Research Progress
A gene variant in maize that increases protein content
Post: 2022-11-17 08:51  View:64

Crops such as maize need nitrogen to create protein, and the yield of many modern crops relies on nitrogen fertilizers. A previously unknown gene variant found in a non-domesticated maize variety enables plants to use nitrogen more efficiently and produce more protein than their modern counterparts.

The problem

Modern cereal crops, such as maize (corn), have been genetically tailored to respond to supplemental nitrogen fertilizer. Although fertilizer treatment can lead to higher grain yield and protein content, farmers often over-fertilize, leading to an overload of nitrogen in rivers and other bodies of water. In view of a growing human population to feed and increasing environmental stresses created by climate change, maintaining high crop yields is essential, but this must be achieved with minimal environmental damage. Understanding how plants take up and use nitrogen will be crucial for crop improvement.

The discovery

Seeds of primitive maize species called teosintes have much higher protein content (about 30% by mass) than seeds of most modern inbred varieties (about 10%)1, even without application of supplemental nitrogen. This contrast provides an opportunity to identify the genetic factors that are responsible for differences in protein content2,3. The genome sequence of B73, an inbred line of maize, is known, but we had to determine that of a teosinte. We crossed a teosinte line (Zea mays subsp. parviglumis) with the B73 variety, and compared the genomes and the amino-acid and nitrogen content of the progeny. Through this approach, we identified several genomic regions that influence seed protein content.

The largest effect on protein content seemed to be associated with a region on chromosome 9, where a gene designated TEOSINTE HIGH PROTEIN 9 (THP9) encodes an enzyme called ASPARAGINE SYNTHETASE 4 (ASN4) that is highly expressed in roots and leaves. ASN enzymes are central to nitrogen metabolism and are responsible for making the amino acid asparagine. Asparagine has an important role in nitrogen recycling and acts as a nitrogen donor in reactions in which amino groups are transferred between molecules. Asparagine levels in plants are thus associated with seed protein content. B73 and several other commonly used inbred maize varieties have a mutant form of THP9 called THP9-B, which leads to undetectable levels of ASN4.

Replacing the inbred varieties’ mutant gene with the teosinte version, THP9-T, led to an increase in seed protein content of about 35%, and an increase in nitrogen content of about 18%, 54% and 94% in the leaves, roots and stems, respectively. As a result, plant biomass was also substantially increased. Field trials involving application of different amounts of nitrogen at normal and reduced nitrogen levels showed that THP9-T enables plants to use nitrogen more efficiently than does THP9-B. Moreover, transferring THP9-T into one of the most commonly grown maize hybrids in China, Zhengdan 958, considerably increased its seed protein content (Fig. 1); nitrogen levels in the roots, stems and leaves; height; and above-ground biomass.

Figure 1

Figure 1 | A variant of the gene THP9 enhances protein content in maize seeds. A gene in maize called THP9 encodes an enzyme involved in nitrogen use. a, Two variants of the modern maize variety Zhengdan 958 were generated and compared. One (Zhengdan 958-T) expresses a form of THP9 called THP9-T, which is found in primitive species of maize called teosintes. The other (Zhengdan 958-B) expresses a form of THP9 called THP9-B, which is from a modern inbred variety. b, Although the two variants show the same yield, Zhengdan 958-T has higher seed protein content than Zhengdan 958-B, suggesting that the introduction of THP9-T into modern maize could improve the crop’s protein content.

The implications

THP9-T promotes nitrogen assimilation and protein synthesis more than THP9-B does, yet has no apparent negative effect on yield. Nevertheless, THP9-T has been relatively under-represented in maize-breeding populations. This might be because the ample application of nitrogen fertilizer has removed selection pressure on THP9 for variation that improves a plant’s nitrogen-use efficiency (NUE). The low NUE of plants with THP9-B means that they need supplemental nitrogen fertilizer to ensure yield and protein content.

THP9-T could potentially be used to improve maize. Introducing THP9-T into modern maize varieties greatly enhanced levels of amino acids, especially asparagine, eventually increasing seed protein content without affecting yield. THP9-T also increases NUE, which is important for promoting a high yield under low-nitrogen conditions.

We determined the genomic sequence of a teosinte and found that the forms of the gene THP9 have the potential to improve protein content in maize seeds and NUE. Future research into the teosinte genomic sequence could reveal other genomic regions that influence seed protein content. The high genomic variation between teosinte and B73 suggests that other agronomically useful genes that were altered during teosinte domestication could be found by mining genomic sequences. — Yongrui Wu is at the National Key Laboratory of Plant Molecular Genetics, CAS Center for Excellence in Molecular Plant Sciences, Shanghai Institute of Plant Physiology & Ecology, Chinese Academy of Sciences, Shanghai, China. Brian A. Larkins is at the University of Arizona, Tucson, Arizona, USA.

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