Weed resistance without changing site

Preliminary results of a study with horse flea biotype region of Palotina, in Paraná, resistant to two mechanisms of action, point out for anatomical changes and/or metabolic processes outside the herbicide site of action (NTSR). As it can result in multiple resistance, a trial with seven herbicides with different mechanisms of action was carried out with a new generation of plants originating from this biotype. The data is being analyzed and will be fundamental to assist in understanding the type of mechanism involved in resistance, as well as in decision-making about control alternatives.

Among among agricultural crops in Brazil, soybean was the one that presented one of the largest productivity growth and expansion in area over the last 40 years. Big part of the gain in productivity occurred due to high investment in production systems, mainly in agricultural inputs, machines and seed technology.

Several factors affect the productivity of soybean crops. The negative effects caused by weed infestation stand out, mainly because these are species with a short life cycle, naturally infesting the agricultural ecosystem, and are subject to great selective pressure resulting from climate, soil conditions and human action. Currently, the use of herbicides is considered the most effective way to manage these species in the field. However, the plants' ability to adapt and the selection pressure caused by the intensive use of herbicides can select weeds that are resistant to these molecules.

Resistance has a negative impact on the profitability of the production sector, reduces productivity, makes harvesting difficult, and increases the costs of crop management. Recently, there has been a major economic impact resulting from the selection of resistant weeds, which requires the application of herbicides with alternative mechanisms of action. This significantly increases the cost of weed control. For example, a study published by Embrapa in August 2017 revealed that, given the presence of resistant weeds in soybean areas, such as buva, ryegrass, and sourgrass, the cost of control can be up to three times higher compared to the cost without the resistant weed management strategy.

Multiple resistor: NTSR type

Herbicide resistance in weeds is the inherited ability of some biotypes within a population to survive and reproduce after exposure to a dose of herbicide that would be lethal to that weed population. Among the resistance mechanisms, the two most important involve: (i) changes in the site of action of the herbicide, “target-site” (TSR) that is, there are changes in the gene that encodes the target protein of the herbicide, causing a reduction in the direct action of the herbicide on its target; (ii) anatomical and/or metabolic changes outside the site of action of the herbicide, known as “non-target-site” (NTSR).

In many cases, this mechanism can rapidly generate the evolution of multiple resistance in weeds, i.e., the selected weed is resistant to herbicides with more than one mechanism of action, generally determined by a single resistance mechanism (metabolism common to all herbicides to which the plant is resistant).

NTSR can be polygenic or monogenic, and is the most difficult type of resistance to control agronomically. Recent studies have shown that the occurrence of NTSR in a population under selective pressure from some herbicides can facilitate the further evolution of NTSR to other herbicides, including herbicides with different mechanisms of action. This phenomenon is more common, especially when doses below those recommended in the package insert, also called underdoses, are used.

NTSR mechanisms are the same as those found in a set of plant responses to abiotic stresses, whether constitutive, stress-induced, or both. The current hypothesis reported in the scientific community is that NTSR can be induced by stress caused by the application of herbicides. Thus, this stress triggers response pathways in all weed individuals, regardless of their sensitivity to the herbicide. The genetic variation of biotypes results in differences in their sensitivity to the herbicide, and this is the basis of the evolution that induces NTSR mechanisms.

The mechanisms that result in NTSR resistance are diverse and different between and within species, mainly because it consists of of a complex process of multigene regulation. These mechanisms can be divided into: 1) reduction of herbicide absorption and translocation; two) herbicide metabolism; and 2) recovery from the phytotoxic effects of herbicide (Figure 3).

Reduced herbicide uptake is associated with differences in the physicochemical properties of the cuticle of resistant plants that cause reduced retention of the herbicide solution in the leaves and/or reduced efficiency of herbicide penetration through the cuticle. Since commercial herbicide formulations are developed to optimize herbicide penetration, this type of mechanism is not expected to confer high levels of resistance and is generally understudied. Reduced herbicide translocation involves restriction of herbicide movement within the plant and/or compartmentalization of the herbicide.

