Herbicide metabolic resistance gene discovery in Lolium rigidum using RNA-Seq

Rapid and widespread evolution of multiple herbicide resistance in economically damaging global weed species endowed by increased capacity to metabolize (degrade) herbicides (metabolic resistance) is a great threat to herbicide sustainability and global food production. Metabolic resistance in the cross-pollinated weed species Lolium rigidum is well known but a molecular understanding has been lacking. We purified a metabolic diclofop-methyl and chlorsulfuron resistant (R) subset from a field evolved resistant L. rigidum population (SLR31). The R, herbicide susceptible (S) and derived F2 populations were used in this study for candidate P450 gene discovery by RNA sequencing (RNA-Seq). A P450 gene CYP81A10v7 was identified with consistently higher expression in R vs. S plants. Transgenic rice overexpressing CYP81A10v7 gene were produced and found to be highly resistant to ACCase-inhibiting herbicides diclofop-methyl and tralkoxydim and the ALS-inhibiting herbicide chlorsulfuron, and moderately resistant to the HPPD-inhibiting herbicide mesotrione, PSII-inhibiting herbicides atrazine and chlorotoluron and the tubulin-inhibiting herbicide trifluralin. This herbicide cross-resistance profile in CYP81A10v7 transgenic rice generally reflects that evident in the R L rigidum subset containing CYP81A10v7. Furthermore, transgenic rice overexpressing the CYP81A10v7 gene displayed enhanced rates of diclofop-methyl and mesotrione metabolism. This is the first characterization of a single P450 gene in a weed species conferring cross-resistance to herbicides of at least five modes of action across seven herbicide chemistries.