Speed Breeding
Author: POOJA YADAV

The ever increasing human population and climate change have always been issues of major concern. We need to pay heed to the slow pace of increase in agricultural production and productivity for various crops in addition limited area availability. A major constraint to the increase in the same is long generation times of crop plants. ‘Speed breeding’, is one such platform which greatly shortens generation time and accelerates breeding and research programmes. Speed breeding can be used to achieve up to 6 generations per year for spring wheat, barley, chickpea and pea, and 4 generations for canola, instead of 2–3 under normal glasshouse conditions. There are 3 speed breeding conditions discussed in the following text.

Speed Breeding I

To evaluate speed breeding as a method to accelerate applied and basic research on cereal species, standard genotypes of spring bread wheat, durum wheat, barley and the model grass Brachypodium distachyon were grown in a controlled environment room with extended photoperiod (22 hours light/2 hours dark). A light/dark period was chosen over a continuous photoperiod to support functional expression of circadian clock genes. Growth was compared with that of plants in glasshouses with no supplementary light or heating during the spring and early summer of 2016 (Norwich, UK). Plants grown under speed breeding progressed to anthesis in approximately half the time of those from glasshouse conditions. Depending on the cultivar or accession, plants reached anthesis in 35–39 days (wheat, with the exception of Chinese Spring) and 37–38 days (barley), while it took 26 days to reach heading in B. distachyon.

Wheat seed counts per spike decreased, although not always significantly, in the speed breeding chamber compared to the glasshouse with no supplementary light and both wheat and barley plants produced a healthy number of spikes per plant, despite the rapid growth. The viability of harvested seeds appeared to be unaffected by speed breeding, with similar seed germination rates observed for all species. Moreover, crosses made between wheat cultivars under speed breeding conditions produced viable seeds, including crosses between tetraploid and hexaploid wheat.

Speed Breeding II

In an alternative, yet similar, protocol for rapid generation cycling, spring wheat, barley, canola and chickpea varieties in Queensland, Australia, in a temperature controlled glasshouse fitted with high-pressure sodium lamps to extend the photoperiod to a day-length of 22 hours. A control treatment in a glasshouse used a natural 12-hour control photoperiod. Both used the same temperature regime. Time to anthesis was significantly reduced for all crop species relative to the 12-hour day-neutral photoperiod conditions, where the average reduction was, depending on genotype, 22 ± 2 days (wheat), 64 ± 8 days (barley), 73 ± 9 days (canola) and 33 ± 2 days (chickpea). Analysis of growth stage progression revealed normal, though accelerated, development for all species compared to the day-neutral conditions.

Wheat plants produced significantly more spikes than those in day-neutral conditions and grain number was unaffected by the rapid development in both wheat and barley. Notably, time to anthesis was more uniform within each species under speed breeding conditions—an important feature, as synchronous flowering across genotypes is desirable for crossing. Additionally, wheat seed was harvested before maturity: 14 days’ post anthesis in speed breeding conditions, and following a 4-day cold treatment seed viability was high, indicating that generation time can be further reduced by harvesting immature seed without the need for labour-intensive embryo rescue. Seed viability of all other species under Speed breeding II conditions was either unaffected or improved compared with day neutral conditions. Since temperature greatly influences the rate of plant development, generation time may be further accelerated by elevating temperature. This may, however, induce stress and affect plant performance. Seed production (g per plant) of canola and chickpea was similar between speed breeding and day-neutral conditions.

The application of speed breeding conditions in a glasshouse fitted with supplementary lighting exemplifies the flexibility of the approach and may be preferred over growth chambers if rapid generation advance is to be applied to large populations, such as in breeding programs. SSD is commonly used in breeding and research programs to facilitate development of homozygous lines following a cross. This process only requires one seed per plant to advance each generation. Therefore, integrating speed breeding and SSD techniques can effectively accelerate the generation of inbred lines for research and plant breeding programs.

