Mechanism of primisulfuron resistance in a shattercane (Sorghum bicolor) biotype

A shattercane biotype resistant to primisulfuron was identified during greenhouse evaluations of seed obtained from southeastern and south-central Nebraska fields previously treated with primisulfuron or nicosulfuron a minimum of 3 consecutive yr. Absorption, translocation, metabolism, and acetolactate synthase (ALS) assay experiments were conducted to determine resistance mechanism(s) by comparing ALS-susceptible forage sorghum (ROX) to resistant shattercane (RS). The ROX had 10 and 12% greater C-14 absorption than RS 24 and 96 hours after treatment (HAT), respectively. Absorption of C-14 increased over time for both ROX and RS, while C-14 translocation from the treated leaf to the shoots and roots was similar for ROX and RS. Unmetabolized primisulfuron accounted for > 80% of the radioactivity recovered 24 h after application for both ROX and RS. The ROX and RS had similar ALS-specific activities and total protein concentrations. K-m values for RS and ROX were 10.4 and 5.8 mM pyruvate, respectively. The ALS isolated from RS was less sensitive than ROX to inhibition by primisulfuron. The I-50 values for RS and ROX were 231 and 0.025 mu M primisulfuron, respectively. The mechanism of primisulfuron resistance in this RS biotype is an altered ALS with decreased sensitivity to primisulfuron.