Speed-Breeding System in Soybean: Integrating Off-Site Generation Advancement, Fresh Seeding, and Marker-Assisted Selection

The growing human population and a changing environment have raised significant concern for global food security, with the current improvement rate of several important crops inadequate to meet future demand 1 . This slow improvement rate is attributed partly to the long generation times of crop plants. Here, we present a method called 'speed breeding', 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 (Triticum aestivum), durum wheat (T. durum), barley (Hordeum vulgare), chickpea (Cicer arietinum) and pea (Pisum sativum), and 4 generations for canola (Brassica napus), instead of 2-3 under normal glasshouse conditions. We demonstrate that speed breeding in fully enclosed, controlled-environment growth chambers can accelerate plant development for research purposes, including phenotyping of adult plant traits, mutant studies and transformation. The use of supplemental lighting in a glasshouse environment allows rapid generation cycling through single seed descent (SSD) and potential for adaptation to larger-scale crop improvement programs. Cost saving through light-emitting diode (LED) supplemental lighting is also outlined. We envisage great potential for integrating speed breeding with other modern crop breeding technologies, including high-throughput genotyping, genome editing and genomic selection, accelerating the rate of crop improvement.

Gene and genome duplications underlie the origins of evolutionary novelty in plants. Soybean, Glycine max, is considered to be a paleopolyploid species with a complex genome. We found multiple homologs of the phytochrome A gene (phyA) in the soybean genome and determined the DNA sequences of two paralogs designated GmphyA1 and GmphyA2. Analysis of the GmphyA2 gene from the lines carrying a recessive allele at a photoperiod insensitivity locus, E4, revealed that a Ty1/copia-like retrotransposon was inserted in exon 1 of the gene, which resulted in dysfunction of the gene. Mapping studies suggested that GmphyA2 is encoded by E4. The GmphyA1 gene was mapped to a region of linkage group O, which is homeologous to the region harboring E4 in linkage group I. Plants homozygous for the e4 allele were etiolated under continuous far red light, but the de-etiolation occurred partially, indicating that the mutation alone did not cause a complete loss of phyA function. The genetic redundancy suggests that the presence of duplicated copies of phyA genes accounts for the generation of photoperiod insensitivity, while protecting against the deleterious effects of mutation. Thus, this phenomenon provides a link between gene duplication and establishment of an adaptive response of plants to environments.