Speed Breeding III

In an additional protocol, we successfully implemented a low-cost speed breeding growth room design, lit exclusively by LEDs to reduce the operational cost of lighting and cooling, which permits 4–5 generations a year, depending on genotype and crossing plans. These results highlight the flexibility of tailoring the speed breeding ‘recipe’ to suit the local purpose and resources.

Applications of Speed Breeding in various research frontiers

Mutation studies

We observed that phenotypes associated with the ethylmethane sulfonate-induced mutation of the awn suppressor B1 locus and the Green Revolution Reduced height (Rht) genes in wheat could be accurately recapitulated in the controlled-environment room conditions.

Disease resistance studies

Clear signs of FHB progression in the susceptible cultivar, and little to no disease progression in the resistant cultivar. Previously, it has been shown that adult plant resistance to wheat leaf rust (caused by Puccinia triticina f. sp. tritici) and wheat stripe rust (Puccinia striiformis f. sp. tritici) can also be scored accurately under speed breeding conditions.

Cytogenetic Studies

Wheat carrying Ph1 and wheat-rye hybrids either carrying or lacking Ph1 in speed breeding and control conditions, and observed no major differences in chromosome pairing and recombination in meiocytes at metaphase I. The chromosome behaviour suggests that both wheat and wheat–rye hybrids are cytologically stable under speed breeding conditions. In summary, all the above adult plant phenotypes could be recapitulated accurately and much faster than in the corresponding glasshouse conditions.

Transformation Experiment

Seeds of barley (H. vulgare cv. Golden Promise) were sown and immature embryos harvested and transformed. The transformation efficiency of speed-grown and normal plants was comparable. Moreover, transformed barley explants could be grown under speed breeding conditions and viable seed obtained > 6 weeks earlier compared to standard control conditions.

Further Implications

Speed breeding is likely to reduce generation time for other crop species, for example, sunflower, pepper, and radish, which have been shown to respond well to extended photoperiod. Speed breeding methods have already been successfully applied to accelerate breeding objectives for amaranth, peanut and pea. In a breeding context, it should be noted that phenotypes studied under controlled or glasshouse conditions do not always correlate with field observations; in particular, complex traits such as yield and abiotic stress tolerance. Yet, phenotypic selection in early segregating generations of wheat (F2 and F3) has been successfully implemented for some desirable traits such as stripe rust resistance and grain dormancy. Alternatively, speed breeding could be used to rapidly generate fixed populations through SSD, which in some species may be cheaper than generating double haploids, for subsequent field evaluation and selection, thus facilitating genetic gain and production of improved cultivars. For genetically well-defined traits, speed breeding could be used to rapidly introgress genes or haplotypes into elite lines using marker-assisted selection.

Conclusion

Recent advances in genomic tools and resources and the decreasing cost of sequencing have enabled plant researchers to shift their focus from model to crop plants. Despite such advances, the slow generation times of many crop plants continue to impose a high entry barrier. Combining these tools and resources with speed breeding will provide a strong incentive for more plant scientists to perform research on crop plants directly, thus further accelerating crop improvement research. In the context of plant breeding, rapid generation advance to homozygosity following crossing will facilitate genetic gain for key traits and allow more rapid production of improved cultivars by breeding programs.

References:
Watson, A., S. Ghosh, M.J. Williams, W.S. Cuddy, J. Simmonds, M.-D. Rey, M. Asyraf Md Hatta, A. Hinchliffe, A. Steed, D. Reynolds, N.M. Adamski, A. Breakspear, A. Korolev, T. Rayner, L.E. Dixon, A. Riaz, W. Martin, M. Ryan, D. Edwards, J. Batley, H. Raman, J. Carter, C. Rogers, C. Domoney, G. Moore, W. Harwood, P. Nicholson, M.J. Dieters, I.H. DeLacy, J. Zhou, C. Uauy, S.A. Boden, R.F. Park, B.B.H. Wulff, and L.T. Hickey. (2018). Speed breeding is a powerful tool to accelerate crop research and breeding. Nat. Plants 4:23"29. doi:10.1038/s41477-017-0083-8


About Author / Additional Info:
Ph.D. Research scholar (PBG)