Mecanismos fisiológicos, bioquímicos y moleculares de tolerancia/resistencia a glifosato en especies de México

La actividad agrícola es afectada por factores bióticos y abióticos. Las malas hierbas son uno de los factores bióticos más importantes que causan grandes pérdidas en el rendimiento de los cultivos (Oerke, 2006), y su manejo se ha basado principalmente en el uso de herbicidas (Heap, 2014). Uno de los herbicidas más importantes para el manejo de malas hierbas a nivel mundial es el glifosato. Es un herbicida de amplio espectro, sistémico, no residual y con aceptable perfil toxicológico (Duke y Powles, 2008). Sin embargo, el uso intensivo de herbicidas ha provocado la evolución de poblaciones de malas hierbas resistentes (Powles y Yu, 2010). Por otra parte, en México existen pocos casos de malas hierbas resistentes a herbicidas (Heap, 2016), y en la mayoría de los casos se desconocen los mecanismos de resistencia involucrados, principalmente debido a la falta de medios científicos para estudiarlos. En 2010 y 2014, Leptochloa virgata y Bidens pilosa, respectivamente, fueron identificadas como resistentes a glifosato en huertos de cítricos en los estados de Puebla y Veracruz (Pérez-López et al. 2014; Heap, 2016). Por otra parte, especies como Canavalia ensiformis, Clitoria ternatea, Neonotonia wightii y Mucuna pruriens presentan tolerancia natural a glifosato (Cruz-Hipolito et al. 2009, 2011; Rojano-Delgado et al. 2012). Estas especies son utilizadas como cubiertas vegetales en cultivos perennes tropicales de México como una herramienta adicional de manejo de malas hierbas, permitiendo menos aplicaciones de glifosato que en sistemas de producción convencionales. Cologania broussonetii es una leguminosa silvestre con tolerancia a glifosato, nativa de regiones de clima templado del país, que puede facilitar el manejo de malas hierbas, si es utilizada como cubierta vegetal en cultivos perennes que se producen en estas condiciones climáticas. Debido a que el glifosato continúa teniendo una gran importancia como herbicida, pero debido a la falta de nuevas materias activas, es necesario alargar su vida útil (Heap, 2014). En este trabajo se han caracterizado por primera vez los mecanismos de resistencia que desarrollaron poblaciones resistentes de L. virgata y B. pilosa colectadas en huertos de cítricos, ya que ambas especies continúan siendo casos inéditos de resistencia a glifosato en el mundo (Heap, 2016). Además, se ha caracterizado el nivel de tolerancia natural a glifosato que posee C. broussonetti

