HERBICIDE resistance occurs at a genetic, molecular and cellular level in ways that challenge some of the most agile scientific minds.

In following interesting lines of enquiry, scientists working to understand the mechanisms that drive herbicide resistance sometimes come across some unexpected findings.

One such finding is the discovery that an insecticide can reverse metabolic resistance to a herbicide, making the resistant population susceptible to the herbicide once more.

Resistance types

Metabolic resistance is the lesser known cousin to target site resistance in the world of herbicide resistance mechanisms.

Target site resistance is comparatively easy to identify and study, being a more direct ‘cause and effect’ type mechanism that usually confers quite high levels of resistance.

Metabolic resistance however is more complex and more difficult to study due to many internal mechanisms involving secondary enzyme production and activity.

This type of resistance is often moderate however it is also frequently effective across multiple herbicide mode of action groups.

It is not uncommon for plants with metabolic resistance to be resistant to herbicides that they have never been exposed to.

This has a dramatic and limiting effect on herbicide choice and makes herbicide rotation a much less powerful control tactic.

In simple terms, metabolic resistance occurs when the plant uses its metabolic pathways to produce enzymes that ‘protect’ target sites from the applied herbicide molecules.

If the herbicide molecule never reaches the target site then the plant survives.

The same enzyme or multiple enzymes can ‘protect’ multiple target sites, resulting in cross-resistant plants.

Ryegrass resistance

Weed surveys in Western Australia have revealed a high rate of multiple-resistance in annual ryegrass populations with 70 per cent of populations possessing both metabolic and target site resistance to herbicides.

Research into gaining a better understanding of one group of enzymes, known as P450s, has uncovered an unlikely synergism between an insecticide and current pre-emergent herbicides to control ryegrass.

Australian Herbicide Resistance Initiative (AHRI) researcher, Roberto Busi, has shown it is possible to reverse metabolic resistance to trifluralin in annual ryegrass using an organo-phosphate insecticide.

Genetic basis of resistance

In conjunction with Colorado State University researcher, Todd Gaines, Dr Busi is working to better understand the genetic basis of metabolic resistance and how this knowledge can be used to control metabolic resistant weeds.

“There are just five types of pre-emergent herbicides, utilising only two modes of action, with no new modes of action in the pipeline,” Dr Busi said.

“The most recent pre-emergent product, pyroxasulfone (Sakura), was commercialised in 2012 yet even before it was brought to market, research had shown its mode of action can be ‘broken’ within just three generations using low application rates to result in 10-fold resistance.”

This means it is very important to find ways to keep current herbicides effective rather than just looking for new modes of action.

Inhibiting enzymes

In the case of trifluralin-resistant annual ryegrass, Dr Busi’s research demonstrated that inhibiting the production of P450 enzymes was the key to reversing resistance to this useful pre-emergent herbicide.

“Inhibiting the production of P450 enzymes requires the suppression of different genes in the plant that are responsible for regulating production of the enzymes at different stages of the plant’s development,” he said.

“In ryegrass there are probably several different P450 enzymes that are active during the plant’s development that are offering protection against the herbicide, so there is a high level of complexity involved in trying to manipulate the genes responsible for herbicide resistance.

“Using the insecticide phorate, applied in granular form to the soil immediately before spraying with trifluralin, we were able to prevent establishment of plants with known resistance to trifluralin.

“But the effect was not as clear for plants that were resistant to Sakura. Phorate is not the solution to metabolic resistance but this proof-of-concept research confirms that it is possible to manipulate and even reverse metabolic resistance with the use of existing pesticides.”

Phorate is not currently registered for use in any crop except cotton and the described use is not permitted in the field.

Phorate is highly toxic and it was used under carefully controlled laboratory conditions for these experiments.

Chemical inhibitors

It is not desirable to turn off P450 production in a crop so chemicals that inhibit P450 production are best suited to use with pre-emergent herbicides.

Current research is investigating ways to design better P450 inhibitor mechanisms using gene technology and to use these mechanisms in future crop breeding programs to confer crop tolerance to certain herbicides.

Further experiments showed that ryegrass plants with metabolic resistance to Sakura use another metabolic pathway involving enzymes known as GST.

In a similar manner, these mechanisms can probably confer cross-resistance for pyroxasulfone (Sakura), prosulfocarb (Boxer Gold) and triallate.

To date, the fifth pre-emergent active ingredient, propyzamide, does not seem prone to metabolic resistance.

“For now, our best advice to growers and agronomists is to rotate between these three groups of pre-emergent herbicides – 1. trifluralin, 2. Sakura, Boxer Gold and triallate and 3. propyzamide – and we are researching the potential benefits of mixing herbicides from these three groups as a means of delaying metabolic resistance. As always, full label rates must be applied,” Dr Busi said.

Source: Weedsmart, www.weedsmart.org.au