A single cytochrome P450 gene from annual ryegrass confers resistance to a broad spectrum of herbicides

Key messages

• A single P450 gene is responsible for resistance to multiple herbicides, including existing (e.g. trifluralin, prosulfocarb, S-metolachlor) and newly registered (bixlozone) pre-emergence herbicides.
• The robustness of herbicide rotations and mixtures involving these herbicides will be compromised in plants with higher expression of this P450 gene and when sublethal herbicide rates are used.
• Cutting rates is risky, and non-chemical weed control tactics need to be incorporated to delay and mitigate herbicide resistance evolution.

Aims

We have recently identified a major P450 gene (CYP81A10v7) from multiple herbicide resistant annual ryegrass and generated transgenic rice plants overexpressing this gene (Han et al 2021). This current study is to further screen CYP81A19v7 transgenic rice with pre-emergence (PRE) herbicides to determine cross-resistance patterns by this specific P450 gene.

Introduction

Evolution of multiple herbicide resistance in annual ryegrass populations due to enhanced metabolic capacity to detoxify herbicides (metabolic resistance) is a major concern for sustainability of herbicide technology and thus grain production (Yu and Powles 2014, Han et al 2016). Metabolic resistance can extend to a wide range of herbicides and even to new and yet-to-be-developed herbicides. Identification of major metabolic genes (e.g. P450) and their cross-resistance spectrum (especially to PRE herbicides) is important for judicious use of herbicide rotations and mixtures to delay and better manage resistance evolution.

Method

Plant materials

Transgenic rice lines overexpressing the CYP81A10V7 gene, and the green florescence protein (GFP) control were generated (Han et al 2021). Seeds were germinated on 0.6% agar for three to four days in a 20°C growth room, and germinating seeds (with visible radicles) were transplanted to 180mm plastic pots containing potting soil (50% peat moss, 25% sand and 25% pine bark) with 20 seeds per pot.

Herbicide screening

The 3-4 leaf stage rice seedlings were treated with commercial formulations of selected PRE herbicides (Table 1) in a 117L/ha spray volume delivered at 200kPa with a cabinet sprayer equipped with two flat-fan nozzles. Pots were covered with 1cm of soil immediately after treatment, kept in the controlled environment rooms (28/20°C, day/night) and lightly watered. Mortality was determined and photos taken three weeks after treatment for comparison between CYP81A10v7 and GFP plants. The experiment was conducted with two CYP81A10V7 versus one GFP transgenic lines and repeated.

Results

Resistance pattern endowed by CYP81A10V7 to PRE herbicides

Resistance screening by PRE herbicide revealed that CYP81A10v7 transgenic rice were clearly resistant to bixlozone and clomazone (Table 1, Figure 1) at and above the field recommended rates, compared to the transgenic GFP control. In addition, transgenic rice also showed a level of resistance to prosulfocarb, S-metolachlor and trifluralin at lower than recommended herbicide rates (Table 1, Figure 1), but remained susceptible to the other four herbicides tested (Table 1).

Conclusion

This single P450 gene isolated from multiple herbicide-resistant annual ryegrass is a major player for metabolic resistance not only to certain post-emergence herbicides (e.g. ACCase/ALS, HPPD, PSII herbicides) (Han et al 2021) but also some PRE herbicides, especially the newly registered herbicide bixlozone (Overwatch) (this study). Evolution of resistance to these PRE herbicides will occur when plants having higher expression of this gene are selected and enriched, and when sublethal herbicide rates are used. Indeed, recent research has shown rapid evolution to Overwatch and Command in annual ryegrass populations after two cycles of recurrent selection with the field rate of Overwatch (Brunton et al 2021). Therefore, using full herbicide label rates, judicious herbicide mixtures and rotations, and implementing integrated weed control strategies is highly recommended to delay and minimise resistance occurrence.

References

Yu Q, Powles SB (2014) Metabolism-based herbicide resistance and cross-resistance in crop weeds: A threat to herbicide sustainability and global crop production. Plant Physiology 166: 1106-1118

Han H, Yu Q, Owen M, Cawthray G, Powles S (2016) Widespread occurrence of both metabolic and target-site herbicide resistance mechanisms in Lolium rigidum populations. Pest Management Science 72: 255-263

Han HP, Yu Q, Beffa R, Gonzalez S, Maiwaid F, Wang J, Powles SB (2021).  Cytochrome P450 CYP81A10v7 in Lolium rigidum confers metabolic resistance to herbicides across at least five modes of action. The Plant Journal 105: 79-92

Brunton DJ, Gill G, Preston C (2021). Resistance to bixlozone and clomazone in cross-resistant rigid ryegrass (Lolium rigidum) populations from southern Australia. Weed Science 69: 284-289

Acknowledgments

This work was financially supported by the Australian Research Council (ARC) linkage grant LP140100909.The research undertaken as part of this project is made possible by the significant support of the GRDC; the author would like to thank them for their continued support.

Contact details

Qin Yu
UWA
35 Stirling Hwy, Crawley, WA 6009
Ph: 6488 7041
Email: qin.yu@uwa.edu.au