Barnyardgrass Biotypes Research Articles

Application of ALS inhibitors at pre-emergence is effective in controlling resistant barnyardgrass biotypes depending on the mechanism of resistance

Echinochloa species are one of the most troublesome weeds in rice crops, and their control is hampered due to herbicide resistance. The aim of this study was to identify the cross-resistance pattern and the differential resistance level for pre- and post-emergence applications of imazethapyr and penoxsulam in populations of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] with different ALS gene mutations. Of 26 biotypes, 23 were imazethapyr-resistant, and 10 were cross-resistant to penoxsulam. The resistance index (RI) to imazethapyr was 5.7–19.5 for Ser653Asn, 26.7–68.3 for Ala122Thr and Ala205Asn, and 70.9–252.9 for Trp574Leu. Only Trp574Leu also resulted in resistance to penoxsulam. The double ALS mutation Ala122Asn + Trp574Leu resulted in a RI for imazethapyr and penoxsulam higher than 2800. The ED50 for penoxsulam applied at pre-emergence was three and six times lower than at post-emergence for the susceptible and resistant biotypes, respectively. The application of imazethapyr at pre-emergence was more effective than at post-emergence only for the biotypes with low resistance level mutation Ser653Asn. The efficacy of the herbicide quinclorac was similar for the application at pre- and post-emergence for susceptible and resistant biotypes. The ALS mutations Ala205Asn and Ala122Asn + Trp574Leu are reported for the first time in barnyardgrass. The use of ALS inhibitors at pre-emergence should be prioritized over post-emergence in fields with resistant barnyardgrass and in need of using these products to control other non-resistant weeds.

Kin Recognition in an Herbicide-Resistant Barnyardgrass (Echinochloa crus-galli L.) Biotype.

Despite increasing evidence of kin recognition in natural and crop plants, there is a lack of knowledge of kin recognition in herbicide-resistant weeds that are escalating in cropping systems. Here, we identified a penoxsulam-resistant barnyardgrass biotype with the ability for kin recognition from two biotypes of penoxsulam-susceptible barnyardgrass and normal barnyardgrass at different levels of relatedness. When grown with closely related penoxsulam-susceptible barnyardgrass, penoxsulam-resistant barnyardgrass reduced root growth and distribution, lowering belowground competition, and advanced flowering and increased seed production, enhancing reproductive effectiveness. However, such kin recognition responses were not occurred in the presence of distantly related normal barnyardgrass. Root segregation, soil activated carbon amendment, and root exudates incubation indicated chemically-mediated kin recognition among barnyardgrass biotypes. Interestingly, penoxsulam-resistant barnyardgrass significantly reduced a putative signaling (-)-loliolide production in the presence of closely related biotype but increased production when growing with distantly related biotype and more distantly related interspecific allelopathic rice cultivar. Importantly, genetically identical penoxsulam-resistant and -susceptible barnyardgrass biotypes synergistically interact to influence the action of allelopathic rice cultivar. Therefore, kin recognition in plants could also occur at the herbicide-resistant barnyardgrass biotype level, and intraspecific kin recognition may facilitate cooperation between genetically related biotypes to compete with interspecific rice, offering many potential implications and applications in paddy systems.

Quick bioassay test from tillers for detecting ALS herbicide resistance of weedy rice and barnyardgrass

Resistance to acetolactate synthase (ALS) inhibitors have increased recently in South Brazil where the major weeds of flooded rice (barnyardgrass and weedy rice) have evolved resistance to imazapyr+imazapic. The aim of this research was to evaluate a growth medium for tissue regeneration of tillers in barnyardgrass, as well as an agar-based bioassays test (also from tillers) to detect susceptible and resistant biotypes of weedy rice and barnyardgrass to imazapyr+imazapic in vitro. Greenhouse experiments were conducted to detect ALS-resistant (R) and susceptible (S) weedy rice and barnyardgrass biotypes, and bioassays were carried out to evaluate an adequate growth medium for barnyardgrass tiller regeneration and determine the concentration of herbicide to distinguish R and S plants. The culture medium that provided a suitable barnyardgrass growth was MS 50% with the addition of benzylamino-purine. The tissue regeneration in vitro with the growth medium containing imazapyr+imazapic allowed to discriminate between R and S barnyardgrass and weedy rice plants. The concentration required for satisfactory control of susceptible barnyardgrass and weedy rice explants grown in vitro was 0.9 μM and 1.3 μM of imazapyr+imazapic herbicide, respectively. The bioassay in vitro using tiller regeneration provides an opportunity to predict effectively imazapyr+imazapic resistance in barnyardgrass and weedy rice.

