Pentatricopeptide repeat proteins in crops: Advances in functional mechanisms and breeding applications

Pentatricopeptide repeat (PPR) proteins constitute a large superfamily of nuclear-encoded proteins characterized by tandem helical repeats. They function as critical coordinators of nucleus-organelle communication by modulating RNA metabolism within chloroplasts and mitochondria. This review summarizes recent advances in understanding the functional mechanisms of PPR proteins in major cereal and oilseed crops, with a focus on their roles in regulating seedling growth, stress responses, seed development, and cytoplasmic male sterility (CMS) restoration. We highlight how chloroplast-localized PPR proteins mediate RNA metabolism to ensure proper chloroplast biogenesis and seedling photosynthesis, while mitochondrial-targeted PPR proteins are crucial for RNA processing events that support respiration, embryo and endosperm development, and fertility restoration in CMS systems. We also describe how certain PPR proteins mediate biotic and abiotic stress responses through their functions in cold, drought, salt, and disease resistance, with specific members localized in chloroplasts or mitochondria. Finally, we outline unresolved questions regarding PPR protein complex assembly and environmental modulation, and highlight the emerging potential of engineered designer PPR (dPPR) proteins as programmable tools for precise RNA targeting and manipulation in organelles.