Sumatran Fleabane (Erigeron sumatrensis) Resistant to PSI-Inhibitor Herbicides and Physiological Responses to Paraquat

Herbicide-resistant weed management is one of the greatest agricultural challenges in crop production. Thus, the quick identification of resistant-herbicide weeds is extremely important for management. This study aimed to evaluate resistance to PSI-inhibitor herbicides (diquat) of Sumatran Fleabane [(Erigeron sumatrensis (Retz.) E.Walker)] and physiological response to paraquat application. The research was conducted with two E. sumatrensis biotypes, one susceptible and the other with multiple resistance to herbicides from five different modes of action (glyphosate, paraquat, diuron, saflufenacil, and 2,4-D). A dose-response assay was carried out to evaluate herbicide resistance to diquat in paraquat-resistant E. sumatrensis biotype. The enzymatic activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), hydrogen peroxide (H2O2) content, and chlorophyll a fluorescence were measured in both biotypes after paraquat (400 g ai ha−1) application. The dose-response assay confirmed resistance of E. sumatrensis to diquat with resistance factor levels of 26-fold and 6-fold for LD50 and GR50 values, respectively, compared with the susceptible biotype. The accumulation of H2O2 occurred faster in the paraquat-susceptible biotype than in the resistant ones. Paraquat treatment caused an increase in SOD and APX activity in the susceptible biotype, but antioxidant enzyme activities were unaffected by paraquat in the resistant one at 5 hours after application (HAA). Chlorophyll a fluorescence increased along the first 4 HAA in both resistant and susceptible biotypes. However, at 24 HAA the resistant biotype showed a decline in fluorescence close to untreated plants while susceptible one died, which can be used to diagnose paraquat resistance at 24 HAA. There is confirmed resistance to diquat in a paraquat-resistant E. sumatrensis biotype. The paraquat-resistant biotype does not induce antioxidative enzymes, as a possible mechanism of resistance to paraquat, but shows a fast recovery of photosynthesis and continuous growth when subjected to paraquat, while the paraquat-susceptible biotype does not survive.

Identification of a Paraquat Resistant Goosegrass (Eleusine indica) Population from a Central Alabama Vegetable Production Field

A goosegrass (Eleusine indica (L.) Gaertn.) population uncontrolled by paraquat (R) in a vegetable production field in St. Clair County, Alabama was collected in Summer 2019. Research was conducted to assess the level of resistance of the suspected resistant population compared to three population with no suspected paraquat resistance (S1, S2, and S3). Visual injury at all rating dates and biomass reduction at 28 days after treatment (DAT) of S populations occurred exponentially to increasing paraquat rates. S biotypes were injured greater than R at 3 DAT with biomass recovery at 28 DAT only occurring at rates < 0.28 kg ha−1. Plant death or biomass reduction did not occur for any rate at any date for R. Paraquat rates that induced 50% or 90% injury or reduced biomass 50% or 90% compared to the non-treated (I50 or I90, respectively) ranged from 10 to 124X higher I50 for R compared to S and 54 to 116X higher I90 for R compared to S biotypes. These data confirm a paraquat resistant E. indica biotype in Alabama providing additional germplasm for study of Resistance to photosystem I-electron diverting (PSI-ED) resistance mechanisms.

AtPQT11, a P450 enzyme, detoxifies paraquat via N-demethylation

Paraquat is one of the most widely used nonselective herbicides in agriculture. Due to its wide use, paraquat resistant weeds have emerged and is becoming a potential threat to agriculture. The molecular mechanisms of paraquat resistance in weeds remain largely unknown. Physiological studies indicated that the impaired translocation of paraquat and enhanced antioxidation could improve paraquat resistance in plants. However, the detoxification of paraquat via active metabolism by plants has not been reported to date. Here we report that an activated expression of At1g01600 encoding the P450 protein CYP86A4 confers paraquat resistance as revealed by the gain-of-function mutant paraquat tolerance 11D (pqt11D), in which a T-DNA with four 35S enhancers was inserted at 1646 bp upstream the ATG of At1g01600. The paraquat resistance can be recapitulated in Arabidopsis wild type by overexpressing AtPQT11 (At1g01600), while its knockout mutant is hypersensitive to paraquat. Moreover, AtPQT11 also confers paraquat resistance in E. coli when overexpressed. We further demonstrate that AtPQT11 has P450 enzyme activity that converts paraquat to N-demethyl paraquat nontoxic to Arabidopsis, therefore detoxifying paraquat in plants. Taken together, our results unequivocally demonstrate that AtPQT11/ CYP86A4 detoxifies paraquat via active metabolism, thus revealing a novel molecular mechanism of paraquat resistance in plants and providing a means potentially enabling crops to resist paraquat.

Enhanced MATE transporter DTX6/PQT15 confers paraquat resistance

ABSTRACTParaquat is one of the most widely used non-selective herbicides and has elicited the emergence of paraquat resistant weeds. However, the molecular mechanisms of paraquat resistance are not completely understood. Here we report an Arabidopsis gain-of-function mutant pqt15-D with significantly enhanced resistance to paraquat and the corresponding PQT15 encoding the MATE transporter DTX6. A point mutation at +932 bp in DTX6 causing the G311E amino acid residue change brings about the enhanced paraquat resistance of pqt15-D. Overexpression of DTX6/PQT15 in the wild type also confers strong paraquat resistance, whereas the DTX6/PQT15 knockout mutants exhibits hypersensitive phenotype to paraquat. Moreover, heterologous expression of DTX6 and DTX6-D in E. coli significantly enhances bacterial resistance to paraquat. DTX6/PQT15 mainly localizes in the plasma membrane as shown by DTX6-GFP and functions as a paraquat efflux transporter as demonstrated by paraquat efflux assays with isolated protoplasts and bacterial cells. Taken together, our results demonstrate that DTX6/PQT15 is an efflux transporter and confers paraquat resistance by exporting paraquat out of cytosol, therefore unraveling a molecular mechanism of paraquat resistance in higher plants and providing a promising candidate of generating paraquat resistance crops.