“Conventional and molecular approaches of apomixis to fix heterosis” ( Seminar Synopsis)

UNIVERSITY OF AGRICULTURAL SCIENCES, GKVK, BENGALURU

DEPARTMENT OF GENETICS AND PLANT BREEDING

SEMINAR – GPB 681 (0+1)

“Conventional and molecular approaches of apomixis to fix heterosis”

Heterosis (Hybrid vigor) has been exploited in the form of commercial hybrid cultivars in selected crops to the greatest extent. However, F1 hybrids being heterozygous, have to be resynthesized every time as they do not breed true. Since apomixis can bypass meiosis and fertilization, F1 hybrids can be made to breed true and thereby enable fixation of heterosis over multiple generations. Apomictic F1 hybrids have been developed conventionally through hybridization. However, its exploitation is restricted to fodder and forage crops. Hence, attempts were made to introduce apomixis through introgression of apomictic genes into maize and pearl millet from their wild relatives Tripsacum dactyloides and Pennisetum squamulatum, respectively. However, it was unsuccessful due to crossing barriers. Hence, an alternative approach is adopted to induce synthetic apomixis by molecular engineering. Genes controlling mechanism of apomixis such as PAIR1, SPO11-1, DFO, REC8, OSD1, PRD1, PAR, ASGR-BBML, MTL1, CENH3 etc. have been identified and characterized and are selectively targeted to engineer apomixis in sexual crops.

Bypassing meiosis (apomeiosis) is one of the critical steps in engineering apomixis in sexual crops that has been achieved by an effective strategy called Mitosis instead of Meiosis (MiMe). MiMe genotype is created by knocking out three meiosis-specific genes that result in unreduced and unrecombined clonal gametes. However, MiMe system in combination with haploid induction is essential to engineer apomixis. Genome editing of MTL1, DMP1, CENH3 genes has led to haploid induction through uniparental genome elimination. Ectopic expression of BABY BOOM1 gene in the egg cell, a member of the AP2 family of transcription factors, has been identified to induce parthenogenesis. Thus, fixation of heterosis through synthetic apomixis is achieved using MiMe coupled with haploid induction either through genome elimination or induced parthenogenesis.

In commercial rice hybrid CY84, editing of four genes (PAIR1, REC8, OSD1, MTL1) was done to obtain plants that could propagate clonally through seeds. In continuation to this study, stable transmission of heterosis over four generations was examined. An investigation was done in an elite commercial rice hybrid, BRS-CIRAD 302, to combine triple MiMe mutations with ectopic expression of BBM1 parthenogenesis inducer. This resulted in achieving clonal seed frequency of F1 hybrid up to 95% across three generations.

Both conventional and molecular approaches have their own benefits and drawbacks in fixation of heterosis. However, a proper look into the major limitations and challenges and overcoming them may aid in its successful application in most of the agriculturally important crop species, which will be a dream come true to the plant breeders and a boon to the farming community.

References:

1. Liu, C., He, Z., Zhang, Y., Hu, F., Li, M., Liu, Q., Huang, Y., Wang, J., Zhang, W., Wang, C. and Wang, K., 2023, Synthetic apomixis enables stable transgenerational transmission of heterotic phenotypes in hybrid rice. Plant Commun., 4(2).

2. Mahalandt, A., Singh, D. K. and Mercier, R., 2023, Engineering apomixis in crops. Theor. Appl. Genet., 136(6): 231.

3. Vernet, A., Meynard, D., Lian, Q., Mieulet, D., Gibert, O., Bissah, M., Rivallan, R., Autran, D., Leblanc, O., Meunier, A. C. and Frouin, J., 2022, High-frequency synthetic apomixis in hybrid rice. Nat. Commun., 13(1): 7963.

4. Wang, C., Liu, Q., Shen, Y., Hua, Y., Wang, J., Lin, J., Wu, M., Sun, T., Cheng, Z., Mercier, R. and Wang, K., 2019, Clonal seeds from hybrid rice by simultaneous genome engineering of meiosis and fertilization genes. Nat. Biotechnol., 37(3): 283-286.

5. Xu, Y., Jia, H., Tan, C., Wu, X., Deng, X. and Xu, Q., 2022, Apomixis: genetic basis and controlling genes. Hortic. Res., 9: 150.