February 14, 2023
Written by: Peter Newman
Express yourself.
“Be who you are and say what you feel, because those who mind don’t matter, and those who matter don’t mind.” Dr Seuss
The enzyme family Aldo-keto reductase is a great fan of Dr. Seuss and this family lives by this quote, never afraid to express themselves, which is now to the downfall of our most precious herbicide, glyphosate, on our most important weed, annual ryegrass.
Back in 2019, we reported on a population of Awnless Barnyard grass that turned out to be the world’s first case of metabolism-based resistance to glyphosate caused by a super family of enzymes, aldo-keto reductase (AKR).
Now our AHRI researchers have confirmed a similar mechanism exists in annual ryegrass. The interesting twist in the story is that the gene for the enzyme is higher expressed when the herbicide is applied to the plant. In other words, the dimmer switch is turned up a bit, and it’s the application of glyphosate that causes this dimmer switch to turn up.
The second twist is that applying 2,4-D to the plant also turns up the dimmer switch on the AKR enzyme which then in turn may increase the tolerance to glyphosate. This may partially explain why 2,4-D is known to antagonise glyphosate.
The research was led by visiting Chinese researcher Feng-Yan Zhou with help from Heping Han, Qin Yu and others from AHRI.
How does it work, and should we consider avoiding mixing glyphosate and 2,4-D?
Metabolism of herbicides means that the plant breaks the herbicide molecule into little pieces.
Until recently, there has been no evidence of glyphosate being metabolised in plants. There are several other known glyphosate resistance mechanisms including gene amplification, reduced movement, and several target site mutations, but the discovery of AKR breaking down glyphosate in Barnyard grass was the first example of metabolism.
The ryegrass
Three populations of annual ryegrass (Lolium rigidum) from Western Australia were involved in this study. They had already been confirmed resistant to glyphosate due to unknown non-target-site based mechanisms.
Applying herbicide increases the gene expression
PCR testing found that the basal level of expression of AKR was similar for the susceptible and resistant populations in plants that were growing with no herbicide being applied to them.
But, applying glyphosate led to a 4.3-fold increase in expression of the enzymes, and applying 2,4-D led to a 3.4-fold increase.
This is pretty unique in the world of herbicide resistance research. It’s not normal of the herbicide to cause changes in the plant that lead to weeds surviving.
Bacteria and Rice testing
The researchers also tested some E. coli bacteria and some rice that was over expressing the AKR gene and found they too were more tolerant to glyphosate.
Two genes tested
Transgenic rice that was over expressing the LrAKR1 and the LrAKR4C10 genes were tested, each resulting in similar levels of glyphosate resistance compared to the control (GFP).
Picture of transgenic rice sprayed with different rates of glyphosate. GFP = control. LrAKR1 and 4C10 is rice with the over expressed AKR gene.
Metabolism
How does it all work? In simple terms, the glyphosate molecule is broken in half by the AKR enzyme. The bits that it’s broken into are called the metabolites. One of these, AMPA, still has some herbicidal activity, so the plant has more work to do to break down AMPA before it’s in the clear.
Summary
This is relatively low level resistance, but this mechanism may occur in conjunction with other mechanisms leading to significant levels of resistance. It is very interesting that 2,4-D also causes over expression of the AKR gene. We know that 2,4-D doesn’t kill annual ryegrass, but applying it can increase the expression of an enzyme that can break down glyphosate. Perhaps we should look to alternatives other than 2,4-D to mix with glyphosate when annual ryegrass is the main target weed.
Dr Seuss would say, “Don’t cry because it’s over. Smile because it happened”. Glyphosate isn’t over, and we’re very glad that glyphosate happened, but sorry Dr Seuss, we’re not smiling about glyphosate resistance, it’s one of the biggest resistance threats that we have.
Paper
Posted in: AHRI Insight, Gene discovery for herbicide resistance