DNA repair under heat: DNA polymerase λ modulates heat stress-induced mutagenesis in plants.

Mutation rates often rise under environmental stress, a process known as stress-induced mutagenesis. Among abiotic factors, heat stress is a potent driver that elevates mutation rates and enhances genetic variation, yet the underlying mechanisms in eukaryotes remain unclear. Here, we investigated how heat stress influences DNA repair and mutagenesis both locally and globally in Arabidopsis thaliana using CRISPR-Cas9-induced DNA breaks, whole genome sequencing, and single-cell transcriptomic analysis. Heat stress greatly enhanced CRISPR editing efficiency across different chromatin contexts, reaching up to a 29.9-fold increase in heterochromatic regions. Moreover, heat stress consistently shifted repair outcomes toward one base pair insertions, independent of chromatin state. We identified a heat-inducible, error-prone DNA polymerase, DNA polymerase λ (DNA Polλ), as the key mediator of these repair profile changes. Genome-wide analyses of somatic mutations further revealed that the heat-induced increase in mutagenesis also depends on DNA Polλ. Single-cell transcriptomic profiling showed that DNA Polλ expression is tightly regulated and enriched in the central zone of the shoot apical meristem. Such tissue-specific patterns suggest that DNA Polλ serves dual functions: maintaining genome integrity in essential stem cell populations while also enabling stress-induced mutagenesis that can be transmitted to progeny.