Exploring quinclorac resistance mechanisms inEchinochloa crus-pavonisfrom China

BACKGROUND Barnyardgrass (Echinochloaspp.) is a global weed in rice fields. Quinclorac is commonly used to control barnyardgrass. However, due to persistent use, quinclorac resistance has evolved. We obtained quinclorac-susceptible (QS) and -resistant (QR1, QR2) lines from the progeny of a single resistantE. crus-pavonisfor a resistance mechanism study. RESULTS Line QR1 exhibited resistance to high quinclorac rates (up to 6400 g ha(-1)), whereas line QR2 exhibited a resistance/susceptibility segregation ratio of 3:1 at the field or lower rates (400, 100 g ha(-1)). Intriguingly, a lower level of(14)C-quinclorac metabolism and hence a higher level of(14)C-quinclorac translocation was observed in QR1 than QS plants. The basal expression levels of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase 2 (ACO2) genes did not differ significantly between the QR1 and QS lines. However, more expression ofACSandACOgenes was induced by quinclorac treatment in QS than in QR1. Basal levels of beta-cyanoalanine synthase (beta-CAS) gene expression were similar in QS and QR1 plants, but a greater level of down-regulation was detected in QS than in QR1 plants after quinclorac treatment. CONCLUSION These results indicate QR plants are less responsive to quinclorac than QS plants in terms of up-regulating quinclorac metabolism and ethylene synthesis. Resistance in thisE. crus-pavonisline is likely controlled by a single major gene, involving possibly an alteration in auxin signal perception/transduction to the ethylene biosynthesis pathway. The beta-CAS is unlikely to play a major role in quinclorac resistance in this particular population.