High-density mutation tracks are associated with proton-beam irradiation patterns in Sorghum bicolor

Authors: Ahn, Ezekiel; Baek, Insuck; LIM, SEUNGHYUN - Orise Fellow; Prom, Louis; Kim, Moon; Meinhardt, Lyndel; MAGILL, CLINT - Texas A&M University

Submitted to: The Plant Genome
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2026
Publication Date: 6/29/2026
Citation: Ahn, E.J., Baek, I., Lim, S., Prom, L.K., Kim, M.S., Meinhardt, L.W., Magill, C. 2026. High-density mutation tracks are associated with proton-beam irradiation patterns in Sorghum bicolor. The Plant Genome. 19(3). Article e70267. https://doi.org/10.1002/tpg2.70267.
DOI: https://doi.org/10.1002/tpg2.70267

Interpretive Summary: Scientists have long used radiation to mutate crop DNA and create new varieties, but we assumed all radiation worked similarly—like a random shotgun blast. This study reveals that modern proton beams behave completely differently from traditional gamma rays. Instead of scattering damage randomly, proton beams act like precision "missiles," concentrating genetic damage into dense, spike-like tracks that smash through specific regions of the sorghum genome. Surprisingly, we discovered that the plant does not try to shield its most important genes from this intense fire; instead, it adopts a strategy of "extreme tolerance," surviving and functioning even when its critical DNA regions are heavily bombarded. This research fundamentally changes our understanding of how to edit crop genomes, providing plant breeders and genetic engineers with a new "heavy artillery" tool to induce rapid, structural evolution in crops, and offering evolutionary biologists a real-world model of how genomes survive catastrophic shock.

Technical Abstract: Induced mutagenesis is a cornerstone of functional genomics, yet the biophysical distinction between high-LET (Linear Energy Transfer) proton beams and low-LET gamma rays remains actively debated. Here, we present a multi-scale genomic analysis of 96 Sorghum bicolor lines, utilizing 192,040 curated SNVs to decode the "geometry of damage." While both sources displayed linear dose-dependency (R2 > 0.8) and conserved 96-channel chemical signatures, their spatial deposition was fundamentally distinct. Proton irradiation generated a highly unequal, "spiky" landscape (Gini ˜ 0.53) characterized by discrete, high-density mutation tracks (~ 500 kb scale) that sequestered ~37% of the total mutational burden, contrasting with the diffuse deposition of gamma rays. Crucially, functional enrichment analysis revealed that coding sequences (CDS) and promoters were not shielded but rather enriched for mutations (Ratio > 1.0) even at high doses, supporting a model of "genomic tolerance" over avoidance. Furthermore, the lack of correlation between mutational load and CDS fraction (r ˜ 0) suggests a state of "evolutionary indifference" where mutation pressure overwhelms immediate purifying selection, establishing proton beams as a potent tool for inducing structural genomic shock.