Current status of community resources and priorities for weed genomics research.

Weeds are attractive models for basic and applied research due to their impacts on agricultural systems and capacity to swiftly adapt in response to anthropogenic selection pressures. Currently, a lack of genomic information precludes research to elucidate the genetic basis of rapid adaptation for important traits like herbicide resistance and stress tolerance and the effect of evolutionary mechanisms on wild populations. The International Weed Genomics Consortium is a collaborative group of scientists focused on developing genomic resources to impact research into sustainable, effective weed control methods and to provide insights about stress tolerance and adaptation to assist crop breeding.

Recurrent Selection of Echinochloa crus-galli with a Herbicide Mixture Reduces Progeny Sensitivity.

Herbicide mixtures are used to increase the spectrum of weed control and to manage weeds with target-site resistance to some herbicides. However, the effect of mixtures on the evolution of herbicide resistance caused by enhanced metabolism is unknown. This study evaluated the effect of a fenoxaprop-p-ethyl and imazethapyr mixture on the evolution of herbicide resistance in Echinochloa crus-galli using recurrent selection at sublethal doses. The progeny from second generations selected with the mixture had lower control than parental plants or the unselected progeny. GR50 increased 1.6- and 2.6-fold after two selection cycles with the mixture in susceptible (POP1-S) and imazethapyr-resistant (POP2-IR) biotypes, respectively. There was evidence that recurrent selection with this sublethal mixture had the potential to evolve cross-resistance to diclofop, cyhalofop, sethoxydim, and quinclorac. Mixture selection did not cause increased relative expression for a set of analyzed genes (CYP71AK2, CYP72A122, CYP72A258, CYP81A12, CYP81A14, CYP81A21, CYP81A22, and GST1). Fenoxaprop, rather than imazethapyr, is the main contributor to the decreased control in the progenies after recurrent selection with the mixture in low doses. This is the first study reporting the effect of a herbicide mixture at low doses on herbicide resistance evolution. The lack of control using the mixture may result in decreased herbicide sensitivity of the weed progenies. Using mixtures may select important detoxifying genes that have the potential to metabolize herbicides in patterns that cannot currently be predicted. The use of fully recommended herbicide rates in herbicide mixtures is recommended to reduce the risk of this type of resistance evolution.

The safener isoxadifen-ethyl confers fenoxaprop-p-ethyl resistance on a biotype of Echinochloa crus-galli.

BACKGROUND: Some herbicides are commercially formulated with safeners to increase crop selectivity. Fenoxaprop-p-ethyl is formulated with the safener isoxadifen-ethyl for Echinochloa crus-galli control in rice. Safeners act on crops by increasing herbicide metabolism, but this effect may also occur in weeds. The objective of this study was to investigate the effect of the safener isoxadifen-ethyl on the resistance to fenoxaprop-p-ethyl in a biotype of E. crus-galli. RESULTS: A screening of 52 biotypes identified lack of control in the biotype SANTPAT-R treated with the recommended dose of 69 g ha-1 of the commercial formulation of fenoxaprop-p-ethyl with the safener isoxadifen-ethyl. While this biotype survived doses greater than 2208 g ha-1 of the formulation fenoxaprop-p-ethyl + isoxadifen-ethyl, it was killed with 69 g ha-1 of fenoxaprop-p-ethyl without the safener. A glutathione-s-transferase (GST) enzymes inhibitor reduced the resistance factor in two dose-response curves. A minor effect of a CytP450 inhibitor was observed. The previous spraying of the safener isoxadifen-ethyl followed by fenoxaprop-p-ethyl induced survival in the resistant but not in the susceptible biotype. The GST1 and GSTF1 genes were up-regulated in the resistant biotype. ACCase gene mutations were not found, and no cross-resistance to other ACCase inhibitors was identified. CONCLUSION: The safener isoxadifen-ethyl present in the commercial herbicide formulation of fenoxaprop-p-ethyl is associated with resistance in the E. crus-galli SANTPAT-R biotype. This resistance is related with herbicide metabolization mediated by GST pathways. This is the first field-selected weed biotype with herbicide resistance due to safener presence in the sprayed formulation. © 2022 Society of Chemical Industry.

