It is India’s largest source of edible oil.
Unlike soyabean, which has only 18 per cent oil content, and groundnut, more than 50 per cent of whose kernels are either consumed directly or exported, rapeseed/mustard is one crop that is a ‘true’ oilseed. With annual production of around 2.4 million tonnes (mt), mustard accounts for a quarter of the country’s average edible oil availability of 9.4 mt from indigenous sources (see chart).
That being the case, mustard would be central to any strategy aimed at reducing India’s reliance on edible oil imports, which, in 2014-15 (November-October), amounted to over 14.4 mt valued at $10.5 billion. The fact that it is a rabi crop with almost 75 per cent area under irrigation — as against barely 25 per cent in groundnut and one per cent for soyabean — and having roughly 40 per cent oil content, makes mustard the most suitable candidate for ushering in a ‘yellow revolution’, similar to what wheat and paddy did for the ‘Green Revolution’.
For years, breeders have exploited a phenomenon known as ‘heterosis’ or hybrid vigour resulting from crossing two genetically dissimilar plant varieties even within the same species. The first-generation or F1 offspring from such crosses tend to have yields higher than what either parent can individually give.
In mustard, though, there have been two major constraints standing in the way of realising yield grains from heterosis that can help considerably boost domestic production.
The first is the narrow genetic base of mustard varieties grown in India. Scientists at the Centre for Genetic Manipulation of Crop Plants (CGMCP) in Delhi University, led by former vice-chancellor Deepak Pental, however, showed that this problem could be addressed by crossing Indian mustard cultivars with juncea lines of East European origin like ‘Early Heera’ and ‘Donskaja’. The combination of the two divergent gene pools enhanced the crossing options; the resultant F1 progeny were found to exhibit significant heterosis.
The second constraint is more basic, having to do with the absence of a natural hybridisation system in mustard. Mustard flowers contain both female (pistil) and male (stamen) reproductive organs, making the plants largely self-pollinating. To the extent that the egg cells of one plant cannot be fertilised by the pollen grains from the stamen of another, it restricts the scope for developing hybrids — which, in crops such as maize, cotton and tomato, is possible through simple emasculation or physical removal of anthers.
Pental’s team at the CGMCP initially developed a cytoplasmic male-sterile or CMS line of ‘Pusa Bold’ Indian mustard. This line (in which the stamen is sterile and cannot produce viable pollen), crossed with the East European ‘Early Heera-2’ variety (which is male-fertile and hence capable of pollinating the former), resulted in DMH-1, India’s first ever mustard hybrid. Its average seed yield, at 2.6 tonnes per hectare, was a fifth higher than the 2.1-2.2 tonnes for existing best ‘check’ varieties, including Pusa Bold, Pusa Jaikisan, Varuna, Rohini and Kranti.
The CGMCP scientists also bred another hybrid DMH-4, from crossing Pusa Bold with ‘S-7’, a derivative from Indian and East European mustard lines. The yields from it were the same as DMH-1, but the new hybrid produced bolder seeds.
The CMS-based hybrid breeding system, however, had drawbacks. It could work only in a single Indian mustard variety (Pusa Bold). Using the same CMS (female) parental line, thus, limited the crossing options. “We tried transferring our CMS system (‘126-1’) to many Indian and East European mustard lines. But in all these cases, the male sterility broke down and such partially fertile lines couldn’t be used for hybrid seed production. The system was also unstable, especially under low temperatures,” explains Pental.
The inherent limitations in CMS-based hybrid breeding, then, led Pental’s team to explore the genetic modification or GM route. The technology they chose involved deploying alien ‘Barnase’ and ‘Barstar’ genes. The ‘Barnase’ gene, isolated from a soil bacterium called Bacillus amyloliquefaciens, coded for a protein that impaired pollen production and rendered the plant into which it was incorporated male-sterile. This plant was, then, crossed with a fertile parental line, containing, in turn, the ‘Barstar’ gene from the same bacterium that blocked the action of the ‘Barnase’ gene. The resultant F1 progeny was high-yielding and could also produce seed/grain, thanks to the ‘Barstar’ gene in the second fertile line.
The CGMCP scientists used the Barnase-Barstar GM technology, originally patented by Plant Genetic Systems (now part of Bayer CropScience), to create a robust and viable hybridisation system in mustard. This system was also used to develop DMH-11 through crossing Varuna (Barnase) with Early Heera-2 (Barstar). This GM hybrid, which has demonstrated up to 30 per cent heterosis in field trials, is currently awaiting approval for commercial cultivation.
“The Barnase-Barstar system enables us to breed hybrids from a wide range of Indian and East European mustards yielding higher levels of heterosis. Besides, we can introduce new traits relating to oil quality (zero erucic acid, low glucosinolates content) or resistance to disease (alternaria blight and stem rot), which wasn’t possible with just a single, unstable CMS parental line,” Pental points out.
CGMCP has, in fact, tied up with the University of Arizona to map the Indian mustard (Brassica juncea), which itself has two constituent genomes ‘AA’ (from the Brassica rapa species) and ‘BB’ (from Brassica nigra), each containing 30,000 genes.
“The rapa genome has already been sequenced, while the work on nigra will be completed by the end of 2016. Once this is done, we will be able to know and locate the genes that code for specific traits, be it oil content, seed size, pod length and density, number of seeds per pod or disease resistance,” adds Pental.
GM technology will only help in developing hybrids with new parental combinations and incorporating these various quantitative, qualitative and disease resistant traits into them.