Cytoplasmic male sterility (CMS) is a widespread trait in flowering plants that has been exploited by breeders for producing hybrid seed for a century or more. Unlike many other 'domestication' traits, CMS is commonly found in the wild. CMS is a maternally inherited trait caused by expression of aberrant mitochondrial genes in anthers leading to a failure to produce viable pollen.

In natural populations, cms genes are balanced by nuclear Restorer-of-fertility (Rf) genes. Most Rf genes encode members of the pentatricopeptide repeat (PPR) family — sequence-specific RNA binding proteins that act in mitochondria or chloroplasts. Rf proteins suppress expression of mitochondrial cms genes by specifically binding to and inducing cleavage of cms transcripts. Rf and cms genes are widely used in three-line hybrid seed production systems in many vegetable and field crops, such as maize, rice, sorghum and canola.

There has been much interest in exploring their use for hybrid seed production in wheat and other Triticeae crops, but development of effective fertility control has proved difficult, in part because of the particularly complex genetics surrounding CMS traits and fertility restoration in Triticeae.

The advent of high-quality chromosome-scale genome assemblies for Triticeae species has revealed unprecedented numbers of Rf-like PPR genes (e.g. ~200 for hexaploid wheat versus ~20–30 for rice and barley; Melonek & Small, 2022). The complexity is compounded by an even more massive and abrupt expansion in genes encoding a second family of organellar RNA-binding proteins known as mTERFs (mitochondrial transcription termination factors) that may also play a role in fertility restoration in cereals.

It seems certain, therefore, that within the Triticeae there are multiple different causes of CMS and multiple Rf alleles active against them and that disentangling them may be difficult. Nevertheless, progress has been made with the identification of Rf1 and Rf3, encoding PPR proteins that bind to and induce cleavage of the CMS-inducing mitochondrial gene orf279 (Melonek et al. 2021).

This CMS gene is from Triticum timopheevii, where its expression is naturally suppressed far more effectively than has been achieved so far in T. aestivum, even by combining Rf1 and Rf3 in the same genotype. It seems therefore probable that one or more additional Rf genes remain to be discovered in T. timopheevii.

Recently we have discovered a characteristic C-terminal sequence in Rf-like proteins that correlates with their ability to induce RNA cleavage (Huynh et al. 2023) that should make it easier and faster to identify Rf candidates from genome or transcriptome data.

Melonek, J., Duarte, J., Martin, J., Beuf, L., Murigneux, A., Varenne, P., Comadran, J., Specel, S., Levadoux, S., Bernath-Levin, K., Torney, F., Pichon, J.-P., Perez, P., & Small, I. (2021). The genetic basis of cytoplasmic male sterility and fertility restoration in wheat. Nature Communications, 12(1), 1036.

Melonek, J., & Small, I. (2022). Triticeae genome sequences reveal huge expansions of gene families implicated in fertility restoration. Current Opinion in Plant Biology, 66, 102166.

Huynh, S. D., Melonek, J., Colas des Francs-Small, C., Bond, C. S., & Small, I. (2023). A unique C-terminal domain contributes to the molecular function of Restorer-of-fertility proteins in plant mitochondria. The New Phytologist, 240(2), 830–845.