Development and innovation of haploid induction technologies in plants.

Haploid induction is one of the main techniques for breeding new varieties of major crops, and its key steps are improving the haploid induction rate and simplifying the induction procedure. With the development and innovation of plant haploid induction technologies, haploid breeding has been widely used in varietal improvement of important crops, showing the advantages of rapid homozygosity of heterozygous genes, shortening breeding period, and improving breeding efficiency. The combination of haploid breeding with crossing breeding, mutation breeding, reverse breeding, and molecular marker-assisted selection will greatly improve the effectiveness of crop breeding. Haploids and doubled haploids have demonstrated their usefulness in production of genetic populations, characterization of gene functions, and transgenic and cytological studies in plants. In this review, we summarize the progress of haploid induction technologies in view of various haploid induction techniques and applications of haploids and double haploids. In particular, the advances on the haploid induction in several major crops by genome editing were briefly described. Finally, we discuss current issues and future perspectives in this field, so as to promote the application of the haploid induction techniques, especially the techniques of creating haploid inducer lines by genome editing in crop breeding.