Speed breeding: A revolutionary method for generation advancement in plant breeding
Pace of the over growing global population and it’s breakneck food demand in upcoming years has questioned the food security. Breeding a new crop via conventional approach requires selection of complementary parental genotype with desired traits, followed by crosses and a series of selection and advancement of superior progenies to release candidate cultivars that meet market demand. Thus, conventional breeding method can take more than 10 years to develop and release an improved variety in the absence of an integrated pre-breeding programme. Speed breeding is such a tool or technique for rapid generation advance that significantly reduces the harvest time of crops in order to speed up the agricultural research and increase the production of food to meet the demand of growing population.
Speed breeding is a technique which includes extending of photo period and controlled growing conditions like temperature, soil media, spacing etc. in greenhouse, enabling the rapid generation advancement by shortening the breeding cycle. This process has shortened the harvest time two to three times quicker than the conventional breeding. Speed breeding is a highly flexible process and can easily be practiced within a low-cost growth room. It can be done in some long day to day neutral crops by extending the light exposure duration to plants for reducing the generation times. Speed breeding technique is less susceptible to adverse biotic and abiotic stresses and allows the breeder more flexibility in the generation of new breeding material.
Method and application of speed breeding:
The setup required for speed breeding for the generation enhancement are:
- Light: PAR region (400-700 nm) ambient lighting with LED, with particular focus on the blue, red and far-red ranges.
- Photoperiod: 22 hours with 2 hours of darkness in 24 hours diurnal cycle.
- Temperature: The optimal temperature regime (maximum and minimum temperatures) should be applied for each crop. A higher temperature should be maintained during the photoperiod, whereas a fall in temperature during the dark period can aid in stress recovery. By contrast, a temperature cycling regime 22/17 degree Celsius for 22 hours light and 2 hours dark is used.
- Humidity: Ideally 60-70%
Speed breeding can be used to shorten generation time and to accelerate crop breeding and research programs of different crops like wheat, barley, rice, pea, canola, chickpea and so on. It can be used integrated with several other technologies such as high-throughput phenotyping and genotyping, marker assisted selection, CRISPR gene editing etc. and could help in accelerating the rate of crop improvement of many crops across the world. The USA inspired by NASA to grow crops in space, experiments in controlled environment were carried out in different cereals, by following three speed breeding methods: SB 1 (controlled environment chamber speed breeding conditions), SB 2 (glasshouse speed breeding conditions) and SB 3 (Homemade growth room design for low-cost speed breeding. Different method has been followed in many crops to shorten the generation and are tabulated below for few crops:
Table 1. Accelerated generation time and protocol of different crop species
|S.N.||Crop||Method||Duration in field or uncontrolled greenhouse||Duration under speed breeding|
|1.||Bread wheat||Speed breeding 1
Speed breeding 2
|2.||Durum wheat||Speed breeding 1||102||62|
|3.||Barley||Speed breeding 1
Speed breeding 2
Modified controlled biotron speed breeding system
Rapid generation advance (RGA)
|5.||Sorghum||In vitro plus in vivo||90-140||80|
|6.||B. distachyon||Speed breeding 2||73||48|
|7.||Canola||Speed breeding 2||171||98|
|8.||Chickpea||Speed breeding 2||115||82|
Opportunities of speed breeding techniques
- Rapid development of homozygous lines for accelerated breeding.
Speed breeding techniques have been used to develop homozygous line rapidly after initial crosses of selected parent with complementary traits on various crops. This breeding technique mainly depends on manipulation of photoperiod, light intensity, temperature, soil moisture, soil nutrition and high-density planting; which help to induce early flowering and seed set, reducing the time taken to generate each generation. This method allows for production of 3 to 9 generations per year.
- Amenability with selection method:
Speed breeding is generally used for generation advancement without phenotypic selection. However, modern technologies like marker assisted selection can be successfully integrated for target traits selection. The combination of speed breeding and effective selection methods should allow for the maintenance of a good breeding population and genetic diversity in the environment that restricts plant growth and for maximum yield production.
Conventional selection methods such as bulk, mass, recurrent, Pedigree and Pure line selection are not suitable for speed breeding as they require long inbreeding and selection cycle. So, the appropriate method for selection in speed breeding are single seed descent (SSD), Single pod descent (SPD) and single plant selection (SPS) method.
Challenges of speed breeding:
The use of speed breeding requires expertise, effective and complementary plant phenomics facilities, appropriate infrastructure and continuous financial support for research and development. However, the most common challenges hampering the use of speed breeding include: (a) access to suitable facilities, (b) staff trained in the protocol, (c) adopting major changes to breeding programme design and operations, and (d) the need for long-term funding.