Benzobicyclon efficacy is affected by plant growth stage, HPPD Inhibitor Sensitive 1 (HIS1) expression and zygosity in weedy rice (Oryza sativa)

Abstract

AbstractBenzobicyclon tolerance in rice (Oryza sativa L.) is dependent on the presence of a functional HIS1 gene, but the level of sensitivity might vary among different cultivars. Greenhouse, laboratory, and field experiments were conducted to further explore the role of HIS1 in cultivated rice tolerance and to exploit findings toward optimizing benzobicyclon activity on weedy rice (unwanted rice; Oryza sativa L.). In a heredity experiment, benzobicyclon tolerance was confirmed to be a semidominant trait conferred by HIS1 based on the intermediate response (ED50 values) of HIS1 heterozygous F1 plants. The spatial–temporal expression of HIS1 was next studied in tissue types (blades, sheaths, and whorls) across tolerant cultivars (‘Roy J’, ‘Diamond’, ‘LaKast’, ‘CLXL745’, and ‘XL753’) and growth stages (2- to 3- compared with 5- to 6-leaf). The relative expression of HIS1 was tissue specific and highest in whorls, followed by blades and then sheaths. Minimal differences in expression across cultivars and growth stages were observed. Furthermore, HIS1 was not largely upregulated at 6 h after benzobicyclon treatment. In the same experiment, cultivar tolerance to benzobicyclon at the label rate of 371 g ha−1 was found to be growth stage dependent. Plant growth was reduced by ∼35% when rice plants were at the 2- to 3- compared with 5- to 6-leaf growth stages. These results show that differences in benzobicyclon tolerance among HIS1 homozygous cultivars is likely not directly correlated with the expression of HIS1. In this research a model was proposed and supported by a field proof of concept study, indicating benzobicyclon efficacy on weedy rice is a function of HIS1 zygosity by growth stage at application. Prior research indicates HIS1 is the dominant allele in weedy rice accessions in Arkansas, and thus, based on our model, benzobicyclon should be applied to weedy rice with ≤2 leaves for suppression.