Maize Genomics and Genetics 2025, Vol.16, No.2, 80-88 http://cropscipublisher.com/index.php/mgg 83 traditional steps, and quickly obtain homozygous for the target trait (Wang et al., 2019; Jacquier et al., 2020). Combined with tools such as molecular markers or genomic selection, the entire breeding process is more accurate and efficient (Chaikam and Prasanna, 2020; Meng et al., 2021). 4 Molecular and Genetic Progress in Haploid Induction 4.1 Discovery of key genes The study of haploid induction has made important progress, and some key genes discovered have brought new possibilities for corn breeding. For example, the MTL (also called ZmPLA1 or NLD) gene, although its loss triggers haploid induction, this was not discovered by chance. Through a large number of genetic studies, this gene was confirmed to play an important role in the haploid induction process (Liu et al., 2019). On the other hand, the discovery of the ZmDMP gene used a different method - map-based cloning, and later it was knocked out by CRISPR-Cas9 technology, and it was found that single nucleotide mutations can increase the haploid induction rate by two to three times, which is very helpful for improving efficiency (Figure 2) (Zhong et al., 2019). Despite this, the genetic mechanism of haploid induction is still complex, and these genes are only the key part of it. Figure 2 Phenotypic evidence of transgenic events that enhanced HIRs (Adopted from Zhong et al., 2019) Image caption: a, Bar plot of the HIR of ZD958 ears pollinated by genotype classes A, H and B derived from transgenic events T0-15 and T0-17. b, Bar plot of the EnA rate (EnAR) of ZD958 ears pollinated by genotype classes A, H and B derived from transgenic events T0-15 and T0-17. n indicates the number of ears used for calculating the HIR and the EnAR of each genotype. c, Performance of ZD958 ears pollinated by genotype classes A (n=30), H (n=46) and B (n=63). Scale bar, 2 cm. Genotype classes A, H and B represent the genotype combinations zmpla1-ZmDMP, zmpla1-heterozygous and zmpla1-(zmdmp-ko), respectively (a-c). d, The blue bar plot shows the fold change of the HIR and the EnAR between genotype classes zmpla1-zmdmp (natural allele) and zmpla1-ZmDMP in the CAU5-CAUHOI F2 population; the orange bar plot shows the fold change of the HIR and the EnAR between zmpla1-(zmdmp-ko) (knockout allele) and zmpla1-ZmDMP in the F2 population. n indicates the number of F2 population. Data represent the mean ± s.d.; **, P< 0.01, ***P< 0.001 (two-sided Wilcoxon rank-sum test) (a,b,d) (Adopted from Zhong et al., 2019)
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