Sexual reproduction in flowering plants involves double fertilization, the union of two sperm from pollen with two sex cells in the female embryo sac. Modern plant breeders increasingly seek to circumvent this process to produce doubled haploid individuals, which are derived from the chromosome-doubled cells of haploid gametophyte. Doubled haploid production fixes recombinant haploid genomes in inbred lines. Technology of haploid induction is based on an inbred called Stock 6 that induces haploids when used as a male, over time inducer have been identified that have ability to induce hapliods having frequencies as high as ~15%2.

The key underlying gene, called MATRILINEAL (MTL/ZmPLA1/NLD), is a patatin-like phospholipase expressed primarily in pollen that causes a change in membrane properties during fertilization thus leading to selective loss of the paternal chromosome. In the zmpla1 haploid inducer lines, single nucleus sequencing revealed an unexpectedly high frequency of sperm DNA fragmentation, suggesting this is a cause of paternal genome loss following fertilization and subsequent production of maternal haploids2. A comprehensive molecular atlas integrating transcriptomes with various proteomes and metabolomes revealed that reactive oxygen species (ROS) burst may lead to sperm DNA fragmentation and HI1.

Loss-of-function mutants of Zea mays PHOSPHOLIPASE D3 (ZmPLD3), one of the members from the phospholipase D subfamily, could trigger maternal HI in maize. Mutations of ZmPLD3 resulted in a haploid induction rate (HIR) similar to that of mtl/zmpla1/nld and showed synergistic effects rather than functional redundancy on tripling the HIR (from 1.19% to 4.13%) in the presence of mtl/zmpla1/nld 3.

The integration of Doubled Haploid (DH) technology with marker assisted selections (MAS), transgenic technology, induced mutagenesis offers new insights to minimize breeding cycles and maximize genetic gains. Deeper understanding of the gene involved in inducing haploids coupled with editing such gene by CRISPR-Cas9 is expected to increase the haploid induction rates.

References:

1 JIANG, C., SUN, J., LI, R., YAN, S., CHEN, W., GUO, L., QIN, G., WANG, P., LUO, C., HUANG, W. AND ZHANG, Q., 2022, A reactive oxygen species burst causes haploid induction in maize. Mol. Plant., 15(6): 943-955.

2 KELLIHER, T., STARR, D., RICHBOURG, L., CHINTAMANANI, S., DELZER, B., NUCCIO, M.L., GREEN, J., CHEN, Z., MCCUISTON, J., WANG, W. AND LIEBLER, T., 2017, MATRILINEAL, a sperm-specific phospholipase, triggers maize haploid induction. Nature, 542(7639): 105-109.

3 LI, Y., LIN, Z., YUE, Y., ZHAO, H., FEI, X., E, L., LIU, C., CHEN, S., LAI, J. AND SONG, W., 2021, Loss-of-function alleles of ZmPLD3 cause haploid induction in maize. Nat. Plant., 7(12): 1579-1588.