Genetic Journey Begins to Understand and Manage Yellow Soybeans

Producers’ ongoing interest in expanding their soybean acreage is understandable, given high prices, solid markets, great yields and the crop’s increasing diversity. However, this desirability is continuing to push production into areas that may not be ideal for soybean production – that is, in fields where crops are particularly susceptible to yellow beans, a result of a condition called iron deficiency chlorosis (IDC). It’s not a new problem – it plagues millions of acres now in the Midwest – but with soybeans’ popularity, experts predict it will only intensify. So, the industry is getting aggressive with a new research program designed to sort it out and give plant breeders new tools to develop resistant varieties.

IDC occurs when iron-starved leaves produce insufficient chlorophyll (the substance which gives them their green color). When iron is not available to plants or is inadequate for normal growth, leaves become pale green, yellow or white. IDC causes reduced growth and yield – in fact, every year it robs farmers of almost 100 million bushels of yield and at least $1 billion in lost income. That’s why growers now consider it the top non-disease production problem in the Midwest.

But asking “What’s new?” in the fight against IDC has long drawn frustrating and difficult answers. It seems each time researchers make a small gain in the drive to understand the problem they become further aware of its complexity. That’s especially true now that painstaking lab and field work has revealed that not just one or two genes are responsible for poor iron uptake in soybeans – rather, perhaps as many as dozens may be involved in different aspects of the problem, such as the way iron moves into the seed and up and down the stem. This is why IDC is proving to be one of the most complicated soybean production problems soybean researchers have ever encountered.

That understanding was a breakthrough in itself, actually. But it’s just the beginning for Midwest farmers, who watch helplessly year after year as apparently healthy crops turn yellow and collapse. “It’s such a difficult problem for plant breeders, because IDC is a complicated condition,” says USDA/ARS research scientist and IDC expert Randy Shoemaker. “Many genes are involved, and just like there’s no magic yield gene, there’s no magic IDC gene, either. There is not a single resistance gene. It’s very challenging.”

Now, though, an unprecedented research journey has begun to untangle how the genes interact, and to understand their respective roles in IDC. Support from the North Central Soybean Research Program is bringing together expertise from the USDA/ARS in Iowa, Minnesota and Maryland, and from North Dakota State University (NDSU), to sort out the genetics driving IDC. Ultimately, the researchers hope to develop tools called molecular genetic markers, to help plant breeders identify varieties that can best tolerate iron deficient soils. “This is the first time there’s been a long-term, concerted effort to work on the problem,” says NDSU research scientist Phil McClean, one of the project leaders along with USDA’s Shoemaker.

It may sound like a pipe dream, given the problem’s magnitude. But thanks to advances in molecular genetics research, a solution is more possible than ever before. Plants breeders have access to the same breakthroughs in molecular genetics technology that have helped the likes of forensic police investigators develop new leads in cold cases. Similar technology helped agricultural researchers identify more than one gene was responsible for IDC. Indeed, it’s no longer science fiction…it’s real, and it’s being used in agricultural laboratories everywhere.

That means once researchers find the precise genes involved in the problem and their function, plant breeders can get busy. They can run molecular tests on hopeful soybean varieties, determine whether they have desirable genes, and have a good idea of their likely performance before going through lengthy field trials. This should help to appreciably shorten the time it takes to get a helpful variety from the lab into farmers’ planters – not just by months, but by years.

It won’t be easy. The IDC problem is a chronic perfect storm, starting with iron-deficient soil found in vast parts of Iowa, North Dakota, Minnesota, Nebraska and Kansas. It’s a natural trait; some soils are simply rich in certain nutrients, and others are not. In this case, some natural chemical properties of the soil make iron less available to the plant. Because the problem is so expansive, virtually nothing can be done to improve the environment itself. “It’s not like some other deficiencies, where you can add something to the soil and fix the problem,” says Shoemaker, In fact, previous research has shown foliar applications of products meant to boost iron were largely ineffective, even with a surfactant to stimulate iron moving into the plant tissue. Shoemaker says increased yields of about a bushel an acre were realized from these treatments, hardly enough to justify their cost.

Worse, irreversible damage is done early, even before the plants turn yellow. And in some cases, the leaves don’t even turn yellow. Still, they’re sufficiently affected by IDC to reduce yield.

Several other factors contribute to this perfect IDC storm. High pH, greater than 7.4, is where yellow beans typically appear. That said, IDC can appear in soils with lower pH scores, too. Compacted and poorly drained soils make iron uptake difficult, as does the presence of phosphorus, manganese and zinc. High calcium levels in soil – a trait called calcareous soils – cause the iron molecules to bind tightly to soil particles, making the iron unavailable for uptake. And finally, SCN, cool soil temperatures and wet weather are implicated in some
IDC situations.

Research scientist McClean says what’s needed is better performance from the genes that transport iron. If researchers can find those genes responsible, then breeders can select plants that lead to varieties exhibiting such traits. The genes will have several functions, he says: they’ll lower the pH levels of the soil around the roots, bring iron into the roots and move iron into the water stream and then into the plant.

Until researchers come up with a genetic answer, they suggest selecting varieties said to be IDC tolerant, and to avoid excessive nitrogen fertilizer application to the crop that precedes soybeans in the rotation.

Ultimately, the scientists believe their work will have implications far beyond the Midwest. Some 200 billion acres of calcareous soils exist globally, and they are prime targets for iron deficiency. Humans, particularly women and children, who are unable to acquire iron through their diet are prone to many debilitating conditions, particularly anemia. Improving breeding tools at home could someday lead to better conditions abroad for millions of people with limited access to this
essential mineral.

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