Cadmium (Cd) accumulation in maize grains is detrimental to human health. Developing maize varieties with low-Cd contents is important for maize grains safe consumption. However, the key genes controlling maize grain Cd accumulation have not been cloned. Here, we identified one major locus for maize grain Cd accumulation (qCd1) using a genome-wide association study (GWAS) and bulked segregant RNA-seq analysis with a biparental segregating population of Jing724 (low-Cd line) and Mo17 (high-Cd line). The candidate gene ZmHMA3 was identified by fine mapping, which encodes a tonoplast-localized heavy metal P-type ATPase transporter. An EMS mutant analysis and an allelism test confirmed that ZmHMA3 influences maize grain Cd accumulation. A transposon in intron 1 of ZmHMA3 is responsible for the abnormal amino acid sequence in Mo17. Based on the natural sequence variations in ZmHMA3 gene of diverse maize lines, four PCR-based molecular markers were developed, which were successfully used to distinguish five haplotypes with different grain Cd contents in the GWAS panel and to predict grain Cd content levels of widely used maize inbred lines and hybrids. These molecular markers can be used to breed elite maize varieties with low grain Cd contents.
Waxy maize has many excellent characteristics in terms of its nutritional and economic value. In recent decades, the waxy maize germplasm has increased dramatically as a result of different selection methods. We collected 200 waxy maize inbred accessions from different origins to study their genetic diversity and phylogenetic relationships, and to identify new waxy mutations. A simple sequence repeat (SSR) analysis revealed wide genetic diversity among the 200 waxy maize accessions. The maize accessions were clustered into three groups. We sequenced the waxy gene from the first to the 14th exon. Nucleotide variation analysis of 167 waxy maize and 14 flint maize lines revealed some nucleotide differences in the waxy gene among different waxy maize groups, and much narrower nucleotide diversity in waxy maize than in flint maize. In a phylogenetic analysis, waxy maize carrying the same mutation allele clustered together, and waxy maize carrying different mutation alleles distributed in different groups; waxy maize was intermixed with flint maize in each branch, and wx-D7 waxy maize separated significantly from waxy maize lines carrying wx-D10, wx-124 and wx-hAT mutant alleles. The wx-hAT was a new waxy mutation identified in this study. It consisted of a 2286-bp transposon inserted into the middle of exon three of the waxy gene. A PCR marker specific for the wx-hAT allele was developed. These results will be useful for the utilization and preservation of the waxy maize germplasm, and the PCR marker has potential uses in waxy maize breeding programs.
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