Cross-resistance of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] to aryloxyphenoxypropionate herbicides

Managing emerged weeds that have evolved resistance to acetyl CoA carboxylase (ACCase)-inhibiting herbicides is a challenging task. A dose-response experiment was conducted on barnyardgrass biotypes resistant (R) and susceptible (S) to three aryloxyphenoxypropionate herbicides cyhalofop-butyl (CyB), fenoxaprop-ethyl (FeE), and quizalofop-ethyl (QuE) along with investigations into the potential resistance mechanism of these biotypes. The tested R barnyardgrass biotypes had strong resistance to CyB and FeE (resistant/susceptible ratio: 7.9–14.4) but weak resistance to QuE (resistant/susceptible ratio: 2.4–3.1). Absorption, translocation, and total metabolism of CyB and QuE were not associated with differences among S and R barnyardgrass biotypes. However, differences between S and R barnyardgrass were observed in production of active acid forms of each herbicide (cyhalofop-acid and quizalofop-acid). Production of cyhalofop-acid was >1.6-fold less in R barnyardgrass (3–8%) for 24 h after herbicide application than in the S barnyardgrass (8–16%). Meanwhile, production of quizalofop-acid was less in R barnyardgrass (< 14%) throughout the study period than in the S barnyardgrass (< 22%). Sequencing results of ACCase gene showed no difference between S and R barnyardgrass. Overall results show that a non-target-site resistance mechanism altering metabolism of CyB and QuE likely contributes to resistance of the barnyardgrass biotypes to these herbicides.

Absorption, translocation, and metabolism of florpyrauxifen-benzyl and cyhalofop-butyl in cyhalofop-butyl-resistant barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.

Dose-response experiments were conducted to assess the sensitivity of one susceptible and three putative resistant (R1, R2, and R3) barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] biotypes to florpyrauxifen-benzyl and cyhalofop-butyl alone and as a formulated premix. Subsequently, potential resistance mechanisms of the barnyardgrass were evaluated. Based on biomass reduction results, resistant/susceptible ratios were calculated for R1 (7.0–50), R2 (7.0–150), and R3 (18–214) biotypes. Absorption and translocation of [14C]-florpyrauxifen-benzyl decreased in R1 and R3 biotypes, but not for [14C]-cyhalofop-butyl. The metabolism of [14C]-florpyrauxifen-benzyl to [14C]-florpyrauxifen-acid was >2-fold less in resistant biotypes (9–11%) than in the susceptible biotype (23%). Moreover, the production of [14C]-florpyrauxifen-acid in susceptible barnyardgrass (not in the R biotypes) increased 3-fold when florpyrauxifen-benzyl and cyhalofop-butyl were applied in mixture compared to florpyrauxifen-benzyl applied alone. The tested barnyardgrass biotypes had no mutation in the Transport Inhibitor Response1, auxin-signaling F-box, and acetyl coenzyme A carboxylase genes. Although further studies on cyhalofop-butyl resistance with respect to analysis of specific metabolites are needed, our findings in this study demonstrates that the evolution of florpyrauxifen-benzyl resistance in multiple resistant barnyardgrass can be related to non-target-site resistance mechanisms reducing absorption and translocation of the herbicide and causing reduced conversion or rapid degradation of florpyrauxifen-acid.

Non-target-site resistance mechanism of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] to florpyrauxifen-benzyl.

Florpyrauxifen-benzyl (FPB) is an arylpicolinate herbicide (Group IV) for barnyardgrass control in rice. One susceptible (Sus) and three putative FPB-resistant (R1, R2, and R3) barnyardgrass biotypes were selected based on resistant/susceptible (R/S) ratios obtained from dose-response tests and used to investigate the potential resistance mechanisms. Based on visual control results, the R/S ratios of barnyardgrass biotypes R1, R2, and R3 were 60-, 33-, and 16-fold greater than the Sus standard, respectively. Sequencing results of TIR1 and AFB genes in the tested barnyardgrass revealed no difference between Sus and R barnyardgrass biotypes. Absorption of [14 C]-FPB in Sus barnyardgrass increased over time and reached 90%, which was >10 percentage points greater than that in R biotypes. The [14 C]-FPB absorption in all R barnyardgrass equilibrated after 48 h. For both Sus and R barnyardgrass, most [14 C]-FPB absorbed was present in the treated leaf (79.8-88.8%), followed by untreated aboveground (9.5-18.6%) and belowground tissues (1.3-2.2%). No differences in translocation were observed. Differences between Sus and R barnyardgrass biotypes were found for FPB metabolism. Production of the active metabolite, florpyrauxifen-acid, was greater in Sus barnyardgrass (21.5-52.1%) than in R barnyardgrass (5.5-34.9%). In conclusion, reductions in FPB absorption and florpyrauxifen-acid production may contribute to the inability to control barnyardgrass with FPB. © 2021 Society of Chemical Industry.

