Researchers may be able to improve corn yields and nutritional value after discovering genetic regulators that synthesize starch and protein in the widely eaten grain, according to a new study.
The research, published in the journal Proceedings of the National Academy of Sciences, could benefit millions of people who rely on corn for nutrition in South America, Africa and elsewhere.
The world's corn supply depends on improving its yield and quality, which relies on the accumulation of starch and proteins in the grain's endosperm, the study says. Endosperm, an important source of human nutrition that contains starch, oils and proteins, is the seed tissue that surrounds embryos.
"We found a novel approach to discover new regulators in the synthesis of starch and protein, which determine grain yield and quality," said study lead author .
The scientists discovered how corn starch and protein are simultaneously synthesized in the endosperm, which could allow them to find a good balance between nutrient quality and yield, the lead said. Corn domestication and modern breeding have gradually increased starch content but decreased protein accumulation in endosperms.
The researchers looked at key proteins in corn kernels known as zeins, which are devoid of lysine, an essential amino acid (a building block of proteins), resulting in poor nutrient quality. During corn breeding over decades, people increased lysine content by cultivating corn with lower levels of zeins. Still, today's lysine levels are too low to meet the needs of the world's rapidly growing population.
So, molecular geneticists and corn breeders are trying to dramatically reduce zein levels to improve corn nutrient quality by focusing on blocking them and so-called transcription factors. Transcription is when the information in a gene's DNA is transferred to RNA, resulting in proteins that play key roles in the body's tissues, organs, structure and functions.
The research team found that two transcription factors (ZmNAC128 and ZmNAC130) play key roles in regulating the synthesis of starch and protein, paving the way for further research to fully understand the balance between nutrient quality and yield at a molecular level.
Knockdown of expression of ZmNAC128 and ZmNAC130 with RNA interference (RNAi) caused a shrunken kernel phenotype with significant reduction of starch and protein. Authors show that ZmNAC128 and ZmNAC130 regulate the transcription of Bt2 and then reduce its protein level, a rate-limiting step in starch synthesis of maize endosperm. Lack of ZmNAC128 and ZmNAC130 also reduced accumulation of zeins and nonzeins by 18% and 24% compared with nontransgenic siblings, respectively.
Although ZmNAC128 and ZmNAC130 affected expression of zein genes in general, they specifically activated transcription of the 16-kDa γ-zein gene. The two transcription factors did not dimerize with each other but exemplified redundancy, whereas individual discovery of their function was not amenable to conventional genetics but illustrated the power of RNAi.
Given that both the Bt2 and the 16-kDa γ-zein genes were activated by ZmNAC128 or ZmNAC130, authors identified a core binding site ACGCAA contained within their target promoter regions by combining Dual-Luciferase Reporter and Electrophoretic Mobility Shift assays. Consistent with these properties, transcriptomic profiling uncovered that lack of ZmNAC128 and ZmNAC130 had a pleiotropic effect on the utilization of carbohydrates and amino acids.