Cytoplasmic male sterility (CMS) is a maternally inherited trait that results in the inability to produce viable pollen. CMS has been identified in numerous plant species and arises due to alterations in the mitochondrial genome, either through spontaneous mutations (autoplasmy) or through incompatibility between nuclear and mitochondrial genomes (alloplasmy) (Kang et al., 2017).

Hybrid crop breeding heavily relies on a limited number of CMS sources, but over-reliance on a single CMS type presents significant risks. If a specific CMS type becomes susceptible to pests or diseases, widespread crop failure can occur, as exemplified by the Southern corn leaf blight epidemic in the 1970s that devastated T-cytoplasm maize hybrids (Levings & Siedow, 1992). The mitochondrial gene T-urf13 was identified as the main factor linking CMS to disease susceptibility in T-cytoplasm maize. Consequently, diversifying CMS sources is a crucial strategy for strengthening hybrid breeding programs.

Characterization and Development of New CMS Sources

When a new CMS source is discovered, detailed characterization is essential to differentiate it from existing CMS lines. For instance, a novel CMS known as "inap CMS" was developed by hybridizing Brassica napus with Isatis indigotica Fort. (Chinese woad) and subsequently performing repeated backcrossing with B. napus. Classical, histological, and genetic analyses confirmed that inap CMS is a distinct source of cytoplasmic male sterility (Kang et al., 2017).

Several CMS-inducing cytoplasms have been extensively studied in different crops such as sorghum, sunflower, and pearl millet. In sunflower, the ARG-2-1-2 CMS (Helianthus argophyllus) exhibited performance similar to the widely used PET-1 (H. petiolaris) CMS in terms of seed yield, oil content, and oil yield. Similarly, in sorghum, A2 CMS was found to be comparable to A1 CMS in grain yield and plant height, with additional benefits such as improved heterosis for flowering time and plant height (Reddy et al., 2007). To understand the effects of alternative CMS sources, researchers develop isonuclear alloplasmic lines—lines with identical nuclear backgrounds but different cytoplasms—to analyze how alien cytoplasm influences seed yield and related traits.

Considerations for Utilizing Alternative CMS Sources

The successful integration of alternative CMS sources into hybrid breeding programs depends on multiple factors, including the stability of male sterility, the presence of restorer genes in the germplasm, and the potential impact of CMS on agronomic traits. The effectiveness of a CMS source should be assessed in terms of its contribution to commercially viable hybrid production. Before large-scale adoption, an alternative CMS source should demonstrate at least equivalent, if not superior, performance compared to the currently used CMS sources (Sharma & Shadakshari, 2022).

Conclusion

Diversification of CMS sources plays a vital role in enhancing the resilience of hybrid breeding programs against biotic and abiotic stresses. By incorporating a broader range of CMS types, breeders can mitigate risks associated with genetic uniformity and develop more stable and productive hybrid crops. Continuous efforts to identify, characterize, and utilize novel CMS sources will be essential for the advancement of hybrid seed production and food security.

References:

  1. Kang, L., Li, P., Wang, A., Ge, X., & Li, Z. (2017). A novel cytoplasmic male sterility in Brassica napus (inap CMS) with carpelloid stamens via protoplast fusion with Chinese woad. Front. Plant Sci., 8: 252832.
  2. Reddy, B.V., Ramesh, S., Reddy, P.S., & Ramaiah, B. (2007). Combining ability and heterosis as influenced by male-sterility inducing cytoplasms in sorghum [Sorghum bicolor (L.) Moench]. Euphytica, 154: 153-164.
  3. Sharma, M., & Shadakshari, Y. G. (2022). Effect of alien cytoplasm on seed yield and its attributing traits in sunflower (Helianthus annuus L.). Mysore J. Agric. Sci., 56(1): 213-220.
  4. Levings, C.S., & Siedow, J. N. (1992). Molecular basis of disease susceptibility in the Texas cytoplasm of maize. Plant Mol. Biol., pp.135-147.