Introgression of Clearfield™ rice crop traits into weedy red rice outcrosses

- Clearfield™ rice technology exceeded the lifespan expectations in the USA.
- After 14 years, Clearfield™ cultivars comprised 44% of rice hectareage in Arkansas.
- Fields with Clearfield™ rice history had 76% herbicide-resistant (HR) weedy rice.
- All HR weedy rice carried the S653N als mutation from Clearfield™ rice.
- The majority of HR weedy rice shattered less, were nondormant, and more crop-like in overall morphology.

Studies were conducted to determine the impact of introgression of crop alleles into weedy rice populations. Seeds of 89 weedy rice accessions collected from Arkansas fields in 11 counties, with a history of Clearfield™ rice production, were planted in 2011 and sprayed twice with imazethapyr (0.071 kg ai ha−1). Survivors were genotyped using an allele-specific PCR assay to detect the presence of resistance-endowing als mutations, S653N and G654E, in herbicide-resistant (HR) weedy rice that could have originated from Clearfield™ cultivars planted in the state. Hybridization between Clearfield™ rice and weedy rice was verified using four DNA markers RM 215, RM 220, and RM 234 microsatellites, and the and RID12 indel. Two to five HR plants, representing different plant types per accession per replication (698 plants total) were characterized for 16 morphological and biological traits. Seventy-nine percent of accessions were resistant to imazethapyr. The HR weedy rice plants carried the S653N mutation and were confirmed as outcrosses with Clearfield™ rice using DNA markers. Ten weedy rice accessions (nearly 25% of confirmed outcrosses) harbored multiple alleles from rice cultivars, indicating previous hybridization events with multiple cultivars or outcrossing with weedy rice. The majority of HR weedy rice accessions were similar to cultivated rice in height, culm angle, flag leaf length and width, days to flower, seed shattering and dormancy. Kernels of 20% of the parent weedy rice accessions were segregating for white and red bran color. Two of the parent weedy rice accessions with gold hull were homozygous for white bran and produced progenies with 100% white-bran kernels; thus, weedy rice was no longer exclusively red. Therefore, HR weedy rice outcrosses generally had acquired crop-like traits, which make it easier to deplete from the soil seedbank, but also difficult to scout.