Barnyardgrass control using tank-mixed herbicides with saflufenacil and its influence in photosynthesis and chlorophyll fluorescence

ABSTRACT: The objectives of this research were to evaluate the interaction between herbicides mixed with saflufenacil for the control of barnyardgrass and to determine the effect on photosynthetic and chlorophyll fluorescence parameters. The experiment was conducted in a greenhouse in a 2x8 factorial scheme, whose factor A tested resistant and susceptible biotypes; and factor B the herbicides: saflufenacil (70 g a.i. ha-1), clomazone (180 g a.i. ha-1), imazapyr + imazapic (73.5 + 24.5 g a.i. ha-1), and cyhalofop (360 g a.i. ha-1), the mixtures of these herbicides with saflufenacil, and control without treatment. Weed control was assessed 7, 14, 21 and 28 days after herbicide application (DAA), as well as shoot dry matter at 28 DAA, photosynthetic parameters using infrared gas analyzer (IRGA), and emission of chlorophyll a fluorescence after 24 and 28 hours of application of treatments, respectively, and interaction of herbicides. Combination of saflufenacil with the herbicides tested in general did not change the response of both barnyardgrass biotypes to the herbicides used. The resistant biotype showed a lower negative effect on chlorophyll fluorescence and photosynthesis parameters in the combination of herbicides with saflufenacil. The herbicide cyhalofop was effective for the control of ALS-susceptible and resistant barnyardgrass.

Fitness of Echinochloa crusgalli var. mitis biotypes susceptible and resistant to imazapyr + imazapic herbicide

The continuous use of herbicides with the same mechanism of action has caused the selection of resistant weed biotype that may present differences in fitness when compared with susceptible biotype. The objective of the present study was to identify and distinguish the fitness of barnyardgrass biotypes resistant and susceptible to imazapyr + imazapic herbicide under controlled and noncompetitive conditions. For that, there were used barnyardgrass biotype susceptible (ECH1 and ECH38) and resistant (ECH14 and ECH44) to ALS inhibitors, collected at Pelotas/RS and Rio Grande/RS cities, respectively, in experiment installed in a greenhouse in a completely randomized experimental design during the months from October to December 2015. The biotype were evaluated during the period of 15 to 120 days after the emergence related to plant height, foliar area, shoot dry matter mass and root dry matter mass. The results showed insignificant differences between the biotypes during the development of the plants. The susceptible biotype ECH44 showed a higher number of panicles per plant and seeds along with the resistant ECH1. There is variability among Echinochloa crusgalli var. mitis biotypes resistant and susceptible to ALS inhibitors, indicating an absence in fitness penalty caused by resistance when cultivated without the selection of pressure (herbicide).

Evaluation of Florpyrauxifen-benzyl on Herbicide-Resistant and Herbicide-Susceptible Barnyardgrass Accessions

AbstractFlorpyrauxifen-benzyl is a new herbicide under development in rice that will provide an alternative mode of action to control barnyardgrass. Multiple greenhouse experiments evaluated florpyrauxifen-benzyl efficacy on barnyardgrass accessions collected in rice fields across Arkansas, and to evaluate its efficacy on herbicide-resistant biotypes. In one experiment, florpyrauxifen-benzyl was applied at the labeled rate of 30 g ai ha−1to 152 barnyardgrass accessions collected from 21 Arkansas counties. Florpyrauxifen-benzyl at 30 g ai ha−1effectively controlled barnyardgrass and subsequently reduced plant height and aboveground biomass. In a dose-response experiment, susceptible-, acetolactate synthase (ALS)-, propanil-, and quinclorac-resistant barnyardgrass biotypes were subjected to nine rates of florpyrauxifen-benzyl ranging from 0 to 120 g ai ha−1. The effective dose required to provide 90% control, plant height reduction, and biomass reduction of the susceptible and resistant biotypes fell below the anticipated labeled rate of 30 g ai ha−1. Based on these results, quinclorac-resistant barnyardgrass as well as other resistant biotypes can be controlled with florpyrauxifen-benzyl at 30 g ai ha−1. Overall, results from these studies indicate that florpyrauxifen-benzyl can be an effective tool for controlling susceptible and currently existing herbicide-resistant barnyardgrass biotypes in rice. Additionally, the unique auxin chemistry of florpyrauxifen-benzyl will introduce an alternative mechanism of action in rice weed control thus acting as an herbicide-resistance management tool.

