Nickel Transport Gene Identified in Cereal Grains

Researchers have finally unraveled a long-standing mystery in plant biology after identifying a key gene responsible for transporting nickel to cereal grains.

The discovery was made possible by studying the role of nicotianamine (NA), a unique amino acid found in plants that plays a crucial part in nickel transport, and its impact on grain yield and vigor.

Led by Chao Daiyin from the Center for Excellence in Molecular Plant Sciences of the Chinese Academy of Sciences, a team of scientists collaborated with researchers at Huazhong Agricultural University to shed light on this complex process.

The study focused on understanding how nickel, an essential micronutrient, is transported over long distances within plants from stems and leaves to grains. This knowledge has been elusive for over 50 years.

A significant milestone in plant nutrition was achieved by Justus von Liebig's "law of the minimum" in 1840, which posits that crop growth can be limited by either macronutrients or micronutrients like nickel.

Researchers identified a key gene responsible for transporting nickel in cereal crops through a genome-wide association study of 1,404 maize accessions collected over multiple years and locations.

The study revealed that natural variation in the regulatory region of this gene, known as ZmNAET1, significantly affects grain nickel content. Those maize varieties with higher expression levels of ZmNAET1 had approximately 60% more nickel in their grains, indicating a potential breeding opportunity for enhanced nutrient uptake.

Further investigation into the function of ZmNAET1 showed that it plays a crucial role in exporting nickel from phloem cells located in stem nodes and ear shanks. Once exported, the nickel is bound to a molecule called NA to form Ni-NA complexes, which are then imported back into phloem cells by other transporters for distribution to grain-producing organs.

The ZmNAET1 gene's influence extends beyond nickel content, controlling key aspects of maize growth and development, including ear weight, grain size, starch content, and germination rates after storage. When the gene's function is impaired, significant reductions in these traits are observed, with mutant seeds showing notably lower germination rates even after extended periods of storage.

The researchers found that supplementing mutant seeds with nickel restored their germination rates, whereas adding zinc had no such effect. This suggests that nickel plays a crucial role in seed development and viability.

Furthermore, applying nickel to the leaves of plants grown in the field helped to restore plant height and urease activity, which were previously reduced due to impaired gene function. These findings indicate that nickel is essential for maintaining optimal plant growth and metabolism.

The study also showed that disrupted nickel transport leads to decreased urease activity in sink organs such as grains, resulting in impaired urea utilization and affecting amino acid synthesis and energy metabolism. This highlights the significance of nickel in nitrogen metabolism and yield formation.

In rice, researchers identified two homologous genes, OsNAET1 and OsNAET2, which are key determinants of grain nickel content. This discovery suggests that the mechanism of nickel transport is conserved across cereal crops, including maize and rice.

The study's findings also reveal differences in long-distance metal transport strategies between grasses and dicotyledonous plants, such as Arabidopsis thaliana. While related genes in Arabidopsis primarily regulate iron, copper, and cobalt transport, the NAET1 gene in maize and rice mainly affects nickel distribution, with a lesser impact on zinc.

The discovery of a key gene in maize and rice has shed light on how cereal crops transport nickel, a crucial nutrient for plant growth. This breakthrough fills a significant knowledge gap in understanding source-to-sink nickel transport, challenging the conventional view that grasses require minimal amounts of nickel.

The study reveals that the NAET1 gene plays a central role in regulating nickel distribution in these crops, with a lesser impact on zinc levels. This new insight opens up opportunities for improving cereal crop yield and seed vigor through targeted nutrient management and molecular breeding strategies. As a result, researchers believe this finding will have significant implications for agriculture and food production.