Speed Breeding: A Propitious Technique for Accelerated Crop Improvement

Abstract

Development of climate-resilient genotypes with high agronomic value through conventional breeding consumes longer time duration. Speed breeding strategy involves rapid generation advancement that results in faster release of superior varieties. In this approach, the experimental crop is grown in a controlled environment (growth chambers) with manipulation provisions for temperature, photoperiod, light intensity, and moisture. The generation of the crop cycle can be hastened by inducing changes in the physiological process such as photosynthesis rate, flowering initiation, and duration. Speed breeding eases multiple trait improvement in a shorter span by integration of high-throughput phenotyping techniques with genotype platforms. The crop breeding cycle is also shortened by the implementation of selection methods such as single-seed descent, single plant selection, and marker-assisted selection.

1. Introduction

The increase in world population coupled with climatic fluctuations such as drought, flood, and high temperature poses a serious threat to food security [1]. Many researchers quoted the importance of enhancing the genetic gain of primary crops at a faster rate to meet the global food demands [2]. It remains a challenging task for plant breeders to evolve resilient varieties in a shorter period by employing conventional approaches. The slow progress in crop improvement is mainly attributed to long breeding cycles/generation [3]. To overcome the drawbacks involved in traditional methods and to safeguard food security, speed breeding concepts are now being adopted at large/small units for realizing a rapid genetic gain in many crop species.

The speed breeding techniques include the use of controlled environments with manipulation provisions for the light duration, intensity, and temperature. This serves as more advantageous for the plant breeder to hasten the crop development in several major photosensitive crops [4]. The concept of stimulating an artificial environment for plant growth was first initiated by a team of botanists several years ago. Around 1980, similar protocols were again adopted by scientists of National Aeronautics and Space Administration (NASA) in collaboration with Utah State University to understand the accelerated crop growth cycle under constant light in the space station [5]. As an outcome, a new dwarf variety USU-Apogee was released by NASA in wheat [6]. In earlier crop improvement programs, the breeders employed few manipulations in conventional approaches such as the single-seed descent method [7], shuttle breeding [8], and haploid technique for rapid delivery of superior varieties. These were upgraded and combined with the use of other innovative technologies under the term speed breeding. Scientists achieved rapid generation advancement through the adoption of novel techniques such as marker-assisted selection, in vitro culture, high-throughput phenotyping, next-generation sequencing, genomic selection, and gene editing in the speed breeding protocols [9]. The speed breeding concept was first employed in Triticum aestivum (wheat) to investigate the seed dormancy trait under controlled conditions [10]. At present, speed breeding protocols are widely employed in several crops, including underutilized species [11]. Around six generations per year have been achieved in crops such as oat [12], barley [13], wheat [14], chickpea [15], faba bean, and lentil [16] through the implementation of speed breeding techniques. Speed breeding protocols allow for the integration of new techniques along with several manipulations in influencing factors (Figure 1), which have been briefly discussed in this chapter.

Figure 1.

Rapid generation advancement through speed breeding. a. Experimental crop grown under controlled environments. b. Use of high-throughput genotyping platforms; advanced phenotyping tools and other modern breeding techniques in speed breeding protocol.

2. Speed breeding techniques

2.1 Crops under controlled environment

Speed breeding techniques involve deliberate manipulation of environmental conditions for the rapid advancement of crop cycle. The use of controlled growth chambers equipped with manipulation provisions for light intensity, temperature regime, photoperiod, soil moisture, carbon dioxide level and nutrition supply will influence/alter the plant physiological growth process [17]. Researchers employ these modifications in a crop improvement program to achieve increased generation per year. The early flowering was induced in IR 64 rice variety by altering the light exposure in the growth chamber [18]. Similarly, a photoperiod of 22 hours of light exposure reduced flowering duration in wheat genotypes [19]. The breeding strategy can be efficiently planned with photosensitive crops through the adoption of light-based speed breeding protocols. The quality of light delivered per day highly influences the photosynthesis rate, gas exchange, transpiration rate, stomatal activity, and other plant developmental processes [20]. Adoption of 360–650 μmol/m² light intensity with 400–700 nm of PAR (photosynthetic active radiation) was found successful in barley, wheat, chickpea, canola, and other major crops for early flowering and seed set [15]. The induction of early flowering was observed in legumes such as chickpea, faba beans, and pea with the use of blue and far-red light spectrums [21]. Early flowering was induced in groundnut by continuous exposure (24 hours) of 450 W lamps 25 days after germination [22].

The temperature variation plays a crucial role in the transition from vegetative to flowering stage in crop plants [23]. It influences the seed germination rate, plant growth, flowering period, seed set per cent, and maturity [24]. A temperature range of 12–30°C for germination and 25–30°C for other developmental processes (growth, flowering, and seed formation) is found suitable for most of the species [25]. Rapid plant development is observed on introducing the crop to altered temperature regime (17°C/32°C) and photoperiod in groundnut [22].