Effect of allelochemical tricin and its related benzothiazine derivative on photosynthetic performance of herbicide-resistant barnyardgrass

Despite increasing knowledge of allelochemicals as leads for new herbicides, relatively little is known about the mode of action of allelochemical-based herbicides on herbicide-resistant weeds. In this study, herbicidal activities of a series of allelochemical tricin-derived compounds were evaluated. Subsequently, a benzothiazine derivative 3-(2-chloro-4-methanesulfonyl)-benzoyl-hydroxy-2-methyl-2H-1,2-benzothiazine-1,1-dioxide with 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) inhibiting activity was identified as a target compound on photosynthetic performance of penoxsulam-resistant versus -susceptible barnyardgrass (Echinochloa crus-galli). Regardless of barnyardgrass biotype, the benzothiazine derivative greatly affected chlorophyll fluorescence parameters (Fv/Fm, ETR1min and NPQ1min), reduced the chloroplast fluorescence levels and expression of HPPD gene. In particular, the benzothiazine derivative interfered with photosynthetic performance of resistant barnyardgrass more effectively than the allelochemical tricin itself. These results showed that the benzothiazine derivative effectively inhibited the growth of resistant barnyardgrass and its mode of action on photosynthesis system was similar to HPPD-inhibiting sulcotrione, making it an ideal lead compound for further development of allelochemical-based herbicide discovery.

Interference of allelopathic rice with penoxsulam-resistant barnyardgrass.

Despite increasing knowledge of allelopathic rice interference with barnyardgrass, relatively little is known about its action on herbicide-resistant barnyardgrass. The incidence of herbicide-resistant barnyardgrass is escalating in paddy fields. Knowledge of the interference of allelopathic rice with herbicide-resistant barnyardgrass and the potential mechanisms involved is warranted. Penoxsulam-resistant and -susceptible barnyardgrass biotypes were identified and segregated from a putative penoxsulam-resistant population occurring in paddy fields in China. Allelopathic rice inhibited the growth of barnyardgrass roots more than shoots, regardless of biotype. In particular, there was a stronger inhibition for resistant barnyardgrass than for susceptible barnyardgrass. Allelopathic rice significantly reduced total root length, total root area, maximum root amplitude and maximum root depth in barnyardgrass. Furthermore, the rice allelochemicals tricin and momilactone B inhibited the growth of both resistant and susceptible barnyardgrass. Compared with root contact, root segregation significantly increased inhibition of barnyardgrass with an increase in rice allelochemicals. Root exudates from barnyardgrass induced the production of rice allelochemicals, but the effect of susceptible barnyardgrass was much stronger than that of resistant barnyardgrass. Allelopathic rice can interfere with the growth of penoxsulam-resistant barnyardgrass through allelochemical-mediated root interactions. This type of allelopathic interference may provide a non-herbicidal alternative for herbicide-resistant weed management in paddy systems. © 2017 Society of Chemical Industry.

Barnyardgrass with Multiple Resistance to Synthetic Auxin, ALS and Accase Inhibitors

ABSTRACT The objective of this research was to confirm the resistance of a barnyardgrass biotype (Echinochloa crus-galli) to herbicides quinclorac (synthetic auxin), penoxsulam (ALS inhibitor) and cyhalofop-butyl (ACCase inhibitor). Two experiments were carried out in a greenhouse located in Itajaí, State of Santa Catarina, Brazil. One experiment evaluating the biotype ECH 18 (known as susceptible) and one with the ECH 141 biotype (suspected to be multiple resistant). The experimental design used was completely randomized, in a 4 x 7 factorial arrangement with four replications. The first factor was the herbicide: cyhalofop-butyl (Clincher(r)), penoxsulam (Ricer(r)), quinclorac (Facet(r)) and propanil (Grassaid(r)) and the second factor consisting of seven rates of each herbicide (0.00; 0.25; 0.50; 1; 2; 4 and 8 times the label rate). The evaluations of percentage of control, plant density per pot and dry mass of shoots were performed 35 days after treatments. The rates to control 50% and 80% and the dose to provide 50% and 80% of the weed biomass accumulation and the resistance factor were estimated based on data analysis. The initial hypothesis was confirmed; the biotype ECH 141 presented multiple resistance (synthetic auxin, ALS and ACCase inhibitor). However, the herbicide propanil controlled biotype ECH 141 adequate.