Herbicide metabolism is certainly the most studied aspect of NTSR, a multistep process involving the coordinated action of several types of enzymes. Basically, the process can be divided into three phases: 1) the herbicide molecule is first transformed into a more hydrophilic metabolite; 2) conjugation with a cellular molecule; 3) export to the vacuole and/or cell wall where further degradation occurs.

Protection against collateral damage from herbicide action in most plants, this is due to an increase in the expression of peroxidases and oxidases that protect cells against oxidative damage caused by the action of the herbicide on the plant. In this case, the herbicide reaches the site of action, performs its function, but the effects triggered in plants are “combated” by the plant protection system.

NTSR Resistance in Brazil and the World

Most cases of NTSR in plants weeds was elucidated for herbicides that inhibit EPSPs, photosystem I, photosystem II, ACCase, ALS and auxin mimics.

 The reduction in absorption and translocation of glyphosate are related to the resistance mechanism of whitegrass biotypes and bittergrass in Brazil, sorghum Aleppo in Argentina and buva in United States and Brazil, and for the latter it has also been diagnosed increased plant metabolism. He comes a positive correlation was also observed between resistance levels and increased glyphosate sequestration in the vacuole in ryegrass (lolium sp.) resistant in Brazil, Chile, Australia and Italy.

The NTSR engine is recognized as the predominant ACCase inhibitor herbicides. The resistance of lolium rigidum (Australia) and Echinochloa phyllopogon (U.S) to ACCase and ALS herbicides is due to increased metabolism involving enzymes of the cytochrome P450 type. In southern Brazil, changes in the pattern of plant metabolism Annual ryegrass, provided resistance to the herbicide clethodim.

Studies with biotypes from the United States of Perennial ryegrass L. spp. multiflorum, Conyza bonariensis and Conyza canadensis indicate that reduced translocation of the herbicide paraquat is the cause of resistance to this molecule. Translocation reduction was also observed in wild turnip biotype (Raphanus raphanistrum) resistant to the herbicide 2,4-D in Australia. In Egypt it was verified reduction of translocation and protection against collateral damage of the paraquat herbicide on horseweed plants.

The best-known case in the world of resistance from NTSR type is lolium rigidum in Australia, which presents resistance to 16 different herbicide molecules, with a total of nine mechanisms of action.

Buva with resistance

Recently two new cases of Conyza sumatrensis with resistance to photosystem-inhibiting herbicides I and PROTOX inhibitors have been reported in Brazil. Following the reports, a fleabane biotype from the Palotina/Paraná region that showed resistance to the two mechanisms described above (Figures 2, 3, 4 and 5) was selected for studies to elucidate the resistance mechanism presented by these plants. The studies are being coordinated by the group of research on Weeds and Pesticides in the Environment, from the Federal University Rural Rio de Janeiro (PDPA/UFRRJ).

Because both herbicides result in side effects similar in plants, preliminary tests were carried out to identify changes in plant metabolism that could be involved with metabolization of herbicides or protection against collateral damage caused by them. From of the preliminary results obtained, the hypothesis that presented the greatest consistency is that the mechanism is of the NTSR type, but detailed research is being developed to confirm this hypothesis.

As the NTSR-type resistance mechanism can often result in multiple resistance, a trial with seven herbicides of different mechanisms of action was carried out with a new generation of plants from this biotype. The data is under analysis and will be essential to assist both in understanding the type of mechanism involved in resistance, as well as in decision-making on the control alternatives that can be used in areas where these plants are present, avoiding more severe damage in production areas.

Practical recommendations to avoid the selection of weeds with multiple resistance, resulting from the NTSR mechanism

• Understand and use knowledge of the biology of the weed infesting the area

• Plant soybeans in clean, using good quality seeds

• Survey and monitoring of plant infestation at all stages of the production process

• Use the recommended dose of herbicide leaflet

• Combine herbicides or formulated mixtures of herbicides with different mechanisms of action

• Diversify the system production and as a consequence diversification of herbicides used in system

• “Full” dose of all herbicides in association or mixture when the herbicides control spectrum different from weeds

• Use integrated management of weeds, integrating cultural and chemical measures

Article published in issue 223 of Cultivar Grandes Culturas, December 2017/January 2018.