A shift from vegetative to reproductive phase is reported in crop plants at increased CO₂ levels [25]. Plants’ response to CO₂ levels highly varies with the genotype of a species. The experimental genotype has to be evaluated with a critical range of CO₂ levels in growth chambers to determine the optimum value for induction of earliness in flowering. The breeding cycle was enhanced up to five generations per year in soybean by manipulating CO₂ supply (> 400 ppm) coupled with light exposure of 14 hours cycle in a growth chamber [26].

Most crop species exhibit early flowering and seed set on subjecting to moisture stress [27]. Modulation of soil moisture status in speed breeding protocol helps in rapid generation advancement of crop species. The high induction of grain filling and maturation is observed in barley, wheat, and chickpea on the gradual decrease of moisture status at the end of the flowering stage [15].

High-density planting is a low-cost strategy in speed breeding as it contributes to rapid generation turnover along with the maintenance of large population size. Crops raised at high density tend to compete with each other resulting in early induction of flowering and seed maturity [28]. The earliness in flowering at high density was reported in rice, sorghum, and cotton [25]. On contrary, many researchers found no deviations in flowering initiation at high-density planting [29]. Therefore, the genotypic responses need to be investigated in each species to validate the use of high-density planting as a component in speed breeding.

Application of plant growth hormones and essential nutrients tend to regulate flowering and seed set under in vitro conditions [30]. More breeding cycles per year can be generated through the use of growth regulators with other approaches. Around eight generations per year were obtained in lentil and faba bean with the use of plant growth regulators viz., auxin, cytokinin, and flurprimidol under modified temperature (22°C light/18°C dark) and photoperiod (18 hr. light/6 hr. dark) in growth chambers [31].

The immature seeds obtained from plants grown under speed breeding protocols with an extended duration of photoperiod (22 h of light) proved to be viable in wheat and barley [15]. A similar finding on early seed harvest was reported in wheat cultivars [32]. The advancement of subsequent generations can be hastened by the adoption of early harvest with other speed breeding techniques. The immature seeds (37 days after postanthesis) from plants grown under CO₂ supplementation exhibited a high germination rate similar to control in soybean [26]. Around 7–8 generations/year is achieved in lentils by integrating early harvest with the application of plant growth regulators [16].

2.2 Accelerated crop improvement through integration of novel approaches

Speed breeding is a feasible platform that allows the integration of modern approaches along with generation advancement techniques. The conventional breeding techniques (pedigree selection, mass selection, pure line selection, bulk selection, and recurrent selection) of line development require more number of inbreeding and selection processes. These methods were not found amenable for inclusion in speed breeding protocols [25]. The use of modern techniques coupled with high-throughput phenotyping platforms in speed breeding would highly augment the crop improvement program. The target-specific traits involved in biotic and abiotic stress can be improved at a faster rate by creating artificial environments with accurate phenotyping.

Few modifications in conventional selection methods proved efficient for inclusion in speed breeding protocol. The single plant selection method was employed in the handling of backcross progeny at earlier generations (F₂ and F₃). A rigid selection for the trait under transfer and characteristics of the recurrent parent was made in segregating generations (F₂ and F₃) after the first and third backcross. Each F₂ selected plant was harvested separately for the advancement of generation (F₃) following the progeny-row method. The inclusion of selection in the early generation reduced the number of backcrosses and thereby saves labor, time, and other resources. The modified backcross method was employed in barley for the rapid development of introgression lines [33]. The European barley cultivar (Scarlett) was crossed with other donor parents to evolve lines exhibiting resistance to blotch and leaf rust. The lines under evaluation were raised under growth chambers with continuous light exposure at 22°C. Similarly, the single plant selection in combination with the speed breeding protocol was followed in wheat for multiple trait improvement [34].

Single-seed descent serves as a promising selection approach for inclusion in speed breeding techniques in field and controlled environments. The attainment of homozygosity is accelerated through constant inbreeding of segregating population by forwarding a single seed of each individual to the next generation. It allows for the advancement of generations in growth chambers and small nursery fields [35]. The single-seed descent method provides the opportunity for high-density planting and proves to be a very effective strategy for resource-limited environments [36]. The popular rice cv. Nipponbare was developed by adopting a single-seed descent method with rapid generation techniques at growth chambers [37]. Around 450 inbred lines evolved rapidly under field conditions following the single-seed descent method in wheat [38]. No selection is imposed in any successive generation which may carry more inferior progenies in a population compared to other selection methods.

A slight deviation from the single-seed descent method was found successful in legume species. The selection of one pod per plant was followed from F₂ to F₄ generation instead of a single seed. Single-pod descent selection provides scope for maintaining each F₂ line in advanced generations compared to the single-seed descent method. It also possesses the advantage of early selection of pods, which is not feasible in the single-seed descent method. The mean yield of progenies developed from single-pod descent (7.96 g / plant) was higher compared to the single-seed descent (6.42 g/plant) selection method in soybean [39]. However, the conduct of preliminary test trials under controlled environments is required to validate the selection efficiency of the single-pod descent method in legume crops [25].