Quantitative proteomics reveals ecological fitness cost of multi-herbicide resistant barnyardgrass (Echinochloa crus-galli L.)

Barnyardgrass (Echinochloa crus-galli) is one of the top 15 herbicide-resistant weeds around the world that interferes with rice growth, resulting in major losses of rice yield. Thus, multi-herbicide resistance in barnyardgrass presents a major threat, with the underlying mechanisms that contribute to resistance requiring elucidation. In an attempt to characterize this multi-herbicide resistance at the proteomic level, comparative analysis of resistant and susceptible barnyardgrasses was performed using iTRAQ, both with and without quinclorac, bispyribac-sodium and penoxsulam herbicidal treatment. A total of 1342 protein species were identified from 2248 unique peptides by searching the UniProt database and conducting data analysis. Approximately 904 protein species with 4774 Gene Ontology (GO) terms were grouped into the categories of biological process, cellular component and molecular function. Among these, 688 protein species were annotated into 1583 KEGG pathways, with 980 protein species relating to metabolism and 93 relating to environmental information processing. A total of 292 protein species showed more than a 1.2-fold change in abundance in the resistant biotype relative to the susceptible biotype. Furthermore, herbicide treatment resulted in 157 protein species that showed more than a 1.2-fold change in the resistant biotype. Moreover, physiological analyses demonstrated an ecological fitness cost in the resistant biotype. SignificanceWhile some studies have shown a fitness cost to be associated with an altered ecological interaction, our understanding of the fitness costs associated with herbicide resistance are limited. Herein, physiological and proteomic analysis demonstrates herbicide resistance associated ecological fitness cost and potential mechanisms of herbicide-resistance in resistant biotypes of E. crus-galli. The results presented herein have revealed differences in ecological adaptation between resistant and susceptible biotypes in E. crus-galli and provide a fundamental basis enabling the development of new strategies for weed control. Lastly, this is the first large-scale proteomics study to examine herbicide stress responses in different barnyardgrass biotypes.

Identification of Herbicide-Resistant Barnyardgrass (Echinochloa crus-galli var. crus-galli) Biotypes in Korea

The continuous use of acetolactate synthase (ALS) and acetyl-CoA carboxylase (ACCase) inhibitors has led to the selection of herbicide resistant barnyardgrass populations in direct-seeded rice fields of Korea. This study was conducted to identify herbicide resistant barnyardgrass biotypes and to determine the cross- and multiple-resistance of them. 25% of the population collected from Taeahn was partially resistant to ACCase inhibitors and 22% collected from Kimjae were partially resistant to ALS inhibitors. However, 8.2% of the population from both sites was resistant to ALS and ACCase inhibitors. Resistance to sulfonylurea herbicide, flazasulfuron was identified from two barnyardgrass accessions collected from both Taeahn and Kimjae. One barnyardgrass accession from both sites was resistant to ACCase inhibitor, sethoxydim. The cross-resistance to ALS inhibitors was identified at one barnyardgrass accession from Taeahn and at two accessions from Kimjae. Further, crossresistance to ACCase inhibitors was also identified at barnyardgrass accessions from Taeahn and Kimjae. Multiple-resistance to flazasulfuron and sethoxydim was determined at four barnyardgrass accessions from Taeahn and at six accessions from Kimjae. Therefore, the herbicide mixture and sequences within a growing season or the herbicide rotation with different modes of actions across growing seasons are recommended to control herbicide-resistant barnyardgrass in infested fields.

