Australian Herbicide Resistance Initiative (AHRI) researchers have identified a key gene in annual ryegrass responsible for the weed’s resistance to seven different herbicide chemistries from five modes-of-action.
Annual ryegrass is considered one of the major weeds in Australian broadacre cropping systems and populations are commonly resistant to one or more herbicides.
The discovery of this gene – more than 35 years after researchers first found this type of herbicide resistance, will assist growers and agronomists identify the most effective herbicide options for use on-farm.
AHRI director and The University of Western Australia (UWA) Professor Hugh Beckie said this was the first identification in the world of a single gene, CYP81A10v7 in ryegrass, which is responsible for metabolic resistance to herbicides across several different modes-of-action.
AHRI scientist and UWA research fellow Heping Han made the discovery of gene CYP81A10v7, as part of an Australian Research Council project in collaboration with Bayer Crop Science and with investment from the Grains Research and Development Corporation (GRDC).
Professor Beckie said the ground-breaking research paved the way for future discoveries of similar genes endowing metabolic resistance in the problematic weed annual ryegrass.
Metabolic resistance occurs when plants internally detoxify the herbicide before it ever reaches its intended site. This mechanism is less studied than target-site resistance. Weeds with metabolic resistance have commonly been found to be resistant to herbicides that they have not even been exposed to.
“One implication of this discovery is the potential to develop a rapid diagnostic marker or group of markers for screening other herbicide-resistant annual ryegrass populations found in growers’ paddocks to identify the most-effective herbicide options available,” Professor Beckie said.
He said to delay the onset of metabolic herbicide resistance, growers needed to use a wide range of herbicides from different groups in rotations or mixtures.
“It is also important to know your enemy, which means testing any surviving annual ryegrass, or other troublesome weeds, for resistance to determine which herbicides or mixtures work,” Professor Beckie said.
“The genes conferring metabolic resistance already exist in a number of populations of ryegrass collected from across Western Australia, so using a diverse range of effective herbicides will, in general, help minimise the selection pressure for resistance evolution.”
Project leader Dr Qin Yu, a principal research fellow at the UWA, said the discovery of the gene comes more than three decades after researchers first found this type of herbicide resistance.
“We now have hard evidence that a single P450 gene can confer resistance to herbicides of dissimilar modes of action or chemistries, and even those that were not applied or not yet on the market,” she said.
“Previous AHRI research demonstrated that metabolic herbicide resistance can arise rapidly in annual ryegrass when herbicides are used at low rates.
“Growers may not intentionally cut their rates, but lower effective rates can result if herbicides are applied in unfavourable conditions, such as windy or hot weather, or even if weeds are too big when they are sprayed.”
Accordingly, Dr Yu said, a lower herbicide rate allowed weeds with the capacity to detoxify herbicides to survive and evolve metabolic resistance, so as a consequence, the advice was not to reduce herbicide rates.
“If we know the majority of the resistance genes, we can make good recommendations for herbicide mixtures and rotations,” she said.
Finding gene CYP81A10v7 involved selecting more than 30 resistant annual ryegrass populations randomly across Australia, confirming the herbicide metabolism, analysing DNA sequences, identifying P450 genes and transferring the genes into the model plant rice to study the cross-resistance patterns.
The Australian Herbicide Resistance Initiative is a GRDC initiative whose mission is to research, develop and communicate innovative herbicide resistant weed science and technology.
Source: Grains Research & Development Corporation
