Next-gen technologies, including gene-editing, should be used. The setback that the hybrid breeding programme has received should not be its death knell.
By Vivian Fernandes
In a development that could perhaps help in commercialisation of pigeon pea (tur) hybrids, researchers at ICRISAT have discovered why certain lines of the plant alternate between male sterility and male fertility with a change in temperature. They have identified the responsible gene. They have found that if the temperature drops below 24 degrees Celsius during certain stages of pollen development, a male sterile plant becomes male fertile. And, that the external application of a naturally-occurring growth hormone can reverse male sterility even if the temperature is higher than the threshold.
“We adopted an integrated approach,” said Rajeev Varshney, Director for the Genetic Gains Research Project at ICRISAT who led the research effort. ICRISAT is an international institute at Hyderabad which researches crops like legumes and millets for the semi-arid tropics.
Some of the male sterile lines used for making pigeon pea hybrids are temperature sensitive. When they become male fertile due to a change in temperature, the plants self-pollinate and cannot be cross-pollinated, says NP Singh, director of the Indian Institute of Pulses Research at Kanpur, which has released two pigeon pea hybrids.
Pigeon pea is a drought-tolerant pulse crop. It is cultivated in regions that depend on rain and is often grown between rows of cotton, jowar and groundnut. It is a socially-useful crop as it fixes nitrogen from the air in root nodules. Its roots also secrete an acid which untangle phosphorous bound with soil particles and make it available for plants to use. Since its roots go up to two metres deep, they can absorb leached nutrients. And, being a tall bushy plant, the leaves that it sheds profusely raise the soil’s organic carbon content.
India’s average annual production of pigeon pea over the past five years, including one during which the crop failed, has been 3.76 million tonnes. On average, about half a million tonnes is imported annually. The productivity varies with the duration of the crop, but on average, has been about 800 kg/ha.
The pulse has been grown in India for hundreds of years. Formal efforts at genetic improvement began in 1931, says Kulbhushan Saxena, a pigeon pea breeder at ICRISAT until his retirement in 2014. (Much of the data in this article has been sourced from Saxena’s research papers). In 1965, the Indian Council of Agricultural Research (ICAR) launched a crop improvement programme and established research centres in various agro-ecological zones. But despite serious efforts, there was no breakthrough in productivity.
Pigeon pea is a hermaphrodite plant. Its flowers have both male and female reproductive organs. Though it is largely self-pollinated, some amount of out-crossing happens when insect pollinators visit the flowers attracted by honey, chemicals called flavones and flavonol, and the scent of pollen. They seem to be more attracted to male fertile plants, that is, those with viable pollen to fertilise the eggs.
To break the yield barrier, ICRISAT decided to use this small window of out-crossing. It launched the pigeon pea improvement programme in 1974. While pursuing conventional breeding, breeders JM Green and D Sharma decided to explore yield improvement through hybrid breeding. It was an approach that had not been tried before.
To produce hybrids in a predominantly self-pollinated crop such as pigeon pea a male sterile source is needed so that they can be fertilised on a large-scale with the pollen of plants with desirable characteristics. Such genetically-controlled male sterile plants normally occur in nature through spontaneous mutation, but in pigeon pea, these have to be bred. Male sterility is a condition in the cytoplasm or area surrounding the nucleus of a cell caused by aberrant DNA. To produce a hybrid, the ‘A’ male sterile line has to be multiplied. This is done by crossing it with the ‘B’ maintainer line which is alike in all respects to A, except that it has viable pollen, or is male fertile. The A-line is then crossed with a male fertility restorer ‘R’ line. This cross produces a hybrid that is fully male fertile. This is known as the three-line A, B, R hybrid breeding system.
The story of pigeon pea hybrid breeding is one of patient and arduous effort over four decades. It is a saga of hope, promise, disappointment and heartbreak. In 1974, ICRISAT’s Subba Reddy discovered a male sterile line. He individually examined 7,000 plant variants (a.k.a germplasm) in the institute’s collection but found a few male sterile plants in a field collection from Andhra Pradesh. A few more genetic male sterile lines were discovered at research centres in Akola and Pantnagar. In all five hybrids based on genetic male sterile lines were released between 1991 and 2002 by ICRISAT, Punjab Agricultural University, Tamil Nadu Agricultural University and Panjabrao Deshmukh Krish Vidyapeeth.
Though they gave better yields compared to the then best available varieties, they could not be commercialised because of certain seed production issues. These could be overcome if male sterile lines, their maintainers, restorers and hybrids could be produced with a high degree of certainty year after year. This was only possible with cytoplasmic nuclear male sterility (CMS).
The first workable CMS system in pigeon pea was produced at Gujarat Agriculture University by SBS Tikka and his team. They crossed a wild relative of pigeon pea with a cultivated variety. It was used to produce a hybrid which was released in 2004. In multi-location trials, it gave an average yield of 1,830 kg/hectare, or 32% more than the then best available variety, Saxena says. But it could not be commercialised due to its inability to produce male fertile flowers uniformly and give good yields across locations and environments.
Saxena’s ICPH 2671 was the first commercial pigeon pea hybrid. It was released in 2010. In all India coordinated trials, Saxena says, it yielded 2,564 kg/ha or 31% more than the best available variety used for comparison. It was based on a cytoplasmic nuclear male sterile line he had developed in 2005. He says some farmers in Maharashtra even obtained about four tonnes per hectare. This hybrid was meant for Madhya Pradesh. Two more hybrids ICPH 2740 and ICPH 3762 were developed for Andhra Pradesh and Odisha.
For seed production, 94 professional seed producers were selected in six states. Ten companies were also roped in. Pravardhan Seeds sold the hybrid seed as ‘Pushkal.’ The production programme was undertaken under the supervision of a team of experts. On average 1,019 kg of hybrid seed was harvested, and the highest yield (1,674 kg/ha) was recorded in MP, says Saxena.
To ensure consistency of performance of hybrids in farmers’ fields, a high level of genetic purity must be maintained year-after-year. In most field crops, the freshly harvested seed is put through standard grow-out tests. This is not possible with pigeon pea because it is a six to nine-month crop.
Saxena says after he left ICRISAT in 2014, there was lax monitoring and the seed production targets were raised manifold. Because of this, the seed purity levels fell below the threshold, and there were many complaints. ICRISAT decided to tamp down on hybrid breeding activity. There was no large-scale seed production or on-farm testing.
Saxena says a genomics-based kit which Varshney developed could identify the genetic purity of the A, B, R lines and the hybrids using accurate molecular markers. It saves time and can accurately determine seed purity at the genetic level. According to Saxena, the integration of this technology with hybrid seed production could have solved the issue and helped farmers in reaping the benefits of hybrid technology.
The temperature-based male sterility is an advance in that the same plant can be male sterile or male fertile at different temperatures. So, a B maintainer line is not needed. Hybrids can be produced with just two lines, A and R, saving time and expense in breeding hybrids.
This is where the latest ICRISAT discovery fits in. But scaling up hybrid seed production can still be an issue. Large isolated plots have to be near areas of lush vegetation and free of insecticide use, for pollinators to visit in sufficient numbers.
Pigeon pea hybrids with more vigour and making them tolerant to infestation by pod borers are two ways of improving pigeon pea yields, says Bhagirath Chaudhry, Director of South Asia Biotechnology Centre. He says next-gen technologies, including gene editing, should be used. The setback that the hybrid breeding programme has received should not be its death knell.
Blogs at www.smartindianagriculture.in. Views are personal