In the following decades, a wide range of haploid generation methods were found, including parthenogenesis (mainly via anther culture), pollen irradiation, seed selection with twin embryos, sparse pollination, alien cytoplasm and wide hybridization. The potential importance of haploids in crop breeding and genetics was quickly recognized, resulting in the first boom in haploid production research. Similar discoveries of haploids soon followed in other species, including Nicotiana tabacum and Triticum compactum. The first observation of a natural haploid in higher plants was in Datura stramonium by Bergner in 1922. This review will promote the study and application of both CENH3- and MTL-mediated haploid induction in plants. We discuss the utility and potential of these two methods in crops by comparing their mechanisms and applications to date in plants. The conservation of CENH3 and MTL in plants indicates wide potential application for HI. In addition, we review the current understanding of MTL-mediated HI in plant.
This model can explain both the findings of HI failure when wild-type and mutant CENH3 genes are coexpressed and the alien centromere loading of CENH3 in stable hybrids. Here, we review the progress of the CENH3-medialed HI and propose a revised centromere-size model by suggesting a competitive loading process between wild-type and mutant CENH3 during HI. The mutation of a pollen-specific phospholipase gene, MATRILINEAL ( MTL) has been revealed to trigger the haploid induction (HI) in maize, which present another promising HI approach by the editing of MTL in plant. Subsequent studies demonstrated that haploids can be efficiently induced either through genetic engineering of CENH3 N-terminal tail or histone fold domain or by replacing CENH3 with an ortholog. In 2010, Ravi and Chan reported that haploid Arabidopsis thaliana plants can be efficiently induced through the introduction of a single genetic alteration in centromere histone H3 (CENH3).
Although diverse strategies have been developed to produce haploids over the past decades, most of them are applicable in only a limited number of plant species. Simple and consistent production of haploid is always an appealing pursuit for both crop breeders and researchers.
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