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Glufosinate is an important and widely used non-selective herbicide active on a wide range of plant species. Evolution of resistance to glufosinate in weedy plant species (including the global weed Eleusine indica) is underway. Here, we established the molecular basis of target site glufosinate resistance in Eleusine indica. Full-length E. indica glutamine synthetase (GS) iso-genes (EiGS1-1 , 1-2 , 1-3 , and EiGS2) were cloned, and expression of EiGS1-1 and EiGS1-2 was higher than that of EiGS2. A novel point mutation resulting in a Ser59Gly substitution in EiGS1-1 was identified in glufosinate-resistant plants. Rice calli and seedlings transformed with the mutant EiGS1-1 gene were resistant to glufosinate. Purified mutant EiGS1-1 expressed in yeast was more tolerant to glufosinate than the wild-type variant. These transgenic results correlate with a more glufosinate-resistant GS in the crude tissue extract of resistant versus susceptible E. indica plants. Structural modelling of the mutant EiGS1-1 revealed that Ser59 is not directly involved in glufosinate binding but is in contact with some important binding residues (e.g. Glu297) and especially with Asp56 that forms an intratoroidal contact interface. Importantly, the same Ser59Gly mutation was also found in geographically isolated glufosinate-resistant populations from Malaysia and China, suggesting parallel evolution of this resistance mutation. [ABSTRACT FROM AUTHOR]

BACKGROUND Trifluralin is widely used in Australia as one of the important pre‐emergence herbicides to control annual ryegrass (Lolium rigidum Gaud.) populations. Trifluralin resistance evolution and mechanisms have been identified in some ryegrass populations. RESULTS: In this study, 21 putative resistant field survey populations from Western Australian were screened with trifluralin, and 90% (19 of 21) contained individuals surviving 480 g ha−1 trifluralin treatment. Twelve populations contained individuals possessing the known α‐tubulin resistance mutations at Val‐202, Thr‐239 and Arg‐243 in TUA4 (alpha‐tubulin 4 n), plus multiple potential resistance mutations in TUA4 pending genetic confirmation. Three populations had only individuals carrying newly identified (but uncharacterized) mutations in TUA3/TUA4. Radioactive work found that six populations evolved metabolic resistance to trifluralin, and at least four of them also possessed the known and/or putative target‐site mutations. CONCLUSION: These results confirm that a high incidence of resistance to the dinitroaniline herbicide (trifluralin) is present, and target‐site tubulin mutations make a major contribution to resistance in these annual ryegrass populations. Co‐evolution of both target‐site and non‐target‐site resistance to per‐emergence herbicides warrants diverse management tactics. [ABSTRACT FROM AUTHOR]

Glyphosate is the most widely used herbicide in world agriculture and for general vegetation control in a wide range of situations. Global and often intensive glyphosate selection of very large weedy plant populations has resulted in widespread glyphosate resistance evolution in populations of many weed species. Here, working with a glyphosate-resistant (GR) Echinochloa colona population that evolved in a Western Australia agricultural field, we identified an ATP-binding cassette (ABC) transporter (EcABCC8) that is consistently up-regulated in GR plants. When expressed in transgenic rice, this EcABCC8 transporter endowed glyphosate resistance. Equally, rice, maize, and soybean overexpressing the EcABCC8 ortholog genes were made resistant to glyphosate. Conversely, CRISPR/Cas9-mediated knockout of the EcABCC8 ortholog gene OsABCC8 increased rice susceptibility to glyphosate. Subcellular localization analysis and quantification of glyphosate cellular levels in treated ABCC8 transgenic rice plants and isolated leaf protoplasts as well as structural modeling support that EcABCC8 is likely a plasma membrane-localized transporter extruding cytoplasmic glyphosate to the apoplast, lowering the cellular glyphosate level. This is a report of a membrane transporter effluxing glyphosate in a GR plant species, and its function is likely conserved in crop plant species. [ABSTRACT FROM AUTHOR]