Molecular mechanisms of herbicide resistance in weeds

Herbicides have become one of the most widespread weed-control tools in the world since their advent in the mid-20th century [1]. Nowadays, they are still being used in most conventional cropping systems in modern agriculture [2]. Unfortunately, the persistent use of herbicides is being threatened by the spread of herbicide resistance, a fast evolutionary process that took place a few years after their arrival into modern agriculture [2,3]. To safeguard their future use in agriculture, there is great interest in understanding the molecular mechanisms conferring resistance or predisposing weeds toward evolving herbicide resistance. Herbicide resistance is governed by target-site resistance (TSR) and non-target-site resistance (NTSR) mechanisms [4]. TSR-based resistance is caused by any gene alteration able to change the interaction with the encoded target protein/enzyme so that the herbicide is not able to sufficiently interfere with it to cause plant death. TSR mechanisms are usually better understood because there is a single well-known target gene, and therefore, they are monogenic [5]. On the other hand, NTSR mechanisms are those not involving the target protein and can decrease the herbicide arriving at the site of action (SoA) into an insufficient amount, so plants can survive; more rarely, any mechanism protecting plants from herbicide damage is also referred as NTSR [5]. NTSR mechanisms are rarely fully understood since they can be quantitative in nature and controlled by several genes (with each gene providing some level of resistance); in other words, NTSR-based resistance can be polygenic [4]. The increase in multiple herbicide resistance to different SoAs, mainly through enhanced metabolism, is of great concern [2]. Multiple herbicide resistance reflects an evolutionary process by which populations or plants can accumulate different resistance mechanisms (TSR and/or NTSR), conferring resistance to several SoAs [6]. Sadly, this process usually occurs because resistance to one SoA provokes switching to another SoA rather than reducing herbicide-selection pressure [7]. Among NTSR mechanisms, enhanced metabolism is the most threatening because, as a generalist mechanism, it can confer cross-resistance to dissimilar herbicide chemistries, even to those never used before [4]. Conversely, TSR is governed by specialist mechanisms, always specific to a single SoA [5]. This Special Issue was focused on the new well-characterized cases of herbicide resistance, both for TSR and/or NTSR (if a molecular basis is reported), as well as studies that identify new gene alterations conferring TSR or the genetic basis involved in NTSR. Both TSR and NTSR can also be divided into different mechanisms depending on their nature. Point mutations, altered expression, or codon deletion of the target-site gene are among the most reported types of TSR mechanisms [5]. NTSR mechanisms usually involve altered patterns of herbicide absorption, translocation, or metabolism. Herbicide-metabolism-based resistances are complex and often involve genes that are members of large gene families, including cytochromes P450 (P450) and Glutathione-S-transferases (GST) [4]. Therefore, this editorial focuses on the nature of the resistance mechanisms of the two major types, TSR and NTSR, described in each of the contributions to this Special Issue.Joel Torra acknowledges support from the Spanish Ministry of Science, Innovation, and Universities (grant Ramon y Cajal RYC2018-023866-I)

Underlying Resistance Mechanisms in the Cynosurus echinatus Biotype to Acetyl CoA Carboxylase-Inhibiting Herbicides

Hedgehog dogtail (Cynosurus echinatus) is an annual grass, native to Europe, but also widely distributed in North and South America, South Africa and Australia. Two hedgehog dogtail biotypes, one diclofop-methyl (DM)-resistant and one DM-susceptible were studied in detail for experimental dose-response resistance mechanisms. Herbicide rates that inhibited shoot growth by 50% (GR50) were determined for DM, being the resistance factor (GR50R/GR50S) of 43.81. When amitrole (Cyt. P450 inhibitor) was applied before treatment with DM, the R biotype growth was significantly inhibited (GR50 of 1019.9 g ai ha-1) compared with the GR50 (1484.6 g ai ha-1) found for the R biotype without pretreatment with amitrole. However, GR50 values for S biotype do not vary with or without amitrole pretreatment. Dose-response experiments carried out to evaluate cross-resistance, showed resistance to aryloxyphenoxypropionate (APP), cyclohexanodione (CHD) and phenylpyrazoline (PPZ) inhibiting herbicides. Both R and S biotypes had a similar 14C-DM uptake and translocation. The herbicide was poorly distributed among leaves, the rest of the shoot and roots with unappreciable acropetal and/or basipetal DM translocation at 96 HAT. The metabolism of 14C-DM, D-acid and D-conjugate metabolites were identified by thin-layer chromatography. The results showed that DM resistance in C. echinatus is likely due to enhanced herbicide metabolism, involving Cyt. P450 as was demonstrated by indirect assays (amitrole pretreatment). The ACCase in vitro assays showed that the target site was very sensitive to APP, CHD and PPZ herbicides in the C. echinatus S biotype, while the R biotype was insensitive to the previously mentioned herbicides. DNA sequencing studies confirmed that C. echinatus cross-resistance to ACCase inhibitors has been conferred by specific ACCase double point mutations Ile-2041-Asn and Cys-2088-Arg

Fitness Cost of Imazamox Resistance in Wild Poinsettia (Euphorbia heterophylla L.)

