The molecular secrets of Palmer Amaranth reveal a disturbing potential

Producers of corn, soybeans and cotton quiver with the idea that Palmer's amaranth invades their fields. The aggressive cousin of waterhemp – himself a formidable opponent – grows extremely fast, produces hundreds of thousands of seeds per plant and resists multiple classes of herbicides, including glyphosate.

Palmer's resistance to PPO inhibitor herbicides, a group of chemicals that disrupt chlorophyll synthesis, is particularly problematic because glyphosate is absent. Farmers turned to OPP inhibitors as an effective alternative, until resistance was discovered in waterhemp in 2001 and Palmer in 2011.

Pat Tranel of the University of Illinois has been working for years to understand the mechanisms of resistance to OPP inhibitors and was the first to discover key mutations in both species of weeds. Now, in two new studies, he goes further to explain the evil genius of Palmer.

"We knew that Palmer had the same molecular mechanism as waterhemp to resist PPO inhibitors, a genetic mutation known as the gly-210 deletion, and at least one more.We now know that the deletion gly-210 has been developed independently, hybridization with waterhemp, "says Tranel, associate director and professor of molecular weed science at the Department of Plant Science at the University of Italy.

This is important in two ways. It's good news that scientists find no evidence of hybridization between the two super-weeds, at least not so far. But the fact that Palmer developed the same mutation independently, and at least one more, shows how cunning the weed is.

Tranel and his team determined the evolutionary origins of the gly-210 mutation by examining the genetics of resistant plants from both species that grew together in a Kentucky field. Being close to each other for several years should have offered a possibility of hybridization, should that happen.

"Laboratory experiments have taught us that they are able to hybridize, and the fact that it does not happen on the ground is a good thing, the more they can and do it, the more we would have." problems, "says Tranel.

Only about one-third of Palmer plants in Kentucky had gly-210 suppression. The others used a different mutation – a substitution with arginine – to avoid the damage caused by the PPO inhibitor.

"The discovery that this population of Palmer has two different mutations is a concern, because if you plan for the future, Palmer is well positioned to manage the OPP's future chemicals – it can use the one that is most effective against a new PPO.

"It is also well placed to combine the two mutations to create a double mutant, with both mutations on the same copy of the chromosome." Chemistry designed to kill plants with gly-210 suppression will not kill double mutants "said Tranel. said. "In my opinion, it's only a matter of time before we see double mutants on the ground."

Tranel 's second new study explains why Palmer' s asbestos took a decade longer than waterhemp to develop the deletion of gly – 210, and reveals another devilish truth about the species: l & # 39; Palmer amaranth appears to be naturally tolerant to the application of PPO inhibitors in post-emergence.

It has long been recognized that the timing of post-emergence PPO application is particularly critical for Palmer amaranth, compared to aquatic hemp. If the Palmer plants are not sprayed before reaching about 4 inches, everything is over.

"If you wait too long, you miss them – and too long can be a bargain one day because Palmer is growing so fast.This can go to a 4 inch factory where you could control it at a factory. 6 inches literally in a day, "says Tranel.

For Tranel, the scheme suggests natural tolerance to PPO inhibitors post-emergence. Tolerance describes the ability of a species to handle a substance, in this case PPO herbicides. Resistance, on the other hand, occurs at the level of the population; Localized populations of the species develop mutations in response to repeated exposure to the substance. For example, corn is tolerant to atrazine. It can be spray treated and does not require mutation to treat in a given population.

The idea is that Palmer amaranth has a natural tolerance to PPO inhibitors and does not need to develop resistance. That's why the gly-210 mutation took longer. But, until now, no one had previously studied Palmer's tolerance to chemistry.

Tranel confirmed it by growing Palmer and Waterhemp plants side by side with and without the gly-210 mutation and applying different formulations of PPO inhibitors in pre-emergence and post-emergence. Post-emergence applications were made early (less than 4 inches) or late (more than 4 inches).

"We found that Palmer's 'palpable' plants without the mutation survive as well as the resistant waterhemp after post-emergence spraying," says Tranel.

On the other hand, the research team found that pre-emergence formulations effectively controlled both species.

"The difference in tolerance between Palmer and waterhemp disappears at the pre-emergence stage," says Tranel. "In the end, it's the message to remember here." If you deal with these weeds, especially Palmer Amaranth, and want to incorporate an OPP inhibitor as an effective mode of action, you you'll have a lot more luck if you use it in a pre-emergence app. "