Research report: Historical breeding reveals graph-based structure dynamics and intracellular DNA transfer in rice mitogenome

Plant mitochondrial genomes have long puzzled biologists because they mutate very slowly at the sequence level while rearranging dramatically in structure. A new study of ten rice cultivars, sampled across more than a century of modern japonica rice breeding from 1882 to 2011, has now traced how that paradox plays out within a documented breeding history. Using long-read sequencing and graph-based assembly, the researchers reconstructed the mitogenomes of cultivars spanning 129 years of artificial selection and compared their structural organization segment by segment.

The analysis showed that within one documented rice lineage in the sample set, the core master graph structure of the mitochondrial genome was conserved across the breeding period. In other words, despite the well-known capacity of plant mitogenomes to shuffle, the overall arrangement of the mitochondrial genome persisted along this branch of the breeding pedigree. The other two sampled breeding branches, however, displayed alternative graph connections, a pattern consistent with rearrangements mediated by repeats, the repetitive sequences that can pair up and recombine to swap genomic segments.

Graph-based assembly proved central to resolving these structures. One cultivar delivered an unexpected finding. Shennong265 is classified as japonica rice on the basis of its nuclear background, yet its mitochondrial genome clearly points to an indica maternal origin.

The study also examined intracellular DNA transfer, the movement of organellar DNA into the nuclear genome. Sequences of chloroplast origin inserted into the nuclear genome, known as NUPTs, and sequences of mitochondrial origin, known as NUMTs, were mapped across the ten nuclear genomes analyzed. The ultra-long organellar insertions, defined as those exceeding 10 kilobases, showed a clustered distribution rather than a random spread across chromosomes.

That clustering had a striking pattern: NUPTs were concentrated on chromosome 4, while NUMTs were enriched on chromosome 12. The most dramatic single example came from the cultivar Akitakomachi, where a 63-kilobase NUMT was supported by ultra-long reads. Detailed inspection showed that this insertion comprised six distinct mitochondrial segments, indicating that multiple independent insertion events contributed to building this very large NUMT rather than a single transfer of one contiguous mitochondrial block.

Such a structure demonstrates how nuclear reservoirs of organellar DNA can accumulate incrementally over time. By combining structural variation analysis with the study of intracellular DNA transfer within a historical breeding framework, the work offers a new perspective on how plant mitogenomes behave under artificial selection. Beyond mitochondrial biology, the authors argue that the approach establishes a foundation for reading crop breeding history from a cytoplasmic perspective.

Subject of Research: Structural dynamics and intracellular DNA transfer in rice mitochondrial genomes across historical breeding

Article Title: Historical breeding reveals graph-based structure dynamics and intracellular DNA transfer in rice mitogenome

Article References: Ma, Y., Zhangshuo, Huang, S., Zhao, Y., Liu, L., Xu, Q., Wu, Z., Nie, L., & Kong, J. (2026). Historical breeding reveals graph-based structure dynamics and intracellular DNA transfer in rice mitogenome. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10144-1

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10144-1

Keywords: rice, mitochondrial genome, japonica rice, long-read sequencing, graph-based assembly, NUMTs, NUPTs, DNA transfer, breeding history, structural variation, BMC Plant Biology, Historical