Wild poinsettia (Euphorbia heterophylla L.) is a difficult-to-control weed in soybean production in Brazil that has developed resistance to herbicides, including acetolactate synthase inhibitors. We investigated the potential fitness cost associated to the Ser-653-Asn mutation that confers imazamox resistance in this weed. Plant height, leaf and stem dry weight, leaf area and seed production per plant as well as the growth indices of specific leaf area, leaf area ratio, relative growth rate and net assimilation in F2 homozygous resistant (R) and susceptible (S) wild poinsettia progenies were pairwise compared. S plants were superior in most of the traits studied. Plant heights for S and R biotypes, recorded at 95 days after planting (DAP), were 137 and 120 cm, respectively. Leaf areas were 742 and 1048 cm2 in the R and S biotypes, respectively. The dry weights of leaves and stems in the S plants were 30 and 35%, respectively, higher than in the R plants. In both biotypes, the leaves had a greater share in dry weight at early development stages, but from 50 DAP, the stem became the main contributor to the dry weight of the shoots. The R biotype produced 110 ± 4 seed plant−1, i.e., 12 ± 3% less seeds per plant than that of the S one (125 ± 7 seed plant−1). The growth indices leaf area ratio and specific leaf area were generally higher in the S biotype or similar between both biotypes; while the relative growth rate and net assimilation rate were punctually superior in the R biotype. These results demonstrate that the Ser-653-Asn mutation imposed a fitness cost in imazamox R wild poinsettia

First Case of Multiple Resistance to EPSPS and PSI in Eleusine indica (L.) Gaertn. Collected in Rice and Herbicide-Resistant Crops in Colombia

Eleusine indica is a highly competitive and difficult-to-control plant in annual and perennial crops. In Colombia, broad-spectrum herbicides, such as paraquat and glyphosate, have begun to present poor levels of control for this weed. The multiple resistance to glyphosate and paraquat, the increase in herbicide performance with adjuvants (Retenol® and Trend® 90), and alternative herbicides were evaluated in a resistant (R) population of E. indica collected in rice fields, which is rotated with herbicide-resistant (HR) crops. Based on plant mortality, the R population was 9.8 and 7.2 times more resistant than susceptible (S) plants to glyphosate and paraquat, respectively. R plants accumulated 4.2 less shikimic acid and had at least 70% less electrolyte leakage than S plants when they were exposed to glyphosate and paraquat, respectively. Both adjuvants increased the foliar retention of herbicides. In addition, adjuvants also increased the performance of glyphosate effectiveness between 22% and 58% and that of paraquat from 61% to 100%. Alternative herbicides (atrazine, clethodim, imazamox, diuron, flazasulfuron, glufosinate, oxyfluorfen, quizalofop, and tembotrione) provided high levels of control in both populations of E. indica. This is the first case of multiple resistant E. indica confirmed in Colombia. Adjuvants improved the leaf retention and efficacy of glyphosate and paraquat. In summary, the alternative herbicides evaluated in this study should be adopted by Colombian farmers and provide additional herbicide modes-of-action to combat future resistance

Resistance Evolution to EPSPS Inhibiting Herbicides in False Barley (Hordeum murinum) Harvested in Southern Spain

A failure of the EPSPS-inhibiting herbicide glyphosate to control several populations of Hordeum murinum subsp. leporinum (or H. murinum) occurred in southern Spain after more than fifteen applications in both crop (olive, orchards, and citrus) and non-crop (dry areas, roadsides and ditches) areas. Eight out of 18 populations studied were resistant (R) to glyphosate with R factors higher than four based on GR50. These populations also had the highest values of LD50 and the lowest levels of shikimic acid accumulation. Two adjuvants tested increased glyphosate efficacy in both susceptible (S) and R populations thanks to better spray foliar retention. Moreover, PS I-, PS II-, and ACCase-inhibiting herbicides, in pre- or post-emergence, proved to be the best chemical alternatives with different sites of action (SoA) to control both S and glyphosate-R populations. This study represents the first report worldwide of glyphosate resistance in H. murinum found in very different crop and non-crop areas from southern Spain. To design chemical strategies to implement integrated weed management programs for glyphosate-R H. murinum, both adjuvants and herbicides with alternative SoA as well as application timings should be considered.This research has also been supported by the Spanish Government, through project AGL2017-83325-C4-2-R (AEI/FEDER/UE), Asociación de Agroquímicos y Medioambiente and project ref PO 4513358935 by Bayer CropScience

