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.

Insights into the Role of Transcriptional Gene Silencing in Response to Herbicide-Treatments in Arabidopsis thaliana.

Herbicide resistance is broadly recognized as the adaptive evolution of weed populations to the intense selection pressure imposed by the herbicide applications. Here, we tested whether transcriptional gene silencing (TGS) and RNA-directed DNA Methylation (RdDM) pathways modulate resistance to commonly applied herbicides. Using Arabidopsis thaliana wild-type plants exposed to sublethal doses of glyphosate, imazethapyr, and 2,4-D, we found a partial loss of TGS and increased susceptibility to herbicides in six out of 11 tested TGS/RdDM mutants. Mutation in REPRESSOR OF SILENCING 1 (ROS1), that plays an important role in DNA demethylation, leading to strongly increased susceptibility to all applied herbicides, and imazethapyr in particular. Transcriptomic analysis of the imazethapyr-treated wild type and ros1 plants revealed a relation of the herbicide upregulated genes to chemical stimulus, secondary metabolism, stress condition, flavonoid biosynthesis, and epigenetic processes. Hypersensitivity to imazethapyr of the flavonoid biosynthesis component TRANSPARENT TESTA 4 (TT4) mutant plants strongly suggests that ROS1-dependent accumulation of flavonoids is an important mechanism for herbicide stress response in A. thaliana. In summary, our study shows that herbicide treatment affects transcriptional gene silencing pathways and that misregulation of these pathways makes Arabidopsis plants more sensitive to herbicide treatment.

Epigenetic regulation - contribution to herbicide resistance in weeds?

Continuous use of herbicides has resulted in the evolution of resistance to all major herbicide modes of action worldwide. Besides the well-documented cases of newly acquired resistance through genetic changes, epigenetic regulation may also contribute to herbicide resistance in weeds. Epigenetics involves processes that modify the expression of specific genetic elements without changes in the DNA sequence, and play an important role in re-programming gene expression. Epigenetic modifications can be induced spontaneously, genetically or environmentally. Stress-induced epigenetic changes are normally reverted soon after stress exposure, although in specific cases they can also be carried over multiple generations, thereby having a selective benefit. Here, we provide an overview of the basis of epigenetic regulation in plants and discuss the possible effect of epigenetic changes on herbicide resistance. The understanding of these epigenetic changes would add a new perspective to our knowledge of environmental and management stresses and their effects on the evolution of herbicide resistance in weeds. © 2017 Society of Chemical Industry.

Recurrent evolution of heat-responsiveness in Brassicaceae COPIA elements.

BACKGROUND: The mobilization of transposable elements (TEs) is suppressed by host genome defense mechanisms. Recent studies showed that the cis-regulatory region of Arabidopsis thaliana COPIA78/ONSEN retrotransposons contains heat-responsive elements (HREs), which cause their activation during heat stress. However, it remains unknown whether this is a common and potentially conserved trait and how it has evolved. RESULTS: We show that ONSEN, COPIA37, TERESTRA, and ROMANIAT5 are the major families of heat-responsive TEs in A. lyrata and A. thaliana. Heat-responsiveness of COPIA families is correlated with the presence of putative high affinity heat shock factor binding HREs within their long terminal repeats in seven Brassicaceae species. The strong HRE of ONSEN is conserved over millions of years and has evolved by duplication of a proto-HRE sequence, which was already present early in the evolution of the Brassicaceae. However, HREs of most families are species-specific, and in Boechera stricta, the ONSEN HRE accumulated mutations and lost heat-responsiveness. CONCLUSIONS: Gain of HREs does not always provide an ultimate selective advantage for TEs, but may increase the probability of their long-term survival during the co-evolution of hosts and genomic parasites.

Evolutionary and social consequences of introgression of nontransgenic herbicide resistance from rice to weedy rice in Brazil.

Several studies have expressed concerns about the effects of gene flow from transgenic herbicide-resistant crops to their wild relatives, but no major problems have been observed. This review describes a case study in which what has been feared in transgenics regarding gene flow has actually changed biodiversity and people's lives. Nontransgenic imidazolinone-resistant rice (IMI-rice) cultivars increased the rice grain yield by 50% in southern Brazil. This increase was beneficial for life quality of the farmers and also improved the regional economy. However, weedy rice resistant to imidazolinone herbicides started to evolve three years after the first use of IMI-rice cultivars. Population genetic studies indicate that the herbicide-resistant weedy rice was mainly originated from gene flow from resistant cultivars and distributed by seed migration. The problems related with herbicide-resistant weedy rice increased the production costs of rice that forced farmers to sell or rent their land. Gene flow from cultivated rice to weedy rice has proven to be a large agricultural, economic, and social constraint in the use of herbicide-resistant technologies in rice. This problem must be taken into account for the development of new transgenic or nontransgenic rice technologies.