BACKGROUND Tubulin, the target site of dinitroaniline herbicides, is encoded by small gene families in plants. To better characterize the mechanisms of target‐site resistance to dinitroaniline herbicides in the globally important weedy species Lolium rigidum, attempts were made to amplify and sequence α‐tubulin transcripts. RESULTS: Four α‐tubulin isoforms (TUA1, TUA2, TUA3 and TUA4) were identified in L. rigidum. Variations in the number and sequence of transcripts encoding these α‐tubulin proteins were found in individuals from the two L. rigidum populations examined. Within and among populations, differences in the 5′‐ and 3′‐untranslated regions of cDNA in TUA3 and TUA4 were identified. Furthermore, a novel double mutation, Arg‐390‐Cys+Asp‐442‐Glu, in the TUA3 transcript was identified and has the potential to confer dinitroaniline resistance. CONCLUSION: This research reveals the complexity of the α‐tubulin gene family in individuals/populations of the cross‐pollinated weedy species L. rigidum, and highlights the need for better understanding of the molecular architecture of tubulin gene families for detecting resistance point mutations. Although TUA4 is a commonly expressed α‐tubulin isoform containing most frequently reported resistance mutations, other mutant tubulin isoforms may also have a role in conferring dinitroaniline resistance. [ABSTRACT FROM AUTHOR]

SUMMARY: Rapid and widespread evolution of multiple herbicide resistance in global weed species endowed by increased capacity to metabolize (degrade) herbicides (metabolic resistance) is a great threat to herbicide sustainability and global food production. Metabolic resistance in the economically damaging crop weed species Lolium rigidum is well known but a molecular understanding has been lacking. We purified a metabolic resistant (R) subset from a field evolved R L. rigidum population. The R, the herbicide susceptible (S) and derived F2 populations were used for candidate herbicide resistance gene discovery by RNA sequencing. A P450 gene CYP81A10v7 was identified with higher expression in R vs. S plants. Transgenic rice overexpressing this Lolium CYP81A10v7 gene became highly resistant to acetyl‐coenzyme A carboxylase‐ and acetolactate synthase‐inhibiting herbicides (diclofop‐methyl, tralkoxydim, chlorsulfuron) and moderately resistant to hydroxyphenylpyruvate dioxygenase‐inhibiting herbicide (mesotrione), photosystem II‐inhibiting herbicides (atrazine and chlorotoluron) and the tubulin‐inhibiting herbicide trifluralin. This wide cross‐resistance profile to many dissimilar herbicides in CYP81A10v7 transgenic rice generally reflects what is evident in the R L. rigidum. This report clearly showed that a single P450 gene in a cross‐pollinated weed species L. rigidum confers resistance to herbicides of at least five modes of action across seven herbicide chemistries. [ABSTRACT FROM AUTHOR]

BACKGROUND Barnyardgrass (Echinochloa spp.) is a global weed in rice fields. Quinclorac is commonly used to control barnyardgrass. However, due to persistent use, quinclorac resistance has evolved. We obtained quinclorac‐susceptible (QS) and ‐resistant (QR1, QR2) lines from the progeny of a single resistant E. crus‐pavonis for a resistance mechanism study. RESULTS: Line QR1 exhibited resistance to high quinclorac rates (up to 6400 g ha−1), whereas line QR2 exhibited a resistance/susceptibility segregation ratio of 3:1 at the field or lower rates (400, 100 g ha−1). Intriguingly, a lower level of 14C‐quinclorac metabolism and hence a higher level of 14C‐quinclorac translocation was observed in QR1 than QS plants. The basal expression levels of 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthase (ACS) and ACC oxidase 2 (ACO2) genes did not differ significantly between the QR1 and QS lines. However, more expression of ACS and ACO genes was induced by quinclorac treatment in QS than in QR1. Basal levels of β‐cyanoalanine synthase (β‐CAS) gene expression were similar in QS and QR1 plants, but a greater level of down‐regulation was detected in QS than in QR1 plants after quinclorac treatment. CONCLUSION: These results indicate QR plants are less responsive to quinclorac than QS plants in terms of up‐regulating quinclorac metabolism and ethylene synthesis. Resistance in this E. crus‐pavonis line is likely controlled by a single major gene, involving possibly an alteration in auxin signal perception/transduction to the ethylene biosynthesis pathway. The β‐CAS is unlikely to play a major role in quinclorac resistance in this particular population. [ABSTRACT FROM AUTHOR]