Extra iron helps stressed out wheat grow up big and strong

Researchers led by Keiichi Mochida of the RIKEN Center for Sustainable Resource Science (CSRS) in Japan have found that prolonged periods of high stress lead to iron deficiency and stunted growth of wheat crops. Experiments show that reducing iron deficiency with a synthetic organic molecule called PDMA results in better growth and healthier plants.

These results are good news for farmers and consumers and could lead to field treatments that improve wheat production during prolonged heat periods. The study is published in the journal Natural communications.

One of the biggest fears of current climate change is that prolonged periods of heat will disrupt food production. Even moderate warming can reduce the yield of cool-season grain crops like wheat, with one global study estimating that wheat production falls by 6% for every 1°C increase.

Not only do these crops photosynthesize less and produce smaller grain grains, but the grains are also less nutritious. While most research on how plants adapt to heat stress has focused on acute stress (very high temperatures over a few days), Mochida and his team reasoned that the greatest threat posed by climate change was prolonged periods of moderately high temperatures.

Researchers characterized what happens to bread wheat after two weeks of moderate heat stress. Compared to wheat grown at normal temperatures, the stressed wheat plants weighed less and analyzes indicated that they carried out less photosynthesis. By checking for nutrient deficiencies, researchers found that the leaves of heat-stressed plants contained less than half the normal amount of iron. Could their stunted growth be the result of iron deficiency?

Genetically, wheat is complex. To delve deeper into biological details, the researchers turned to a genetically simpler grass called false purple brome (Brachypodium distachyon), which is often used as a model plant for grain crop studies.

Typically, model plants for experiments come from biobanks that store specific specimens that can then be used by researchers around the world, ensuring consistent genetics every time. For a widely used model like B. distachyon, biobanks contain many individual samples, each with a code name and their own slightly different genetics, just like humans.

In experiments, the model grass responded to heat stress almost the same as wheat. But the degree of grass disease varied from sample to sample, as did iron deficiency. For example, grass sample Bd21 had extremely low biomass, very yellow leaves, and 91% less iron than plants grown at normal temperatures. On the other hand, sample Bd21-3 had somewhat milder symptoms and only 61% iron deficiency.

With the simpler genome, the researchers were able to compare these two model grass samples and identify BdTOM1, the gene responsible for the difference.

Plants cannot extract iron from the soil as it is. Instead, they make organic compounds called mugineic acids and release them into the soil. Once these compounds bind to iron in the soil, plants can then absorb it through their roots. The BdTOM1 gene is responsible for the production of mugineic acids.

The analysis showed that after two weeks of heat stress, the Bd21-3 grass sample contained significantly more deoxymuginic acid in its roots than Bd21, explaining why Bd21 had greater iron deficiency and indicating that variations in BdTOM1 likely led to variations in sensitivity to heat stress.

The researchers then reasoned that they could alleviate iron deficiency and improve growth by giving heat-sensitive plants more deoxymuginic acid. They tested this hypothesis in model grass and wheat using a synthetic deoxymugineic acid called PDMA. Their hypothesis was correct; Under heat stress, PDMA treatment led to improved photosynthesis and biomass, provided that the PDMA concentration was not too high.

Mochida is optimistic that these results can be tested in the field. “In the short term,” he says, “this research offers a new approach to improve crop tolerance to heat stress, demonstrating the potential to optimize iron absorption and improve agricultural productivity.”

“In the long term, breeding efforts targeting genes involved in nutrient homeostasis could contribute to food security and sustainable agriculture, taking into account climate scenarios, societal needs and competition for resources between sectors such as agriculture and energy.”