Abstract Amaranthus palmeri is an invasive weed present in Spanish territory. Populations resistant to ALS and EPSPS inhibitors have been described. It is suspected that the accumulation of multiple resistance to both modes of action (MoA) within the same population. In this study we evaluate the herbicide responses in two roadside populations of A.palmeri from Catalonia. In a dose-response essay, most of the plants treated with 540 g a.i./ha of glyphosate survived in both populations, and some individuals even survived the field dose of 1080 g/ha. Single-dose treatments with ALS inhibitors reveal high resistance to thifensulfuron-methyl (6 g a.i. ha-1) and low resistance to imazamox (40 g a.i. ha-1) in both populations. Target-site resistance mechanisms are identified as the primary cause of survival for both modes of action in both populations. Glyphosate survivors show increased EPSPS gene copies but no mutations in the target protein. Surprisingly, survivors of ALS inhibitor treatment have multiple allelic mutations at codon: Pro-197-Ser, Pro-197-Thr, Asp-376-Glu, and Trp-574-Leu. Given the selection pressure history and the high incidence of detected mutations, it is reasonable to assume an ex situ selection of these populations to these MoA. This scenario presents a significant challenge for Spanish agriculture, indicating the ongoing introduction of A. palmeri populations with multiple resistance, complicating field management.

Resumen Amaranthus palmeri es una mala hierba invasora presente en territorio español. Se han descrito poblaciones resistentes a los inhibidores de la ALS y a los de la EPSPS. Se sospecha la presencia de resistencia múltiple a los dos modos de acción en unas mismas poblaciones. En este trabajo se ensaya la respuesta a estos modos de acción (MoA) en de dos poblaciones de margen de carretera procedentes de Catalunya. En un ensayo dosis respuesta la mayoría de las plantas tratadas con 540 g i.a./ha de glifosato sobrevivía en las dos poblaciones y algunos individuos incluso a la dosis de campo de 1080g/ha. Tratamientos con dosis única con inhibidores de la ALS destacan una alta resistencia a thifensulfuron-metil (6 g a.i. ha−1) y baja a imazamox (40 g a.i. ha−1) en ambas poblaciones. El mecanismo de resistencia target-site se ha descrito como la principal causa de supervivencia para las dos poblaciones a los dos modos de acción. Las plantas supervivientes a glifosato muestran un aumento en el número de copias del gen EPSPS y ninguna mutación en la proteína diana. Sorprendentemente, las supervivientes a tratamiento con inhibidores de la ALS presentan distintas mutaciones génicas en la diana de los herbicidas: Pro-197-Ser, Pro-197-Thr, Asp-376-Glu y Trp-574-Leu. Dado el historial de presión de selección y la alta incidencia de mutaciones detectadas, es razonable suponer que ambas poblaciones hayan desarrollado dicha resistencias ex situ. Este escenario plantea un desafío importante para la agricultura española, ya que supone una continua introducción de poblaciones de A. palmeri con resistencia múltiple, complicando su manejo en campo.

Abstract Herbicide resistance in weeds is an evolutionary process. Although there is a great global diversity of weeds, independent origins of herbicide resistance evolution have been shown to converge into similar molecular and physiological resistance mechanisms in geographically distant weed populations. Amaranthus species have shown an extraordinary ability to evolve herbicide resistance and invade new environments at a global scale, which represents an opportunity for identifying adaptive evolutionary patterns. The most frequent cases of herbicide-resistant Amaranthus species have been identified in North America, where A. hybridus, A. palmeri, A. tuberculatus and A. retroflexus comprise more than 90% of them. Meanwhile, A. retroflexus, A. hybridus and A. palmeri have been the most reported species in South America. Around 70% of the cases of herbicide-resistant Amaranthus species have been identified in global soybean and corn crops. The higher fecundity and adaptability of plants to a broad range of environments would make populations more likely to persist and be selected for herbicide resistance. Co-evolution of multiple herbicide resistance mechanisms at the plant and/or population level is evident in weed species. For Amaranthus spp., resistance cases highlight evolutionary responses to herbicide use with clear patterns of selection for multiple herbicide resistance in particular regions and spread to new areas within and between global cropping systems. Seed-mediated gene flow is an important component to the spread of herbicide resistant Amaranthus spp. populations. Reduction of the intensity of herbicide selection by combining diverse and integrated weed control practices should be a common goal in weed management programs.

Abstract RNA interference (RNAi) is a next-generation technology for weed management. Weeds would be sprayed with small RNAs (sRNAs) capable of inducing gene silencing (referred to as spray-induced gene silencing, SIGS) without involving the use of transgenes, reaching traditional targets in chemical control, targets that are not today more sensitive to herbicides and even new targets. Here we present the main challenges and opportunities for using SIGS as a practical tool for weed management. The development of SIGS in weed science has been slower compared to other crop protection areas such as entomology and plant pathology due to the difficulty of obtaining stable molecules that easily enter the plant, without off-target risks to crops, and that in small amounts guarantee systemic silencing with effectiveness. To overcome the challenges, it is necessary to achieve the synthesis of sRNAs on a large scale, making the field application practical and economical, develop formulations that protect sRNAs inside and outside the plant, and substantially increase the genomic and transcriptomic information available for weeds. Once these barriers have been overcome, SIGS technology could be similarly used in the field as herbicides are used today, spraying directly on the crop and selectively controlling weeds. This will provide a new tool for weed management, herbicide resistance management, and potentially exploration of new plant enzyme targets never before achieved by chemical control.

Auxin governs dynamic cellular processes involved at several stages of plant growth and development. In this review, we discuss the mechanisms employed by auxin in light of recent scientific advances, with a focus on synthetic auxins as herbicides and synthetic auxin resistance mechanisms. Two auxin receptors were reported. The plasma membrane receptor ABP1 (Auxin Binding Protein 1) alters the structure and arrangement of actin filaments and microtubules, leading to plant epinasty and reducing peroxisomes and mitochondria mobility in the cell environment. The second auxin receptor is the gene transcription pathway regulated by the SCFTir/AFB ubiquitination complex, which destroys transcription repressor proteins that interrupt Auxin Response Factor (ARF) activation. As a result mRNA related with Abscisic Acid (ABA) and ethylene are transcribed, producing high quantities of theses hormones. Their associated action leads to high production of Reactive Oxygen Species (ROS), leading to tissue and plant death. Recently, another ubiquitination pathway which is described as a new auxin signaling route is the F-box protein S-Phase Kinase-Associated Protein 2A (SKP2A). It is active in cell division regulation and there is evidence that auxin herbicides can deregulate the SKP2A pathway, which leads to severe defects in plant development. In this discussion, we propose that SFCSKP2A auxin binding site alteration could be a new auxinic herbicide resistance mechanism, a concept which may contribute to the current progress in plant biology in its quest to clarify the many questions that still surround auxin herbicide mechanisms of action and the mechanisms of weed resistance.