On August 23, 2024, the collaborative team of Kejian Wang and Qian Qian from China National Rice Research Institute published an article titled "Large-scale production of rice haploids by combining superior haploid inducer with PTGMS lines" in Plant Communications. The study successfully developed a rice haploid inducer line with an induction efficiency of 12.4%, and applied it to the cultivation of two-line rice sterile lines, realizing the large-scale application of doubled haploid technology in rice breeding.
Double haploid technology can quickly produce genetically completely homozygous lines between 1 and 2 generations, which greatly shortens the breeding time compared to the 6-10 generations of self-pollination or backcrossing required by traditional breeding. Corn is one of the most successful crops to use double haploid technology, mainly due to the discovery of the haploid induction line Stock6. In 1959, it was first reported that Stock6 could obtain 2-3% of maternal haploids through self-pollination or hybridization as a male parent, but due to its limited induction efficiency, Stock6 was not widely used in corn breeding. Subsequently, scientists continued to work hard to improve the efficiency of the induction line to 7-15%, greatly promoting the commercial application of double haploid technology in corn. In 2017, the main genetic basis of maize haploid induction lines was analyzed, and it was found that its induction ability was caused by mutations in the MTL gene (also known as ZmPLA1 or NLD). This gene is highly conserved in cereals, and knocking out homologous genes in rice can induce haploids, providing broad prospects for the application of double haploid technology in rice. However, the induction efficiency of haploid induction lines with different backgrounds obtained so far is low, for example, 3.57% for indica rice 93-11, 2.65% for indica rice Minghui 63, 1.26% for japonica rice Nipponbare, and 3.63% for hybrid rice Chunyou 84, which limits the application of this technology in rice breeding.
In order to create a high-efficiency haploid inducer line for rice, the study hybridized the haploid inducer line “Chunyou 84 Osmatl” created in the early stage with 20 germplasm resources including 12 cultivated varieties, 7 local varieties and 1 wild rice. In the F2 generation, homozygous Osmatl mutant strains (exogenous components removed) with different genetic backgrounds were screened out through genotype identification, and the offspring were propagated by single-plant propagation, and finally 112 haploid induced strains (i.e., Osmatl mutant strains) with diverse genetic backgrounds were obtained. After induction rate identification, it was found that the haploid induction rate of most induced strains was low, ranging from 0.7% to 4.4%. Excitingly, among the 10 hybrid offspring of “Chunyou 84 Osmatl” and the local variety “Caiyuanzhong”, the haploid induction rate of 4 strains reached 11.8~15.1%, and the ‘high-efficiency induction’ trait was stably inherited. The main agronomic traits were comprehensively investigated, and one strain with a relatively high plant height and relatively high fruiting rate was selected from the four strains, named HI285, for subsequent application research.
Figure 1. Large-scale haploid production in rice through the integration of PTGMS and HI. (Liu, et al., 2024)
The application of double haploid technology in rice breeding includes key links such as hybrid induction of haploids, screening and identification of haploids, and haploid doubling. Since rice is a self-pollinating plant, its "self-pollination" trait becomes an important obstacle in the hybrid induction of haploids, which seriously limits the large-scale application of double haploid technology in rice. At the same time, the cultivation of two-line sterile lines plays an important role in the development of new two-line hybrid rice varieties. In order to quickly cultivate new two-line sterile line materials, according to the characteristics of two-line sterile lines, hybridization between different two-line sterile lines was carried out under low temperature conditions to obtain heterozygous two-line sterile lines; under high temperature conditions, the high-efficiency haploid induction line HI285 was hybridized with the heterozygous two-line sterile line for seed production. Through practical verification, this study successfully obtained a large number of two-line rice sterile line haploid materials with different recombination, which provides a successful case for the large-scale application of double haploid technology in rice breeding.
