Research - Satoshi Iwakami
The history of crop cultivation is also the history of the battle between humans and weeds.
Humans have developed new technologies one after another in an effort to eliminate weeds from agricultural land. Weeds, on the other hand, have “evolved” traits to get around such attacks. Mimicry to crops, morphology that withstands weeding pressure, germination characteristics that shift the timing of germination to prevent rounding up, and so on.
Herbicides are the most powerful technology ever created by the human side. By cleverly exploiting small genetic differences between crops and weeds, they are designed to be lethal only to weeds. However, weeds have evolved resistance to this extremely powerful stress in the same way, or rather in a shorter period of time, than they had before.
I hope to decipher the battle between humans and weeds that has been waged in agricultural fields through the analysis of crop and weed genomes and genes.
Metabolism-based Super Weeds
The evolution of “multiple herbicide-resistant” weeds is a significant issue in the realm of herbicide resistance. Multiple resistance can occur by accumulating resistance mechanisms against various herbicides, although this evolutionary process typically requires a considerable amount of time. However, there are known cases where resistance to multiple herbicides can be acquired through a single evolutionary event. This phenomenon is known as metabolism-based resistance. It has been observed that weeds that acquire mechanisms to metabolize (detoxify) a specific herbicide often exhibit resistance to completely different herbicides as well. However, the specific details of this phenomenon were not well understood.
Our research has focused on analyzing the resistance mechanisms of a grass weed called late watergarss (Echinochloa phyllopogon), discovered in the late 1990s in California, USA. This particular late watergarss has been found to exhibit resistance not only to herbicides that were being used in California at the time but also to newly introduced herbicides with distinct chemical structures and modes of action.
Through biochemical experiments, we have deduced that the oxidative enzyme cytochrome P450 is involved in resistance. However, due to the extensive number of P450 genes and the lack of genomic information in this weed, pinpointing the specific gene responsible for resistance has proven to be a challenging task. Although several weeds with similar resistance mechanisms were reported, the identification of genes associated with resistance remained elusive.
Nevertheless, we have achieved a significant breakthrough by successfully identifying the cytochrome P450 genes associated with resistance. In resistant late watergarss, we have discovered the activation of two specific cytochrome P450 enzymes, namely CYP81A12 and CYP81A21. These enzymes have demonstrated the ability to metabolize a diverse range of herbicides.
While plant P450 enzymes typically exhibit a relatively high substrate specificity, these identified P450s have shown the capacity to deactivate herbicides with distinct chemical structures. Furthermore, our research has also revealed the presence of multiple CYP81A genes in the late watergarss genome, suggesting the possibility of evolutionary adaptations involving these genes.
Currently, our research efforts are focused on elucidating the regulatory mechanisms governing the expression of these identified P450 enzymes, as well as investigating the general applicability of this resistance mechanism.
Related Works
- Iwakami et al. 2014 Plant Physiology
- Iwakami et al. 2019 New Phytologist
- Guo et al. 2019 Plant Science
- Dimaano et al. 2020 Plant Molecular Biology
- Suda et al. 2023 Plant Physiology
Convergence of resistance evolution
Convergent evolution refers to the phenomenon where distinct populations/species with different genetic lineages independently evolve similar traits. A notable example of convergent evolution is the evolution of herbicide resistance in weeds. Many resistance cases arise from structural changes in the target proteins of herbicides caused by an amino acid replacement. The presence of identical mutations in the same genes across multiple species highlights the role of gene-level convergence (molecular convergence) in resistance evolution. Essentially, this implies that the genetic factors driving evolution are limited. Building upon this knowledge, advancements have been made in genetic diagnostic techniques for resistance detection and the development of herbicides that effectively combat resistant weeds (i.e., herbicides with resistance management strategies).
Related Works
- Iwakami et al. 2020 Pesticide Biochemistry and Physiology
- Tanigaki et al. 2021 New Phytologist プレスリリース
Crops are tolerant to herbicides！
While weed resistance to herbicides poses a significant problem, there are also numerous agricultural fields where weed control has been successfully achieved through herbicide application. The key to effective weed control with herbicides lies in their selectivity. Many herbicides used in agricultural fields are designed to kill weeds while having minimal impact on crop growth. Even though crops and weeds may belong to the same plant species, herbicides exploit the differences between crops and weeds to achieve selectivity. However, in many cases, the underlying differences remain poorly understood.
Our research aims to unravel the genetic-level differences between crops and weeds that are exploited by selective herbicides. By deciphering these distinctions, we strive to enhance our understanding of the selectivity mechanisms employed by herbicides and potentially develop improved selective herbicides in the future.
Related Works
- Saika et al. 2014 Plant Physiology
- Guo et al. 2021 Pest Management Science
- Ha et al. 2022 Pest Management Science
