AFTER decades of work, research geneticist Ram Singh crossed a popular soybean variety ("Dwight," or Glycine max) with a related wild perennial plant to produce the first fertile soybean plants that are resistant to soybean rust, soybean cyst nematode and other soybean pathogens.

Singh works in the Soybean/Maize Germplasm, Pathology & Genetics Research unit in the University of Illinois at Urbana-Champaign department of crop sciences. The unit is a division of the U.S. Department of Agriculture's Agricultural Research Service (ARS).

His efforts to introduce the desirable attributes of wild, perennial Glycine species into soybean plants began in 1983 and followed a path that involved thousands of experiments, development of a hormone treatment that "rescued" immature hybrid seeds from sterility and multiple back-crosses of hybrid plants with their "recurrent parent," Dwight.

Collaborator Randall Nelson, research leader of the ARS soybean/maize research unit, plants seeds from Singh's most promising experiments, grows the plants and distributes their seeds to other scientists, who screen them for desirable traits and conduct their own breeding experiments.

Soybeans are the number-two field crop by acreage in the U.S., worth more than $4 billion annually.

Current soybean varieties are susceptible to an array of pests and pathogens. Among them, the parasitic roundworm known as soybean cyst nematode attacks soybean roots and stunts their growth. Soybean rust, a fungus first detected in the U.S. in 2004, taints leaves and eventually defoliates the plants.

Scientists have known for decades that some wild, perennial soybean relatives had desirable traits, Singh said.

"There are 26 wild species of Glycine perennials that grow in Australia," he said, adding that one species, Glycine tomentella, was of particular interest because it has genes for resistance to soybean rust and soybean cyst nematode.

"Many people tried to hybridize it with soybean plants starting back in 1979 at the University of Illinois," Singh said, but the hybrids produced only sterile plants, so "they decided it was impossible."

He continued to experiment, however, and eventually developed a hormone treatment that interrupted the process that caused the hybrid seeds to abort. He also developed a tissue culture method for producing several embryos — and, thus, several plants — from each seed. The plants were grown in a greenhouse, allowed to flower and crossed again with Dwight.

Singh eventually settled on a G. tomentella known as PI 441001 for these experiments because the wild plant is immune to both rust and soybean cyst nematode, as well as to Phytophthora root rot. It also can tolerate salt and drought.

As the experiments continued, Singh noted that each generation of hybrids had different numbers of chromosomes, reflecting their blend of soybean and tomentella chromosomes.

The goal, he said, was to isolate each of tomentella's 39 chromosomes and add them one at a time to the soybean plant's 20 pairs of chromosomes. That way, all of the genetic richness of tomentella could be captured in the hybrid soybean plants.

Further crosses have introduced the tomentella genes into the soybeans, creating soybean plants with 40 chromosomes and some of the most desirable tomentella traits.

So far, the effort has yielded plants resistant to soybean rust, soybean cyst nematode or Phytophthora root rot. Some of the new plants produce more soybeans per plant than Dwight, and some have higher protein content than Dwight.

As the research continues, soybean breeders now have access to dozens of new soybean lineages, each with some of the traits of the wild Australian plants.

The genetic material in wild Glycine species "is just like a treasure that is locked inside," Singh said. "With this method, we are unlocking the treasure."

A report of this work appears in the journal Theoretical & Applied Genetics.

The Illinois Soybean Assn., the University of Illinois Soybean Disease Biotechnology Center and the United Soybean Board provided partial funding for this work.

Volume:87 Issue:20