Program Approaches to Control Herbicide-Resistant Barnyardgrass (Echinochloa crus-galli) in Midsouthern United States Rice

The goal of this research was to develop herbicide programs for controlling acetolactate synthase (ALS)–, propanil-, quinclorac-, and clomazone-resistant barnyardgrass. Two applications of imazethapyr alone at 70 g ai ha−1failed to control the ALS-resistant biotype more than 36%; however, when imazethapyr at 70 g ha−1was applied early POST (EPOST) followed by imazethapyr at 70 g ha−1plus fenoxaprop at 120 g ai ha−1immediately prior to flooding (PREFLD), barnyardgrass control improved to 78% at 10 wk after planting. When imazethapyr was applied twice following PRE or delayed PRE applications of clomazone at 336 g ai ha−1, quinclorac at 560 g ai ha−1, pendimethalin at 1,120 g ai ha−1, or thiobencarb at 4,480 g ai ha−1control was 92 to 100%. A single-pass program consisting of a delayed PRE application of clomazone at 336 g ha−1plus quinclorac at 560 g ha−1plus pendimethalin at 1,120 g ha−1plus thiobencarb at 4,480 g ha−1controlled all herbicide-resistant barnyardgrass biotypes at the same level as a standard multiple application program.

Determination of ACCase Sensitivity and Gene Expression in Quizalofop–Ethyl-Resistant and -Susceptible Barnyardgrass (Echinochloa crus-galli) Biotypes

Mechanisms of herbicide resistance were studied in a quizalofop–ethyl-resistant barnyardgrass biotype. Acetyl-coenzyme A carboxylase (ACCase) sensitivity to quizalofop-p-ethyl was measured by high-performance liquid chromatography and the trend in ACCase gene expression over time was determined using real-time polymerase chain reaction. The results showed that an insensitive ACCase was present in Geqiushan resistant plants (R), with a resistance index of 106. The basal ACCase activities in Geqiushan R and Geqiushan susceptible plants (S) were similar, at 1.20 and 1.17 ng malonyl-CoA min−1µg−1extract protein, respectively. Basal ACCase gene expression in Geqiushan R was similar to that in Geqiushan S. The relative expression of ACCase gene decreased after spraying quizalofop–ethyl at 60 g ai ha−1in Geqiushan S, whereas it was almost not changed in Geqiushan R. From these results we concluded that plastid ACCase sensitivity change might be responsible for the resistance and gene overexpression does not play a role in this resistance.

De novo Assembly and Characterization of the Barnyardgrass (Echinochloa crus-galli) Transcriptome Using Next-Generation Pyrosequencing

BackgroundBarnyardgrass (Echinochloa crus-galli) is an important weed that is a menace to rice cultivation and production. Rapid evolution of herbicide resistance in this weed makes it one of the most difficult to manage using herbicides. Since genome-wide sequence data for barnyardgrass is limited, we sequenced the transcriptomes of susceptible and resistant barnyardgrass biotypes using the 454 GS-FLX platform.Results454 pyrosequencing generated 371,281 raw reads with an average length of 341.8 bp, which made a total length of 126.89 Mb (SRX160526). De novo assembly produced 10,142 contigs (∼5.92 Mb) with an average length of 583 bp and 68,940 singletons (∼22.13 Mb) with an average length of 321 bp. About 244,653 GO term assignments to the biological process, cellular component and molecular function categories were obtained. A total of 6,092 contigs and singletons with 2,515 enzyme commission numbers were assigned to 151 predicted KEGG metabolic pathways. Digital abundance analysis using Illumina sequencing identified 78,124 transcripts among susceptible, resistant, herbicide-treated susceptible and herbicide-treated resistant barnyardgrass biotypes. From these analyses, eight herbicide target-site gene groups and four non-target-site gene groups were identified in the resistant biotype. These could be potential candidate genes involved in the herbicide resistance of barnyardgrass and could be used for further functional genomics research. C4 photosynthesis genes including RbcS, RbcL, NADP-me and MDH with complete CDS were identified using PCR and RACE technology.ConclusionsThis is the first large-scale transcriptome sequencing of E. crus-galli performed using the 454 GS-FLX platform. Potential candidate genes involved in the evolution of herbicide resistance were identified from the assembled sequences. This transcriptome data may serve as a reference for further gene expression and functional genomics studies, and will facilitate the study of herbicide resistance at the molecular level in this species as well as other weeds.

