The perception pathway for endogenous auxin has been well described, yet the mode of action of synthetic auxin herbicides, used for >70 years, remains uncharacterized. We utilized transcriptomics and targeted physiological studies to investigate the unknown rapid response to synthetic auxin herbicides in the globally problematic weed species Erigeron canadensis. Synthetic auxin herbicide application consistently and rapidly down-regulated the photosynthetic machinery. At the same time, there was considerable perturbation to the expression of many genes related to phytohormone metabolism and perception. In particular, auxin herbicide application enhanced the expression of the key abscisic acid biosynthetic gene, 9-cis-epoxycarotenoid deoxygenase (NCED). The increase in NCED expression following auxin herbicide application led to a rapid biosynthesis of abscisic acid (ABA). This increase in ABA levels was independent of a loss of cell turgor or an increase in ethylene levels, both proposed triggers for rapid ABA biosynthesis. The levels of ABA in the leaf after auxin herbicide application continued to increase as plants approached death, up to >3-fold higher than in the leaves of plants that were drought stressed. We propose a new model in which synthetic auxin herbicides trigger plant death by the whole-scale, rapid, down-regulation of photosynthetic processes and an increase in ABA levels through up-regulation of NCED expression, independent of ethylene levels or a loss of cell turgor.

Halauxifen-methyl is a new synthetic auxin herbicide for control of broadleaf weeds, including preplant applications for corn (Zea maysL.) or soybean [Glycine max(L.) Merr.]. The objective of this study was to investigate the efficacy of halauxifen-methyl in comparison to the current auxin standards, 2,4-D and dicamba, on glyphosate-resistant (GR) horseweed (Erigeron canadensisL.) at different plant heights. In field experiments, a foliar application of halauxifen-methyl at the recommended use rate of 5 g ae ha−1resulted in 81% control. Dicamba applied at 280 g ae ha−1provided a comparable level of efficacy of 80%, while 2,4-D at 560 g ae ha−1resulted in 49% control. The addition of glyphosate improved GRE. canadensiscontrol with 2,4-D more than with halauxifen-methyl or dicamba, possibly due to the higher level of control observed with halauxifen-methyl or dicamba alone. Even though applied at 50 to 100 times lower application rates, the efficacy of halauxifen-methyl onE. canadensiswas similar to dicamba and greater than 2,4-D. Thus, halauxifen-methyl should be an effective tool for management of GRE. canadensisbefore planting both conventional and herbicide-resistant soybean varieties, and it precludes the extended preplant application interval required for dicamba in some soybean management systems.

Halauxifen-methyl is an auxin herbicide for broadleaf weed control in preplant applications to corn and soybean. Our objective for this research was to characterize the phytotoxicity of halauxifen-methyl on horseweed, relative to 2,4-D and dicamba, in terms of weed height, the response to an auxin synergist, and root activity. The 50% reduction in plant growth (GR50) value for halauxifen-methyl on rosette-sized plants was 0.05 g ae ha−1, 100 times less than the labeled use rate of 5 g ae ha−1, compared with 36 and 31 g ha−1 for 2,4-D and dicamba, respectively. In a whole-plant bioassay, 240 g ae ha−1 of 2,4-D was calculated as the GR50 value on horseweed 20-cm tall, whereas applications of only 53 and 0.40 g ae ha−1 were necessary for dicamba and halauxifen-methyl, respectively, to achieve the same response. As weed size decreased, there was a concomitant reduction in the estimated herbicide dose for the GR50 with similar differences observed between halauxifen-methyl and the other two auxin herbicides. The addition of diflufenzopyr, an auxin synergist, to 2,4-D and dicamba resulted in a synergistic response on horseweed. However, the addition of diflufenzopyr to halauxifen-methyl resulted in an additive or antagonistic effect, depending on rate of diflufenzopyr, demonstrating a distinctive physiological pathway for halauxifen-methyl compared with 2,4-D and dicamba. In the agar-based bioassays, GR50 values for horseweed root length for 2,4-D and dicamba were 0.16 and 0.19 µM, respectively, whereas only 0.004 µM halauxifen-methyl was required for a comparable root response. These results indicate that horseweed exhibits a high level of sensitivity to halauxifen-methyl and suggest the activity of halauxifen-methyl is different from that of 2,4-D and dicamba. These differences in herbicide activity may reflect differential absorption, translocation, metabolism, or targeting of auxin receptors found in horseweed.

The topic of sustainability is popular in mainstream media and a common discussion theme, particularly for the agriculture discipline that serves the entire world. Individuals and corporations often have a desire to be sustainable in their practices, but the commentary on “being sustainable” can be confusing in terms of realistic practices. To define if weed science is sustainable it must first be identified what resource or object is desired to be sustained at a certain level. From a historical perspective, weed control in the United States over the past forty years has revolved around no-tillage row crop acres. The implementation of no-till or reduced till has undeniable benefits in sustaining natural resources, especially two of our most valuable resources: soil and water. While the overall trend toward chemical weed control has been shown to decrease agriculture’s impact on the environment, depending solely on herbicides is not sustainable long term with the rise in herbicide-resistant weed species. We also consider the benefits and challenges associated with agronomic trends within the context of sustainability and expand consideration to include emerging technology aligned to human health and environmental stewardship. The key to improving farming is producing more and safer food, feed, and fiber on less land while reducing adverse environmental effects, and this must be accomplished with the backdrop of human population growth and the desire for an improved standard of living globally. Emerging technologies provide new starting points for sustainable weed management solutions, and the weed science community can initiate the conversation on sustainable practices and share advancements with our colleagues and community members. In addition to broadening the sustainability concept, targeted and relevant communication tools will support the weed science community to have successful and impactful discussions.