Abstract Precise modulation of gene expression through cis -regulatory editing holds promise for nontransgenic crop improvement. However, the sequence-to-function relationships that govern plant promoter activity remain poorly understood. Here we develop a massively parallel reporter assay in Sorghum bicolor to systematically measure the effects of >30,000 mutations spanning deletions, substitutions and motif insertions accessible through CRISPR editing across entire native promoters and 5′ untranslated regions of 3 photosynthesis genes: PsbS , Raf1 and SBPase . We find that gene expression is most tunable within a ~500-bp core promoter region. The mutational effects are reproducible across biological replicates and predictive of protein output. Within these regions, we identify compact deletions and motif insertions that strongly increase protein production (>30-fold relative to wild type), outperforming transgenic enhancer elements. Mutation–effect relationships are gene specific, highlighting the need for tailored regulatory maps. Our results establish a high-throughput strategy for cis -regulatory fine-mapping that may enable crop improvements through minimal, precise and nontransgenic gene edits.
Groover et al. (2026) studied this question.