Australian Herbicide Resistance Initiative (AHRI) researchers have identified a gene in annual ryegrass responsible for conferring resistance to seven different herbicide chemistries from five modes of action.

AHRI director and University of Western Australia (UWA) Professor Hugh Beckie says this is the first indentification in the world of the single gene, CYP81A10v7, in the cytochrome P450 mono-oxygenase enzyme family that endows metabolic herbicide resistance in annual ryegrass.

“The study paves the way for future discoveries of similar genes endowing metabolic resistance in this economically important weed,” Professor Beckie says.

“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.”

Have any weed escapes tested to develop an effective weeds management strategy. Photo: Nicole Baxter

Metabolic resistance occurs when a plant has the capacity to detoxify a herbicide before it ever reaches its intended site of action. Weeds with metabolic resistance have been commonly found to be resistant to herbicides that have never been applied.

Target site resistance occurs when there has been a modification of a herbicide’s site of action in the plant that prevents the herbicide from effectively binding to it. Less is known about metabolic resistance compared to target site resistance.

To delay the onset of metabolic herbicide resistance, Dr Beckie says it is critical for growers to use the widest-possible diversity of effective herbicides from different herbicide groups in rotations or mixtures.

“It is also important to know your enemy. That is, to have any surviving annual ryegrass, or other troublesome weeds, tested for resistance to determine which herbicides or mixtures will work,” he says.

“The genes conferring metabolic resistance already exist in numerous populations of ryegrass collected from across WA, 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 UWA, says the discovery of the gene comes more than 35 years 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,” Dr Yu says.

“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, she says, a lower herbicide rate allows weeds with the capacity to detoxify herbicides to survive and evolve metabolic resistance. As a consequence, the advice is not to cut herbicide rates.

Dr Yu says growers need to take care when rotating and mixing metabolisable herbicides.

“Generally, selective herbicides are metabolisable, whereas knockdown herbicides cannot be easily metabolised,” she says. “If we know the majority of the resistance genes, we can make good recommendations for herbicide mixtures and rotations.”

Another suggestion, Dr Yu says, is to include pre-emergent herbicides to manage annual ryegrass, because the annual ryegrass populations involved in the study were found to develop cross resistance to the most commonly used post-emergence herbicides.

“The development of resistance to pre-emergence herbicides is relatively slow, because genes that can metabolise pre-emergent herbicides are rare compared to the genes that metabolise post-emergence herbicides,” she says.

“Eventually resistance will appear, so you really have to use a diverse range of weed management methods including non-herbicide tactics to delay resistance evolution.”

Dr Yu says AHRI scientist and UWA research fellow Dr Heping Han discovered the CYP81A10v7 gene responsible for conferring cross resistance in annual ryegrass.

Dr Han’s work is part of an Australian Research Council linkage project, in collaboration with Bayer Crop Science. This project also has GRDC investment.

Finding the gene involved:

• first isolating it from the South Australian annual ryegrass population SLR31;
• validating it in some WA annual ryegrass populations;
• confirming the herbicide metabolism with radioactive-labelled assays;
• analysing the target-site DNA sequence to exclude target-site resistance;
• identifying candidate P450 genes and validating their expression levels in resistant and susceptible ryegrass populations; and
• transferring the genes into rice to study the cross-resistance patterns.

The work showed:

• 79 per cent of the resistant ryegrass populations evolved metabolic herbicide resistance;
• 91 per cent of the populations also contained plants with target-site resistance mutations;
• 70 per cent of the populations exhibited both metabolic resistance and target-site mutations; and
• one P450 CYP81A10v7 gene was identified from a resistant population conferring resistance to diclofop-methyl, chlorsulfuron, tralkoxydim, mesotrione, atrazine, chlorotoluron and trifluralin.

Dr Yu says there are other P450 genes within annual ryegrass populations that can confer resistance to herbicides from multiple groups.

“Our next steps are to identify more P450 and other metabolic genes in herbicide resistant annual ryegrass, and other weed species, and to understand how they are regulated,” she says.

“We are also keen to identify if there are any chemical inhibitors that can be applied to selectively reduce the activity of the P450 metabolic enzymes in weeds without affecting crops.”

In the meantime, Dr Han says an effective way to delay metabolic resistance evolution in annual ryegrass is to ensure herbicides are applied at recommended rates.

“It is very challenging to manage an annual ryegrass population that contains this gene. Ensure annual ryegrass weed escapes are tested so herbicides can be chosen carefully and correctly,” he says.

“If metabolic resistance has been found in an annual ryegrass population on your farm, it is critical to use non-herbicide tactics such as harvest weed seed control for management.”

The Australian Herbicide Resistance Initiative is a GRDC investment with a mission to research, develop and communicate innovative ways to manage herbicide resistant weeds with science and technology.

Learn more about the gene discovery.

More information: Hugh Beckie, 08 6488 4615, hugh.beckie@uwa.edu.au; Qin Yu, 08 6488 7041, qin.yu@uwa.edu.au; Heping Han, 08 6488 7980, heping.han@uwa.edu.au