IN recent research conducted by the Australian Herbicide Resistance Initiative (AHRI), the same glufosinate resistance mechanism appeared in China and Malaysia at the same time, which could mean a common target-site resistance mechanism to the herbicide.
Glufosinate (Liberty or Basta) is known as the other knockdown herbicide and while not a lot of it is used in Australia at present, growers are starting to with the release of Liberty Link crops which can tolerate the herbicide.
Globally, glufosinate has been used for 30 years and its use has been increasing in tolerant crops and as a knockdown herbicide in countries where paraquat has been banned, with low levels of resistance to this herbicide.
AHRI researchers Qin Yu and Heping Han recently teamed up with some visiting Chinese researchers and found the same single point, target-site mutation, Ser59Gly in resistant Eleusine indica (Crowsfoot grass) populations from Malaysia and China.
While the plants had relatively low-level resistance, the mutation evolved independently in different countries which may represent a common target-site resistance mechanism to glufosinate.
From an Australian perspective, AHRI researchers once again put us in the fortunate position of understanding resistance to a herbicide before its widespread use in the country.
AHRI content writer Peter Newman said when plants metabolise nitrogen, they join Ammonia onto glutamate to form glutamine.
"The enzyme glutamine synthase is a critical part of this reaction," Mr Newman said.
"The herbicide glufosinate works like most other herbicides, it binds to an enzyme, and in this case the enzyme is glutamine synthase.
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"When this reaction stops, Ammonia builds up to toxic levels and the plant dies."
The Ser59Gly rotation is a single point mutation on the glutamine synthase enzyme.
While the mutation isn't right in the catalytic centre of the enzyme, as resistance mutations often are, and is instead near the catalytic centre, the mutation is still just enough to indirectly influence the binding of glufosinate.
To confirm the Ser59Gly mutation was the cause of resistance, AHRI researchers used what is now a common lab technique to develop rice which contains the exact mutation using transgenic techniques.
"The glufosinate resistance was only about 2.5 fold," Mr Newman said.
"The dose response curve showed the susceptible (wild type) rice population versus plants with the Ser59Gly mutation and there was a definite shift in dose response, albeit low level resistance.
"However, glufosinate is known to be a product that often requires two sequential applications of product at robust rates in good conditions to kill the weeds, so any shift in dose response is significant."
Glufosinate could soon become a useful herbicide in Australia as growers adopt glufosinate tolerant crops.
With that in mind, it's useful to know that internationally, resistance to this herbicide is rare and when the issue is faced, researchers know where to start looking to determine the mechanism.