Email: gilangbintangfajar.suhono@research.uwa.edu.au

Hybrid breeding based on cytoplasmic male sterility (CMS) utilises a mitochondrial gene that induces pollen abortion and prevents self-pollination. Pollen production in the F1 generation is restored by Restorer-of-fertility (Rf) genes, which suppress the CMS phenotype.

A well-functioning CMS-Rf system in bread wheat (Triticum aestivum) would enable large-scale hybrid seed production at a low cost. Recently, the CMS-causing mitochondrial gene in wheat carrying Triticum timopheevii CMS has been identified as orf279, and the sequences of two Rf genes (Rf1 and Rf3) have been cloned1. Rf1 and Rf3 proteins, which belong to the family of pentatricopeptide repeat (PPR) proteins, were shown to bind to the orf279 transcript following the co-called ‘PPR-code’ and induce its cleavage.

However, the suppression of orf279 RNA by both Rf proteins differs in various wheat cultivars. In vitro analyses indicated that the RNA-binding affinity of Rf1 and Rf3 proteins to the RNA target is weaker than other well-characterised PPR proteins.

I hypothesised that the poor RNA binding of natural Rf proteins could be the direct cause of inconsistent levels of fertility restoration. I proposed synthetic Rf proteins as a novel tool to improve the suppression of orf279 through stronger binding affinity. In vitro RNA-binding assays showed that designer synthetic Rf3 proteins bind their RNA targets at least fifty times stronger than the natural ones.

The best candidate synthetic Rf3 gene has been transformed to Fielder*CMS wheat and future in planta RNA cleavage analyses will determine whether the stronger binding directly translates into more consistent cleavage of the CMS gene in wheat mitochondria and better fertility restoration in the F1 generation for commercial scale hybrid wheat breeding program.

Reference:

1. 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 and Small I (2021) The genetic basis of cytoplasmic male sterility and fertility restoration in wheat. Nature Communications. 12 (1), 1036.However, lineage-specific trends were observed in spike morphology: UmbL1w accessions have relatively long awns, and UmbL2 accessions have relatively short inter-spikelet rachis lengths.