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Research Highlights

Research Highlights
Discovery and Deployment of Novel Genes for Durable Resistance to Multiple Nematode Populations in Soybean

Figure 1. Drs. Zenglu Li and Melissa Mitchum from the University of Georgia in the greenhouse.

By Zenglu Li, University of Georgia 

Soybean cyst nematodes (SCN) and root-knot nematodes (RKN) are the most yield-limiting soybean pests in the U.S. SCN is by far the No. 1 yield-limiting pest, with an estimated yield loss of over 128 million bushels in 2018. RKN is a major problem, particularly in the sandy or sandy loam soils in the southern U.S., with yield loss estimates of over 13.5 million bushels in 2018. Although cultural practices in soybean production fields can help reduce some yield loss from nematode damage, these soilborne pathogens have a wide range of hosts and can survive for many years in the soil. Currently, the development of nematode-resistant soybean varieties is the most economical and effective means of preventing nematode damages in soybean production.

Over 95% of SCN-resistant cultivars used in U.S. soybean production derive their resistance from only two sources: PI 88788 or Peking. Unfortunately, continued use of the same two resistance genes is causing selection pressure on SCN field populations, which are starting to develop resistance to these two genes, and a shift in the SCN HG types. This has been observed in infested fields. Identifying and implementing novel sources of resistance before the nematodes develop immunity to the two existing sources is critical, and increasing the available genetic diversity for SCN and RKN resistance is critical for long-term sustainability of the soybean crop.

The prevalence and impact of these pests vary by location due to environmental factors that influence the distribution of these pests and the stress they impose on the growing plants. Soybean researchers need to develop genetic resistance to key pests in each region. This project aims to identify and utilize novel nematode resistance genetics from cultivated, exotic or wild soybean sources to develop multiple nematode-resistant germplasm lines and breeder- friendly DNA marker assays for marker-assisted selection to support commercial breeding efforts. The project was funded for $310,170 in 2019, and involves six scientists from five soybean-producing states, covering maturity groups 0 through VIII.

During the period from 2017 to 2019, a total of 24 nematode-resistant germplasm/cultivars derived from diverse genetic backgrounds across maturity groups (MG 0-VIII) were released. These lines include some that are resistant to both SCN and RKN, and others resistant to multiple HG types of SCN as well as fungal diseases. Some of these lines contain SCN resistance from sources other than PI88788 and Peking. A total of 125 MTAs were signed, and 86 germplasm were transferred to commercial and public breeders. The nematode-resistance breeding team published 17 articles in peer-reviewed journals and presented 30 posters, oral presentations and field days.

Two major QTLs associated with resistance to Southern RKN were mapped to chromosomes 10 and 18, and DNA markers associated with the candidate genes were developed and widely employed for marker-assisted selection. Robust and breeder-friendly marker assays were developed for Rhg1 and Rhg4 SCN-resistance genes, which have been routinely used in marker- assisted selection.

Over this same time period, several new sources of unique nematode-resistant lines have been identified using a novel genomic strategy. This is the first step toward finding new genes that will provide broader and more durable resistance in soybean. More than 10 genetic populations incorporating these novel sources of resistance have been developed. These populations will be used for mapping QTLs/genes for nematode resistance, and for developing improved soybean germplasm with resistance to multiple nematode species. Currently, some of these populations are being used in QTL mapping, and a strong pipeline of soybean germplasm lines with resistance to SCN or RKN and competitive yield across all maturity groups (MG 0-VIII) is in place.

Figure 2. Root-knot nematode infection on soybean roots.

Soybean nematode resistance is one of the most important traits for new commercial varieties. Results from this project include new resistance genes, DNA markers and enhanced soybean germplasm and improved soybean cultivars, which will directly benefit both commercial and public soybean breeding programs, as well as soybean producers, by providing new genes or new resistant sources to develop high- yielding and resistant varieties. Soybean growers will be able to reduce yield loss from nematodes by using these improved varieties, and the fact that they are adapted to local growing conditions will enhance yields even more, and add value to the entire soybean value chain. DNA markers and QTL information generated from this project will benefit all soybean researchers seeking a better understanding of the genetics and mechanisms underlying resistance. Finally, it will produce desirable source materials for commercial and public breeders to use in their crossing programs for development of high-yielding and nematode-resistant germplasm and cultivar.

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.