Teosinte,
the wild ancestor of maize, has three times the seed protein content of
most modern maize strains. Researchers tracked down the mechanisms
responsible for the declining seed protein content in maize hybrids and
inbred lines. Their findings open up new avenues for maximizing seed
protein content and quality for future maize breeding, with implications
in nitrogen-use efficiency and food security. The researcher’s findings were published in Nature.
“There
is economic and environmental pressure to maintain high-yielding maize
while reducing the level of nitrogen applied to the soil,” said the
study author. “Therefore, it is crucial to identify genetic factors that
increase nitrogen-use efficiency.”
Over
millennia, plant breeders genetically altered plant species to create
seeds with greater proportions of metabolites to improve nutritional
value and utility. As corn became a major source of feed for livestock,
plant breeders prioritized starch content and yield while protein
content and flavor became secondary concerns. The use of nitrogen
fertilizer further reduced the importance of seed nitrogen content.
Consequently, modern maize hybrids contain only 5–10% protein; by
contrast, teosinte has a protein content of 20–30%, according to the
study.
Scientists
can trace the decline of maize seed protein content, but the genetic
mechanisms remained elusive. The team set out to identify the genes
responsible for the protein content discrepancy between teosinte and
maize by creating a complete teosinte genome sequence. By cross-breeding
teosinte with maize and analyzing the progeny, the researchers were
able to identify the quantitative trait locus (QTL), or the specific
chromosomal regions that are linked to the traits in question.
“Because
modern maize was domesticated from teosinte, we reasoned that
characterizing the genes responsible for the high-protein trait in
teosinte might reveal a more diverse set of QTLs than those found in
recent inbred maize populations,” said the author. “The results might
also help us to understand the reasons for the decrease in seed protein
content during the domestication of maize.”
The researchers zeroed-in on a significant high-protein QTL on chromosome 9. The teosinte high protein 9 (THP9)
QTL not only demonstrated the strongest effect during QTL mapping, but
also encoded an enzyme called asparagine synthetase 4 (ASN4) which plays
an important role in the metabolism of nitrogen. Previous studies on
rice, wheat and barley showed that changes in the expression of these
genes alter plant growth and nitrogen content.
While the THP9-teosinte (THP9-T)
gene variant (allele) is highly expressed in teosinte roots and leaves,
this is not the case the corresponding maize inbred, owing to
mis-splicing of gene transcripts, said the author.
“This
might be one of the factors that leads to differences in nitrogen
assimilation,” said the author. “Amino acids are essential substrates
for protein synthesis, and their levels in the plant are influenced by
soil nitrogen availability and the nitrogen use efficiency of the
plant.”
Through field trials, the team verified that THP9-T allele could increase the nitrogen-use efficiency in both normal- and low-nitrogen conditions. Further analysis suggested that THP9-T has the potential to improve the protein content of maize seeds and plants through plant breeding.
“Our research shows the possible value of hybrids that contain the THP9-T allele,
although larger field trials in multiple geographical locations will be
needed to fully establish its potential for improving seed protein
content and nitrogen-use efficiency in maize breeding,” said the author.
https://www.eurekalert.org/news-releases/971327