The double life of trichomes: understanding their dual role in herbivory and herbicide resistance

External features of organisms often serve as the first line of defense in their immediate environments. Trichomes are hair-like appendages on plant surfaces that can defend against damaging agents such as pathogens, herbivores, and UV radiation. It is currently unknown if the variation observed in trichome traits represents dual or conflicting roles against different types of stressors. Here, we assess whether trichomes serve as an herbicide resistance trait and how it coincides with the conventionally studied defensive strategy of herbivory resistance. In a series of experiments, we exposed the annual invasive velvetleaf ( Abutilon theophrasti ) to glyphosate (active ingredient in the herbicide ‘Roundup’) to investigate whether trichome traits (shape and density) are linked to herbicide resistance and to test whether herbicide alters selection on plant trichomes. We found that an increased proportion of branched trichomes positively impacted herbicide resistance as well as chewing herbivory resistance. We also found evidence that glyphosate imposes positive selection on branched trichomes in velvetleaf. Overall, our results indicate that branched trichomes can contribute to both herbicide and herbivory resistance, serving as a dual structural form of resistance reducing plant injury. If our results are to be applied more broadly, it would suggest that herbicide exposure can alter the composition of plant trichomes, potentially impacting trichome-mediated defenses against various external stressors.