Physiological and Molecular Basis of Acetolactate Synthase-Inhibiting Herbicide Resistance in Barnyardgrass (Echinochloa crus-galli)

Barnyardgrass biotypes from Arkansas (AR1 and AR2) and Mississippi (MS1) have evolved cross-resistance to imazamox, imazethapyr, and penoxsulam. Additionally, AR1 and MS1 have evolved cross-resistance to bispyribac-sodium. Studies were conducted to determine if resistance to acetolactate synthase (ALS)-inhibiting herbicides in these biotypes is target-site or non-target-site based. Sequencing and analysis of a 1701 base pair ALS coding sequence revealed Ala₁₂₂ to Val and Ala₁₂₂ to Thr substitutions in AR1 and AR2, respectively. The imazamox concentrations required for 50% inhibition of ALS enzyme activity in vitro of AR1 and AR2 were 2.0 and 5.8 times, respectively, greater than the susceptible biotype. Absorption of ¹⁴C-bispyribac-sodium, -imazamox, and -penoxsulam was similar in all biotypes. ¹⁴C-Penoxsulam translocation out of the treated leaf (≤2%) was similar among all biotypes. ¹⁴C-Bispyribac-treated AR1 and MS1 translocated 31- 43% less radioactivity to aboveground tissue below the treated leaf compared to the susceptible biotype. ¹⁴C-Imazamox-treated AR1 plants translocated 39% less radioactivity above the treated leaf and aboveground tissue below the treated leaf, and MS1 translocated 54 and 18% less radioactivity to aboveground tissue above and below the treated leaf, respectively, compared to the susceptible biotype. Phosphorimaging results further corroborated the above results. This study shows that altered target site is a mechanism of resistance to imazamox in AR2 and probably in AR1. Additionally, reduced translocation, which may be a result of metabolism, could contribute to imazamox and bispyribac-sodium resistance in AR1 and MS1.

Características micromorfológicas de biótipos de capim‑arroz resistente e suscetível ao quinclorac

O objetivo deste trabalho foi avaliar o efeito do quinclorac nas características anatômicas de folhas e raízes de biótipos de capim‑arroz (Echinochloa spp.), resistente e suscetível a este herbicida, por meio de avaliações micromorfométricas. As plantas foram cultivadas em vasos de plástico com capacidade de 250 cm³. As imagens dos cortes foram obtidas com microscópio de luz equipado com sistema U‑Photo, acoplado à câmera digital conectada a microcomputador. Na seção transversal da lâmina foliar e das raízes, utilizou-se objetiva de 10X, tendo-se avaliado 20 cortes/campos por planta, com dez medições por corte e quatro repetições por tratamento. Observaram-se diferenças entre os biótipos resistente e suscetível, tanto na ausência quanto na presença do quinclorac. O biótipo resistente apresenta lâmina foliar pouco afetada pelo herbicida; no entanto, há modificação na constituição dos tecidos radiculares com a formação mais acentuada de aerênquima. O biótipo suscetível também apresenta formação de aerênquima quando tratado com o quinclorac, mas em quantidade inferior à observada no biótipo resistente.

Does Resistance to Propanil or Clomazone Alter the Growth and Competitive Abilities of Barnyardgrass (Echinochloa crus-galli)?

Barnyardgrass biotypes resistant (R) to propanil (PR) or clomazone (CR) have been confirmed in rice production systems in Arkansas. However, it is not clear whether resistance to these herbicides impose any fitness cost on theRbiotypes compared to susceptible barnyardgrass (S). The overall objective of this experiment was to determine if the growth and competitiveness of barnyardgrass is altered by resistance to propanil or clomazone and to establish a competitive hierarchy among theS,PR, andCRbiotypes. A replacement series study was conducted in a greenhouse using five proportions ofSandRbiotypes (0 : 100, 25 : 75, 50 : 50, 75 : 25, and 100 : 0). The study was carried out in a completely randomized design (CRD) with four replications. The variables, including plant height, number of tillers, number of leaves, and shoot dry weight, were used for quantifying the differences in competitive abilities. Replacement series indices were calculated to explore the competitiveness. Expected (He) and observed (Ho) values for relative yield (RY) and relative yield total (RYT) were compared for number of tillers, number of leaves, and shoot dry weight for each biotype comparison. Other replacement series indices including competitive ratio (CR), relative crowding coefficient (RCC), and aggressiveness index (AI) also were calculated for these variables. The results showed that there were no major differences among theSandRbiotypes for these variables, indicating that in the absence of selection pressure, resistance to propanil or clomazone does not influence the growth and competitiveness of barnyardgrass. The findings will be useful for predicting the dynamics of resistant populations in the absence of herbicide selection and for designing suitable management strategies.