There’s no doubt that herbicide resistance is a problem. It damages profits and the environment, but how does it happen? In a recent article in AoB PLANTS, Ghanizadeh and Harrington suggest that we could get a better understanding of herbicide resistance in weeds by investigating the evolutionary physiology involved. They list three ways they think plant physiology can contribute to understanding herbicide resistance.
1. Interactions among alleles during evolution
Herbicides kill plants, usually. What happens when they don’t? Ghanizadeh and Harrington examine evidence of sub-lethal doses and find that, for plants, what doesn’t kill them can make them stronger. Killing some of the plants removes some alleles from the gene pool and allows for ‘allele stacking’, where certain variants of genes become much more frequent in a population. When you combine certain alleles, they interact with alleles of other genes better than others. It’s possible that it’s then a combination of genes gives resistance
The example Ghanizadeh and Harrington use is work on Lolium rigidum, where two genes contribute to the herbicide resistance. They note we should be looking at how these genes are selected for, when they’re not in a plant with the key allele of the other gene.
They note that ‘minor’ genes may play a role in this. It’s not just the genetic ingredients of the plant that need studying but also these interactions.
2. Epigenetic Factors
Applying herbicide to weeds might seem like a textbook case for epigenetics, but Ghanizadeh and Harrington state that we don’t really have much idea of how epigenetics affect the evolution of herbicide resistance in weeds.
Epigenetic mechanisms work when a plant is put under stress, like having some poison dumped on it. This changes the expression of genes. The authors note that repeated applications of herbicide are recurrent selection events, so there’s an ongoing pressure on the plant to select for certain factors.
Ghanizadeh and Harrington look into how RNA-sequencing is highlighting variation in gene-expression and also at how epigenetic factors might alter proteins in plants. This in turn, can alter how an enzyme works, changing how a plant reacts to stress.
Epigenetics is an important topic because this is one situation where evolution can be almost Lamarckian. Stressing the parent plant can affect the genetic make-up of the offspring. The authors argue that we should be examining if this indeed is what is happening and if herbicides are causing trans-generational effects. If they are, are these changes irreversible?
3. Fitness and evolutionary physiology
This might seem an obvious target, evolution has been described as ‘survival of the fittest’. There are costs in being ‘fit’ for herbicides. Ghanizadeh and Harrington suggest you could look at the costs of maintaining this fitness. When there are no environmental pressures, is herbicide resistance all cost with no benefit? However, they also argue while it’s possible to look at plants that have evolved herbicide resistance, there’s also a need to examine the evolutionary process by investigating herbicide resistance as it is evolving. What are the steps that happen to get a plant to a resistant form? Are there intermediary steps?
Here Ghanizadeh and Harrington argue for looking at correlations and connections between the phenotype and physiological fitness. In particular, they’re interesting in phenotypic variation. The alleles that work on physiology could interact with the phenotype, so a combination of a certain physical form of a plant and its physiology could interact to produce increased fitness.
To understand this kind of interaction, it will be necessary to understand the variation in a population before the selective pressure of herbicide application takes effect. Ghanizadeh and Harrington point out that the ability of plants to select alleles for herbicide resistance is going to depend on the frequency of those alleles in the population before the herbicide application starts.
Ghanizadeh, H., & Harrington, K. C. (2017). Perspectives on non-target site mechanisms of herbicide resistance in weedy plant species using evolutionary physiology. AoB PLANTS, 9(5). https://doi.org/10.1093/aobpla/plx035