Genomic insights into the evolution of Echinochloa species as weed and orphan crop.

As one of the great survivors of the plant kingdom, barnyard grasses (Echinochloa spp.) are the most noxious and common weeds in paddy ecosystems. Meanwhile, at least two Echinochloa species have been domesticated and cultivated as millets. In order to better understand the genomic forces driving the evolution of Echinochloa species toward weed and crop characteristics, we assemble genomes of three Echinochloa species (allohexaploid E. crus-galli and E. colona, and allotetraploid E. oryzicola) and re-sequence 737 accessions of barnyard grasses and millets from 16 rice-producing countries. Phylogenomic and comparative genomic analyses reveal the complex and reticulate evolution in the speciation of Echinochloa polyploids and provide evidence of constrained disease-related gene copy numbers in Echinochloa. A population-level investigation uncovers deep population differentiation for local adaptation, multiple target-site herbicide resistance mutations of barnyard grasses, and limited domestication of barnyard millets. Our results provide genomic insights into the dual roles of Echinochloa species as weeds and crops as well as essential resources for studying plant polyploidization, adaptation, precision weed control and millet improvements.

The safener isoxadifen does not increase herbicide resistance evolution in recurrent selection with fenoxaprop.

Safeners are chemical compounds used to improve selectivity and safety of herbicides in crops by activating genes that enhance herbicide metabolic detoxification. The genes activated by safeners in crops are similar to the genes causing herbicide resistance through increased metabolism in weeds. This work investigated the effect of the safener isoxadifen-ethyl (IS) in combination with fenoxaprop-p-ethyl (FE) on the evolution of herbicide resistance in Echinochloa crus-galli under recurrent selection. Reduced susceptibility was observed in the progeny after recurrent selection with both FE alone and with FE + IS for two generations (G2) compared to the parental population (G0). The resistance index found in G2 after FE + IS selection was similar as when FE was used alone, demonstrating that the safener did not increase the rate or magnitude of herbicide resistance evolution. G2 progeny selected with FE alone and the combination of FE + IS had increased survival to herbicides from other mechanisms of action relative to the parental G0 population. One biotype of G2 progeny had increased constitutive expression of glutathione-S-transferase (GST1) after recurrent selection with FE + IS. G2 progeny had increased expression of two P450 genes (CYP71AK2 and CYP72A122) following treatment with FE, while G2 progeny had increased expression of five P450 genes (CYP71AK2, CYP72A258, CYP81A12, CYP81A14 and CYP81A21) after treatment with FE + IS. Repeated selection with low doses of FE with or without the safener IS decreased E. crus-galli control and showed potential for cross-resistance evolution. Addition of safener did not further decrease herbicide sensitivity in second generation progeny; however, the recurrent use of safener in combination with FE resulted in safener-induced increased expression of several CYP genes. This is the first report using safener as an additional factor to study herbicide resistance evolution in weeds under experimental recurrent selection.

Negative cross-resistance to clomazone in imazethapyr-resistant Echinochloa crus-galli caused by increased metabolization.

Herbicide resistance is frequently reported in E. crus-galli globally with target and non-target site resistance mechanism to acetolactate synthase (ALS)-inhibiting herbicides. However, resistance to certain herbicides can result in increased sensitivity to other herbicides, a phenomenon called negative cross-resistance. The objective of this study is to identify the occurrence of negative cross-resistance (NCR) to the pro-herbicide clomazone in populations of E. crus-galli resistant to ALS inhibitors due to increased metabolization. Clomazone dose-response curves, with and without malathion, were performed in imazethapyr-resistant and -susceptible E. crus-galli biotypes. CYPs genes expression and antioxidant enzymes activity were also evaluated. The effective dose to reduce 50% (ED50) of dry shoot weight obtained in the clomazone dose-response curves of the metabolic based imazethapyr-resistant and -susceptible biotypes groups were 22.712 and 58.745 g ha-1, respectively, resulting in a resistance factor (RF) of 0.37, indicating the occurrence of NCR. The application of malathion prior to clomazone increased the resistance factor from 0.60 to 1.05, which indicate the reversion of the NCR. Some CYP genes evaluated were expressed in a higher level, ranging from 2.6-9.1 times according to the biotype and the gene, in the imazethapyr-resistant than in -susceptible biotypes following clomazone application. Antioxidant enzyme activity was not associated with NCR. This study is the first report of NCR directly related to the mechanism of resistance increased metabolization in plants. The occurrence of NCR to clomazone in E. crus-galli can help delay the evolution of herbicide resistance.

