Allelic dosage across four functional <scp> <i>ALS</i> </scp> loci shapes <scp>ALS</scp> ‐inhibitor resistance in allotetraploid <scp> <i>Pontederia vaginalis</i> </scp>

Abstract BACKGROUND Pontederia vaginalis , carrying four functional ALS genes, is a major weed in Asian rice paddies. Acetolactate synthase (ALS)‐inhibitor resistance is caused by target‐site mutations, but how resistant allelic dosage affects whole‐plant susceptibility in weeds with more than three functional ALS loci remains unclear. This study quantified the effect of resistant allele number on herbicide susceptibility in P. vaginalis using progeny of a cross between two lineages carrying Pro 197 Ser substitutions in different ALS loci. RESULTS Hybridization between lineages carrying Pro 197 Ser in MvALS1 or MvALS3 yielded F 2 progeny with varying resistant allelic dosages, and genotyping confirmed independent segregation of these loci. Dose–response assays with imazosulfuron and bensulfuron‐methyl revealed that susceptibility decreased sharply as resistant alleles increased from zero to two (ED 90 (effective dose to have 90% reduction of dry weights) shifts ~3000‐fold for imazosulfuron), whereas additional alleles beyond two conferred only marginal further resistance (~four‐ to five‐fold). For genotype‐confirmed one‐resistant‐allele plants ( n = 25), the bootstrap 95% confidence interval (CI) for ED 90 was 16.7–292.0 g active ingredient (a.i.) ha −1 (median: 92.6 g a.i. ha −1 ), encompassing the registered field dose (90 g a.i. ha −1 ) and non‐overlapping with the CIs of the zero‐ and two‐allele classes. The F 2 segregation ratios matched theoretical expectations, suggesting non‐target‐site resistance does not substantially distort inheritance patterns. CONCLUSION These results demonstrate a clear allelic dosage effect on ALS‐inhibitor resistance in P. vaginalis , with diminishing returns beyond two resistant alleles. The CI for one‐resistant‐allele plants encompasses the registered field dose, indicating genuinely uncertain control outcomes at standard application rates. These findings challenge fixed dominant/recessive classifications and highlight the need to quantify allelic dosage effects in polyploid weeds for accurate resistance risk assessment. © 2026 Society of Chemical Industry.