The precise identification of candidate genes has become feasible due to recent advancements in genotypic platforms and high-throughput phenotyping techniques. The development of mapping population (F₂, recombinant inbred line (RIL), and backcross) requires a longer generation time on conventional approaches. The inclusion of the speed breeding technique promotes rapid identification and validation of QTL (quantitative trait loci) [21]. It facilitates minimal backcross (1–2) to introgress the target gene in a superior genotype (over 99% of the recurrent genome). The use of marker-assisted selection (MAS) in speed breeding protocol facilitates gene discovery at a faster rate and thereby meets the challenges associated with food production. The SNP marker-assisted selection is combined with speed breeding protocols for rapid development of mapping population (BC₃F₃) associated with salinity tolerance in rice [40].

The marker-assisted selection is efficient only with a few QTLs exhibiting a major effect on the trait of interest. At present, researchers employ a genomic selection approach in the breeding strategies, which is effective for complex trait improvement. It paves way for the identification of several minor QTLs, which is involved in the governance of biotic and abiotic stress resistance. With the development of next-generation sequencing (NGS) technologies, the cost and time involved in genomic selection are drastically reduced [41]. The genomic-estimated breeding values (GEBVs) of individuals are estimated based on genotype and phenotype datasets of a training population. It results in high accuracy of measuring the genetic worth of an individual compared to other selection methods [42]. The rapid genetic gain was realized in wheat through the implementation of genomic selection with other speed breeding protocols [43]. Several haplotypes related to yield improvement have been identified in rice and many other species. Introgression of haplotype into superior cultivars requires more breeding cycles and is highly time-consuming. The haplotype breeding can be accelerated by the integration of speed breeding protocols with the genomic selection approach [9]. Speed breeding also serves as a promising strategy for the rapid advancement of generations in transgenic crops [44].

3. Challenges in adoption of speed breeding protocols

The use of speed breeding techniques for crop improvement demands high infrastructure equipped with control facilities for temperature, photoperiod, humidity, and other factors. It requires the need for expertise/skilled technicians for the maintenance of experimental crops in controlled conditions [45]. Lack of modern tools/techniques in underdeveloped countries, lack of continuous financial assistance, and unsupportive policies add up the concern toward adoption of speed breeding protocol in practice. Many experimental fields have reduced access toward a continuous supply of electricity. The use of energy-efficient LED bulbs and air conditioners under solar power with battery support may help to some extent for small infrastructures. The limited number of crosses and population is maintained under speed breeding due to high input and maintenance costs for infrastructure. Integrated research employing scientists from different organizations is needed to avoid duplications of work, minimize investments on resources, and help in support/sharing of specialized equipment.

4. Conclusion

The adoption of speed breeding protocols in crop improvement programs will hasten the breeding cycle to a great extent with improved selection efficiency. It promotes the rapid delivery of resilient varieties by integrating modern breeding techniques with generation advancement protocols. The superior genotype with improvement over multiple traits such as yield, quality, biotic, and abiotic stress resistance can be developed at a minimal period with the inclusion of high-throughput genotyping and phenotyping platforms in speed breeding. Many superior varieties have been rapidly developed in economically important species through the exploitation of speed breeding techniques. The inclusion of genomic selection approaches in speed breeding paved the way for the improvement of complex traits governing resistance. Few modified conventional approaches viz., single plant selection, single-pod descent, and single-seed descent are included in speed breeding protocols which greatly reduced the limitations of long generation time, cost, and labor. The evolution of advanced genomic techniques coupled with rapid gene fixation approaches offers faster realization of genetic gain in crop breeding programs. In addition to accelerated progression toward the attainment of homozygosity, the speed breeding protocols also prove efficient in the rapid evaluation of genetically modified/transformed lines of a crop species. The standardized speed breeding protocols suitable for small environments are now available with modification provisions to meet the local needs. However, it still remains a less adopted choice in many developing countries due to cost-expensive infrastructure development, lack of trained professionals, unsupportive policies, no proper financial support from the public domain and lack of essential resources. With the coordination of multidisciplinary organization, speed breeding becomes an efficient tool to meet ever-challenging food demand under changing climatic conditions.

Acknowledgments

The authors are highly thankful to Dr. M. Raveendran, Director of Research, Tamil Nadu Agricultural University (TNAU), for his valuable suggestions toward this chapter. We also acknowledge Dr. R. Sudhagar, Associate Professor and Head, Sugarcane Research Station, TNAU, and Dr. K. Ganesamurthy, Professor and Head (Retd.), Department of Rice, TNAU, for rendering supportive documents.

Conflict of interest

The authors declare no conflict of interest in this chapter.

Thanks

The authors express their sincere gratitude to the Department of Plant Breeding and Genetics, Tamil Nadu Agricultural University, for providing scientific assistance on speed breeding techniques.