Review and new perspectives on non-layered manganese compounds as electrode material for Sodium-Ion batteries

After more than 30 years of delay compared to lithium-ion batteries, sodium analogs are now emerging in the market. This is a result of the concerns regarding sustainability and production costs of the former, as well as issues related to safety and toxicity. Electrode materials for the new sodium-ion batteries may contain available and sustainable elements such as sodium itself, as well as iron or manganese, while eliminating the common cobalt cathode compounds and copper anode current collectors for lithium-ion batteries. The multiple oxidation states, abundance, and availability of manganese favor its use, as it was shown early on for primary batteries. Regarding structural considerations, an extraordinarily successful group of cathode materials are layered oxides of sodium, and transition metals, with manganese being the major component. However, other technologies point towards Prussian blue analogs, NASICON-related phosphates, and fluorophosphates. The role of manganese in these structural families and other oxide or halide compounds has until now not been fully explored. In this direction, the present review paper deals with the different Mn-containing solids with a non-layered structure already evaluated. The study aims to systematize the current knowledge on this topic and highlight new possibilities for further study, such as the concept of entatic state applied to electrodes

Ile-1781-Leu and Asp-2078-Gly Mutations in ACCase Gene, Endow Cross-resistance to APP, CHD, and PPZ in Phalaris minor from Mexico

Herbicides that inhibit acetyl coenzyme A carboxylase (ACCase) are commonly used in Mexico to control weedy grasses such as little seed canarygrass (Phalaris minor). These herbicides are classified into three major families (ariloxyphenoxypropionates (APP), cyclohexanodiones (CHD), and, recently, phenylpyrazolines (PPZ)). In this work, the resistance to ACCase (APP, CHD, and PPZ) inhibiting herbicides was studied in a biotype of Phalaris minor (P. minor) from Mexico, by carrying out bioassays at the whole-plant level and investigating the mechanism behind this resistance. Dose-response and ACCase in vitro activity assays showed cross-resistance to all ACCase herbicides used. There was no difference in the absorption, translocation, and metabolism of the 14C-diclofop-methyl between the R and S biotypes. The PCR generated CT domain fragments of ACCase from the R biotype and an S reference were sequenced and compared. The Ile-1781-Leu and Asp-2078-Gly point mutations were identified. These mutations could explain the loss of affinity for ACCase by the ACCase-inhibing herbicides. This is the first report showing that this substitution confers resistance to APP, CHD, and PPZ herbicides in P. minor from Mexico. The mutations have been described previously only in a few cases; however, this is the first study reporting on a pattern of cross-resistance with these mutations in P. minor. The findings could be useful for better management of resistant biotypes carrying similar mutations

Herbicide Resistance in Brazil: Status, Impacts, and Future Challenges

Brazil is a large producer and exporter of crops in global terms. Weeds may be responsible for ~14% of crop losses, depending on the crop system. Herbicides occupy 58% of the Brazilian pesticide market; however, the continuous use of these products and the high selection pressure have led to the emergence of weeds resistant to herbicides. Today, there are 51 weed species reported as being resistant to herbicides in Brazil, of which 17 involves cross and multiple-resistance. Acetolactate synthase (ALS), acetyl coenzyme A carboxylase (ACCase) and 5-enolpiruvylshikimate-3-phosphate synthase (EPSPs) inhibitors are the herbicidal groups with the most resistance cases. Soybean, corn, rice, wheat and cotton present 30, 12, 10, 9 and 8 cases, respectively, occurring mainly in herbicide-resistant crop fields from the Southern and Central West regions of the country. To better understand the dimensions of herbicide resistance, in this chapter, we will explore the size of agricultural activity in Brazil, the pesticide market and the use of herbicides in the main crops. In addition, the agronomic, scientific-technical and economic aspects that have contributed, directly or indirectly, to the selection of resistant weeds will be discussed in order to have an overview of the economic impact of herbicide resistance management