Decoding the science behind genetically modified mustard

 

To modify any variety of a crop so that it incorporates the desirable qualities from another variety, the obvious approach is to breed one with the other.

At the core of DMH-11, the genetically modified mustard variety that has been granted environmental clearance, is the principle of removing male fertility in one parent and restoring it in the offspring. The hybrid has been developed by Centre for Genetic Manipulation of Crop Plants, Delhi University, targeting higher yields than varieties that are currently grown in India.

The principle

To modify any variety of a crop so that it incorporates the desirable qualities from another variety, the obvious approach is to breed one with the other. In mustard, however, crossing two different breeds is a challenge because the crop is self-pollinating, which means that the offspring are produced by the male and female organs of the same plant.

The solution is genetically modifying one of the breeds to make it male-sterile, so that it can no longer breed within itself. It is then cross-bred with another variety, which itself is genetically modified to serve another purpose: ensure that the offspring is not sterile.

The technology used in DMH-11 involves the introduction of three genes called barnase, barstar, and bar. Found in a soil bacterium called Bacillus amyloliquefaciens, barnase in nature protects Bacillus from competing bacteria; while barstar protects the bacterium itself from barnase. The two genes have been harnessed for scientific purposes beyond crop modification, including cancer therapy.

When introduced as a genetic modification, barnase is aimed at making mustard male-sterile while barstar is to help restore male fertility. Bar, the third gene introduced, protects the mustard from a herbicide called phosphinothricin, commercially sold as Basta.

The technology for introducing male sterility using barnase and barstar was developed by scientists in Belgium in the early 1990s. A crop modified with barnase is cross-bred with one modified with barstar.

To make the genes work as targeted, the technology depends on what is known as a promoter. A promoter is a DNA region where key forms of molecular synthesis take place. In plants, bacterial genes can only express themselves under plant promoters.

The genes encoding for barnase and barstar express themselves under a promoter specific to the plant region called tapetum, a layer of cells in the anthers (male organs) of the flower. The tapetum’s role is to produce compounds that are essential for the development of mature pollen. When the barnase gene is introduced, the tapetum tissue degrades and mature pollen cannot develop. This flower will then act as the female.

The other flower, which will act as the male, is engineered with the barstar gene, aimed at restoring male fertility in the offspring. As in bacteria, barstar binds tightly with barnase and makes it ineffective in mustard too. This means that the hybrids grown by farmers will be fully fertile.

DMH-11, which also uses bar, improvises on what is known as the barnase-barstar system. One of the parents (known as Event Var bn 3.6) contains the bar and barnase genes. This is the male-sterile parent.

The other parent (Event EH-2 modbs 2.99) also contains the bar gene, besides barstar for restoration of fertility. DMH-11 contains all three genes: bar, barnase, and barstar.

Concerns and solutions

The crossing happens through wind pollination or bee pollination. Whether the mustard will attract bees is one of the areas of concern. Another concern is whether the genetically engineered plants can potentially introduce the genes into wild populations.