Allelopathic effect of aqueous extracts of different organs of three sunflower cultivars on germination of radish / Efeito alelopático de extratos aquosos de diferentes órgãos de três cultivares de girassol sobre a germinação de nabo

This study aimed to evaluate the allelopathic potential of aqueous extracts of different organs of three sunflower cultivars on the germination and initial development of radish. The study was conducted in a Laboratory of Plant Physiology of the Federal University of Santa Maria, Brazil. The design was completely randomized in a threefactor scheme with four replications. The first factor consisted of three sunflower cultivars: Olisun 3, Olisun 5, and Aguará 6. The second factor consisted of different organs: leaves, stems, and roots. The third factor consisted of the extracts concentrations: 0, 25, 50, 75, and 100%. The experimental unit (EU) used was gearboxes with two sheets of germitest paper that were moistened with the extracts. The EUs were placed in BOD germination chamber at 25 °C. Daily count of germinated seeds was performed. The radicle length and hypocotyl were measured on the 10th day. The variables analyzed included germination (G); germination speed index (GSI); radicle length (RL); and hypocotyl length (HL). Sunflowers have allelopathic potential on the radish. The extract concentration of 75% of all organs and cultivars were sufficient to significantly reduce the variables. The allelopathic activity differs between organs (leaves, stems, and roots) and sunflower cultivars. The amount of allelopathic effects vary in the order of root, stem, and then leaf. Extracts from the roots showed the greatest allelopathic effect on germination and initial development on the radish; however, this depends on the cultivar used. Field studies should be performed to verify such allelopathic activities.

Este estudo teve como objetivo avaliar o potencial alelopático de extratos aquosos de diferentes órgãos de três cultivares de girassol sobre a germinação e o desenvolvimento inicial de rabanete. O estudo foi realizado em Laboratório de Fisiologia Vegetal da Universidade Federal de Santa Maria, Brasil. O projeto foi inteiramente casualizado, em um esquema trifatorial, com quatro repetições. O primeiro fator consistiu de três cultivares de girassol: Olisun 3, Olisun 5, e Aguará 6. O segundo fator consistiu de diferentes órgãos: folhas, caules e raízes. O terceiro fator consistia nas concentrações dos extratos: 0, 25, 50, 75, e 100%. A unidade experimental (UE) utilizada foi caixas do tipo gerbox, com duas folhas de papel germitest na base, que foram umedecidas com os extratos. As EUs foram colocadas em câmara de germinação BOD a 25 °C. Diariamente após semeadura realizou-se a contagem diária de sementes germinadas. O comprimento da raiz e do hipocótilo das plântulas foram medidas no décimo dia. As variáveis analisadas foram: germinação (G); índice de velocidade de germinação (IVG); comprimento radicular (CR); e comprimento de hipocótilo (CH). Girassóis apresentam potencial alelopático sobre o nabo. A concentração do extrato de 75% de todos os órgãos e cultivares de girassóis foram suficientes para reduzir significativamente as variáveis, em comparação com a testemunha. A atividade alelopática difere entre órgãos (folhas, caules e raízes) e cultivares de girassol. A quantidade de efeitos alelopáticos varia na ordem de raiz, caule, folha. Extratos das raízes apresentam maior efeito alelopático na germinação e desenvolvimento inicial sobre o nabo; no entanto, isso depende da cultivar usada. Estudos a campo devem ser realizados para verificar tais atividades alelopáticas.