A Val-202-Phe alpha-tubulin mutation and enhanced metabolism confer dinitroaniline resistance in a single Lolium rigidum population

The research by AHRI PhD student Jinyi Chen (now Dr Jinyi Chen!) documents both target site and non-target site resistance to dinitroaniline herbicides in Lolium rigidum (annual ryegrass) individuals. We are long accustomed to documenting multiple mechanisms of resistance existing within L. rigidum individuals/populations because of the high genetic diversity and obligate cross-pollination between L. rigidum, which enables multiple resistance mechanisms to be a frequent occurrence. In this particular L. rigidum population, resistance to dinitroaniline herbicides is due to a mutation in the alpha-tubulin gene (Val-202-Phe) and non-target site enhanced rates of dinitroaniline herbicide trifluralin metabolism. Unequivocal evidence for the target site Val-202-Phe mutation endowing resistance was obtained by expressing in transgenic rice, where it endowed resistance.

Background

A Lolium rigidum population collected from Western Australia was previously reported as highly resistant to dinitroaniline herbicides mainly due to a Val-202-Phe substitution in the target site 𝜶-tubulin protein. To further determine the contribution of the 202 mutation to resistance, two sub-populations, respectively comprising the 202 mutant and wild-type (WT) individuals, were isolated from within the same resistant population and subject to dinitroaniline herbicide doses. A rice transgenic study was conducted to demonstrate whether the amino acid substitution at the 202 residue confers resistance.

In addition, as indicated in the phenotyping and genotyping study, non-target enhanced trifluralin metabolism was further examined in the same population.

Results

The 202 mutants were more resistant than the wild-type plants. Rice calli transformed with the L. rigidum mutant 𝜶-tubulin gene (Val-202-Phe) were more resistant to dinitroaniline herbicides relative to calli transformed with the wild-type gene. Also, enhanced trifluralin metabolism was detected in the 202 mutants in comparison to the susceptible seedlings.

Conclusion

Both target-site Val-202-Phe 𝜶-tubulin mutation and non-target-site enhanced trifluralin metabolism co-exist in this dinitroaniline-resistant L. rigidum population.