiburg. The team has engineered Japonica rice variety by adding three genes, two of which are new to genetic engineering and are derived from daffodil (Narcissus pseudonarcissus). The third gene was obtained from bacteria, Erwinia uredovara.
Rice endosperm is capable to synthesise geranyl geranyl pyrophosphate which is used to produce colourless carotene known as phytoene by expressing enzyme phytoene synthase. However, synthesis of β-carotene in rice endosperm still requires three additional plant enzymes. First is phytoene desaturase and second Z-carotene desaturase.
These desaturase enzymes introduce two double bonds and third enzyme lycopene β-cyclase encodes by Icy gene. The bacterial carotene desaturases are employed in engineering strategy to facilitate introduction of all four double bonds for the conversion of phytoene to lycopene.
Agrobacterium mediated transformation was carried out to introduce β-carotene biosynthesis pathways into rice endosperm using three reactions. The first vector pB19hpc comprises of plant phytoene synthase (psy) gene, derived from daffodil and bacterial phytoene desaturase (crtl) originated from Erwinia uredovora under the control of endosperm specific glutenin (gtl) and CaMV 35 promoter respectively.
Signal peptide sequence was attached to both psy and crtl gene which direct the formation of lycopene in endosperm plastids, the site of geranyl geranyl pyrophosphate. Subsequently, co-transformation was carried out with vectors PZpsc and PZLCYH vector PLpsc carries Psy and Crtl without selectable marker (aph IV) and vector PZLCYH carries lycopene β-cyclases (Icy) gene derived from daffodil under the control of rice glutenin promoter and cam35 promoter for selectable marker. Lycopene β-cyclases carries transit peptide signal for expressing in plastids (Fig. 17.9).
The single transformed plants exhibits 3: 1 (coloured/uncoloured) segregation pattern, when co-transformants (PLPsc/PZLCYH) shows variable segregation. The transgenic rice accumulate 1.6 pµ/g carotenoid in the endosperm, against the actual goal of at least 2 µg/g provitamin A in homozygous lines, which is correspondence to 100 µg retinol equivalents a day.
Several assessments are still indispensable to see that golden rice is successful in the field. Engineering strategy is still to be extended to other varieties of rice. Monsanto is producing beta carotene in mustard, facilitate readily adsorbed by human being. The monsanto company ownes, through Calgene, the patent on the beta-carotene produced in rape seed under the control of napin promoter.
Although golden rice is providing impetus to combat night blindness problem. But there was an apprehension about acceptability of golden rice by poor people. This much hyped rice will merely grow at the expense of fruits and vegetables.
Besides, just providing single micronutrient such as vitamin A in the rice will do little to overcome micronutrient deficiency. Presence of golden rice gives false impression that VAD exists due to lack of vitamin A food source.
The purchase power from poorest of the poor of the developing countries unlikely to get benefit from this golden rice. Therefore, considering long term solution, it is feasible to encourage improved dietary habits, increased production and consumption of beta-carotene rice foods.
Because, it is very ironic that vast population of people suffering from Xerapthalmia blindness due to vitamin A are surrounded by rice diversity, fruits and vegetables which they contain abundant vitamin A.