Getting to the root of how plants tolerate too much iron
Iron is essential for plant growth, but with heavy rainfall and poor aeration, many acid soils become toxic with excess iron. In countries that experience dramatic flood seasons, such as West Africa and tropical Asia, toxic iron levels can have disastrous consequences on the availability of staple foods, such as rice.
Despite dozens of attempts over the past two decades to discover the genes responsible for iron tolerance, they have remained elusive until recently. Now, Salk scientists have found a major genetic regulator of iron tolerance, a gene called GSNOR. The results, published in Nature Communications on August 29, 2019, could lead to the development of crop species that produce higher yields in soils containing excess iron.
"This is the first time that a gene and its natural variants have been identified for iron tolerance," says Wolfgang Busch, associate professor, lead author of the article and member of the Salk Molecular and Cellular Plant Biology Laboratory and Integrative Biology Laboratory. "This work is exciting because we now understand how plants can grow under stressful conditions, such as high iron levels, which could help us make crops more resistant to stress.
In plants such as rice, high levels of iron in the soil cause direct cellular damage by damaging fats and proteins, reducing the ability of roots to grow. However, some plants seem to have an inherent tolerance for high levels of iron; scientists wanted to understand why.
"We believed that there were genetic mechanisms underlying this resistance, but it was not clear which genes were responsible," says first author Baohai Li, a postdoctoral researcher at Busch Laboratory. "To investigate this question, we used the power of natural variation of hundreds of different plant strains to study genetic adaptation to high levels of iron."
Scientists first tested a number of strains of a small mustard plant (Arabidopsis thaliana) to see if there were natural variations in iron resistance. Some of the plants had tolerance to iron toxicity, so the researchers used an approach called GWAS to locate the responsible gene. Their analyses identified the GSNOR gene as the key to allowing plants and roots to grow in iron-rich environments.
The researchers also discovered that the iron tolerance mechanism is, to their great surprise, linked to the activities of nitric oxide, a gaseous molecule that plays various roles in plants, particularly in response to stress. High levels of nitric oxide induced cellular stress and decreased the tolerance of plant roots to high levels of iron. This occurred when plants did not have a functional GSNOR gene. GSNOR probably plays a central role in the metabolism of nitric oxide and regulates the ability of plants to react to stress and cell damage. This nitric oxide mechanism and the GSNOR gene have also affected iron tolerance in other plant species, such as rice (Oryza sativa) and a legume (Lotus japonicus), suggesting that this gene and its activities are probably critical in many, if not all, plant species.
"By identifying this gene and its genetic variants that confer tolerance to iron, we hope to help plants, such as rice, become more iron resistant in areas where iron levels are toxic," says Dr. Busch. "Since we have discovered that this gene and pathway have been preserved in several plant species, we suspect that they may be important for iron resistance in all higher plants. In addition, this gene and pathway may also play a role in humans and could lead to new treatments for conditions associated with iron overload."
Li will then start his own laboratory at Zhejiang University in China. He plans to identify relevant genetic variants in rice and observe whether iron tolerance variants could increase crop yields in flooded Chinese fields.
