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Research Highlights
Re-Engineering the Soybean Flower to Capture Hybrid Vigor

By Nicole Szeluga, Cornell University

Hybrid breeding has significantly elevated the productivity of maize and rice, and hybrid vigor, the phenomenon in which offspring outperform their parents, has been used for over a century to increase crop yields, improve abiotic and biotic stress tolerance, and enhance seed quality. Hybrid vigor increases crop yields by up to 15-50%, enabling growers to achieve “more yield with less field.” A full 65% of the total maize cultivation is now hybrid. Field trials using soybean that was hybridized by hand increased yields by 10-20% — a promising solution for future agricultural needs.

However, soybean is a self-fertilizing plant that does not readily hybridize with other plants and, therefore, lacks the benefits of hybrid vigor. Soybean makes small, inconspicuous flowers that fail to efficiently attract pollinators or disperse pollen. One current technique to promote hybrid breeding is to cross male-sterile flowers with male-fertile flowers. To generate male-sterile flowers, the anthers must be manually removed, which is inefficient and labor-intensive. Other strategies include using potentially hazardous chemicals or male-sterile genetic mutants that are not 100% effective, and may take years to integrate. This $60,000 project aims to transform soybean from an inbreeding to an outcrossing crop.

Biotechnology offers an environmentally sustainable, cost-effective, and time-efficient approach for producing hybrid soybeans that have been re-engineered to attract pollinators. Honeybees are the most likely insect pollinator for soybean, and they prefer sweeter, larger purple flowers over smaller white flowers. The ideal soybean for hybrid breeding would contain large, purple flowers that are visible to pollinators and have sweet, sucrose-rich nectar.

To generate this ideal plant, the genes that control flower color, growth size, and sucrose concentration in the nectar have been identified. Using dead Cas9 (dCas9, a variant of CRISPR), the activation of the purple petal gene, the petal size gene, and the sucrose concentration gene have been increased.

Figure 1. Integration of (A) Barnase/Barstar system for male-sterile lines and (B) floral trait modifications to create (C) hybrid soybean seed.

Large purple flowers may increase pollinator visits, but more is required. Self-fertilization will be blocked by engineering single-generation male-sterility into soybean using Barnase/Barstar, a two-component system that is currently used by Bayer to successfully promote hybrid breeding in canola, another self-fertilizing crop. The Barnase/Barstar system involves the production of male-sterile “Barnase” plants that express a ribonuclease that blocks pollen maturation and cannot self-fertilize, and male-fertile “Barstar” plants that express a ribonuclease inhibitor that is able to rescue male fertility, and therefore self-fertilization, in the Barnase-Barstar hybrid offspring.

The integration of floral trait modifications and the Barnase/Barstar system will result in a complete hybrid seed germination system for soybean that can directly increase overall yield. Future field trials will examine pollinator visitation and overall performance of these new hybrids.

This project is one of the most innovative research projects funded by USB in 2019, and is a part Genomic Tools to Enable Trait Discovery iPOP. Once developed, these tools will be protected by patents where appropriate and then made available to public and private researchers. This program also has the potential to receive royalties if the discoveries are significant enough for companies to license.

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.