Efficient gene targeting in soybean using Ochrobactrum haywardense-mediated delivery of a marker-free donor template.

Gene targeting (GT) for precise gene insertion or swap into pre-defined genomic location has been a bottleneck for expedited soybean precision breeding. We report a robust selectable marker-free GT system in soybean, one of the most economically important crops. An efficient Oh H1-8 (Ochrobactrum haywardense H1-8)-mediated embryonic axis transformation method was used for the delivery of CRISPR-Cas9 components and donor template to regenerate T0 plants 6-8 weeks after transformation. This approach generated up to 3.4% targeted insertion of the donor sequence into the target locus in T0 plants, with ∼ 90% mutation rate observed at the genomic target site. The GT was demonstrated in two genomic sites using two different donor DNA templates without the need for a selectable marker within the template. High-resolution Southern-by-Sequencing analysis identified T1 plants with precise targeted insertion and without unintended plasmid DNA. Unlike previous low-frequency GT reports in soybean that involved particle bombardment-mediated delivery and extensive selection, the method described here is fast, efficient, reproducible, does not require a selectable marker within the donor DNA, and generates nonchimeric plants with heritable GT.

Development of an efficient marker-free soybean transformation method using the novel bacterium Ochrobactrum haywardense H1.

We have discovered a novel bacterium, Ochrobactrum haywardense H1 (Oh H1), which is capable of efficient plant transformation. Ochrobactrum is a new host for Agrobacterium-derived vir and T-DNA-mediated transformation. Oh H1 is a unique, non-phytopathogenic species, categorized as a BSL-1 organism. We engineered Oh H1 with repurposed Agrobacterium virulence machinery and demonstrated Oh H1 can transform numerous dicot species and at least one monocot, sorghum. We generated a cysteine auxotrophic Oh H1-8 strain containing a binary vector system. Oh H1-8 produced transgenic soybean plants with an efficiency 1.6 times that of Agrobacterium strain AGL1 and 2.9 times that of LBA4404Thy-. Oh H1-8 successfully transformed several elite Corteva soybean varieties with T0 transformation frequency up to 35%. In addition to higher transformation efficiencies, Oh H1-8 generated high-quality, transgenic events with single-copy, plasmid backbone-free insertion at frequencies higher than AGL1. The SpcN selectable marker gene is excised using a heat shock-inducible excision system resulting in marker-free transgenic events. Approximately, 24.5% of the regenerated plants contained only a single copy of the transgene and contained no vector backbone. There were no statistically significant differences in yield comparing T3 null-segregant lines to wild-type controls. We have demonstrated that Oh H1-8, combined with spectinomycin selection, is an efficient, rapid, marker-free and yield-neutral transformation system for elite soybean.

A giant NLR gene confers broad-spectrum resistance to Phytophthora sojae in soybean.

Phytophthora root and stem rot caused by P. sojae is a destructive soybean soil-borne disease found worldwide. Discovery of genes conferring broad-spectrum resistance to the pathogen is a need to prevent the outbreak of the disease. Here, we show that soybean Rps11 is a 27.7-kb nucleotide-binding site-leucine-rich repeat (NBS-LRR or NLR) gene conferring broad-spectrum resistance to the pathogen. Rps11 is located in a genomic region harboring a cluster of large NLR genes of a single origin in soybean, and is derived from rounds of unequal recombination. Such events result in promoter fusion and LRR expansion that may contribute to the broad resistance spectrum. The NLR gene cluster exhibits drastic structural diversification among phylogenetically representative varieties, including gene copy number variation ranging from five to 23 copies, and absence of allelic copies of Rps11 in any of the non-Rps11-donor varieties examined, exemplifying innovative evolution of NLR genes and NLR gene clusters.

Uniform Expression and Relatively Small Position Effects Characterize Sister Transformants in Maize and Soybean.

Development of transgenic cell lines or organisms for industrial, agricultural, or medicinal applications involves inserting DNA into the target genome in a way that achieves efficacious transgene expression without a deleterious impact on fitness. The genomic insertion site is widely recognized as an important determinant of success. However, the effect of chromosomal location on transgene expression and fitness has not been systematically investigated in plants. Here we evaluate the importance of transgene insertion site in maize and soybean using both random and site-specific transgene integration. We have compared the relative contribution of genomic location on transgene expression levels with other factors, including cis-regulatory elements, neighboring transgenes, genetic background, and zygosity. As expected, cis-regulatory elements and the presence/absence of nearby transgene neighbors can impact transgene expression. Surprisingly, we determined not only that genomic location had the least impact on transgene expression compared to the other factors that were investigated but that the majority of insertion sites recovered supported transgene expression levels that were statistically not distinguishable. All 68 genomic sites evaluated were capable of supporting high-level transgene expression, which was also consistent across generations. Furthermore, multilocation field evaluation detected no to little decrease in agronomic performance as a result of transgene insertion at the vast majority of sites we evaluated with a single construct in five maize hybrid backgrounds.