ASHS 2025 Conference

Herbicide tolerance in plants is an increasingly valuable trait due to the high labor and costs associated with weed control in agriculture. Herbicide application remains the most effective and widely used weed management strategy, making the development of tolerant plants essential. First discovered in the 1970s and commercially grown since 1984, herbicide resistant crops have become a key tool in agriculture, with increasing demand for new tolerant varieties. Chemical mutagenesis and CRISPR-mediated gene editing have been used to induce mutations and develop herbicide tolerant plants. Chemical mutagenesis involves treating plant tissue with mutagens such as ethyl methanesulfonate (EMS) to induce random mutations, followed by screening to identify tolerant mutants. This conventional approach has played a significant role in breeding programs and remains widely used for developing herbicide tolerant crops. EMS mutagenesis has successfully generated ALS-resistant varieties in several agronomic crops, including Clearfield® maize, rice, and wheat, which are resistant to imidazolinone (IMI) herbicides without being classified as genetically modified (GM). It is particularly effective for developing crops resistant to ALS- and ACCase-inhibiting herbicides, as these mutations typically require only minor changes in the target genes. CRISPR-mediated gene editing, using tools such as CRISPR-Cas9, base editing (CBE, ABE), and prime editing, enables precise modifications in plant genomes to confer herbicide tolerance. These advancements have revolutionized crop development through their efficiency, precision, and cost-effectiveness. By targeting herbicide receptor genes such as ALS, ACCase, and EPSPS, CRISPR-based systems have produced herbicide tolerant varieties in several agronomic crops. CRISPR is particularly valuable for engineering tolerance to non-selective herbicides, such as glyphosate, due to the complex genomic architecture of the EPSPS gene. Chemical mutagenesis facilitates the discovery of novel mutations and is particularly useful in understudied species lacking the genomic information required for CRISPR-based modification. In contrast, CRISPR-based genome editing provides a highly precise and efficient method for developing herbicide tolerant crops, especially when targeting complex genes. Integrating chemical mutagenesis with CRISPR-mediated gene editing expands the range of available herbicide tolerance traits and offers new opportunities for sustainable weed management. These advances in agronomic crops provide a strong foundation for extending herbicide tolerance studies to horticultural and specialty crops, where research has been more limited despite similar weed management issues.