Research Article Received: 17 November 2008 Revised: 15 January 2009 Accepted: 20 January 2009 Published online in Wiley Interscience: 16 April 2009 (www.interscience.wiley.com) DOI 10.1002/ps.1760 Are herbicide-resistant crops the answer to controlling Cuscuta? Talia Nadler-Hassar, Dale L Shaner,∗ Scott Nissen, Phill Westra and Baruch Rubin Abstract BACKGROUND: Herbicide-resistant crop technology could provide new management strategies for the control of parasitic plants. Three herbicide-resistant oilseed rape (Brassica napus L.) genotypes were used to examine the response of attached Cuscuta campestris Yuncker to glyphosate, imazamox and glufosinate. Cuscata campestris was allowed to establish on all oilseed rape genotypes before herbicides were applied. RESULTS: Unattached seedlings of C. campestris, C. subinclusa Durand & Hilg. and C. gronovii Willd. were resistant to imazamox and glyphosate and sensitive to glufosinate, indicating that resistance initially discovered in C. campestris is universal to all Cuscuta species. Glufosinate applied to C. campestris attached to glufosinate-resistant oilseed rape had little impact on the parasite, while imazamox completely inhibited C. campestris growth on the imidazolinone-resistant host. The growth of C. campestris on glyphosate-resistant host was initially inhibited by glyphosate, but the parasite recovered and resumed growth within 3–4 weeks. CONCLUSION: The ability of C. campestris to recover was related to the quality of interaction between the host and parasite and to the resistance mechanism of the host. The parasite was less likely to recover when it had low compatibility with the host, indicating that parasite-resistant crops coupled with herbicide resistance could be highly effective in controlling Cuscuta. Published 2009 by John Wiley & Sons, Ltd. Keywords: Cuscuta; dodder; herbicide resistance; transgenic crops; amino acid inhibitors 1INTRODUCTION accessory pigments;10–12 however, these plastids are probably Members of the Cuscuta family are considered among the most more functional than previously thought with respect to protein damaging parasites worldwide.1 Cuscuta spp. are above-ground, synthesis.13 Understanding the importance of the chloroplast absolute parasites that must make contact with a susceptible host in a parasite’s metabolism is a significant issue, as pathways in order to survive. Once a rootless parasite seedling makes contact inhibited by some of the main herbicide families are located in the with a host, it develops haustoria2,3 and connects to the host’s chloroplast (e.g. photosynthesis, amino acid, chlorophyll and lipid vascular bundles,4 forming a highly efficient absorption system biosynthesis). that enables the parasite to divert resources from the host.2,5 The The three amino acid biosynthesis inhibitors examined in parasite can also divert and accumulate phloem-mobile herbicides this study all inhibit biosynthetic pathways located in the absorbedbythehost.Thecombinationofherbicide-resistantcrops chloroplast. Glyphosate inhibits aromatic amino acid biosynthesis and phloem-mobile herbicides has been used to control parasitic byinhibitingenolpyruvylshikimate-3-phosphatesynthase(EPSPS), plants such as Orobanche spp.6 and Striga spp.7 while a large group of acetolactate synthetase (ALS) inhibitors stop This same combination of technologies has been less successful the biosynthesis of branched chained amino acids. Glufosinate for control of Cuscuta spp. Cuscata campestris Yuncker was able inhibits glutamine synthetase, the initial enzyme involved in the to recover from glyphosate and sulfometuron applications when assimilation of inorganic nitrogen into organic forms.14 attached to herbicide-resistant sugar beet (Beta vulgaris L.).8 These Although C. campestris has not been reported as a problem in findings suggest that herbicide-resistant crop technology may oilseed rape, this crop was chosen as a model host because it not be as successful in managing Cuscuta spp. compared with its could be easily infested with Cuscuta spp., but more significant success for root parasites. This initial work also did not determine is the availability of glyphosate-, imidazolinone- and glufosinate- if the resistance of C. campestris to glyphosate and sulfometuron resistant varieties. The primary objective of this research was to resulted from metabolism, lack of translocation within the parasite, sequestration or insensitive sites of action in the chloroplast. The extent to which the basic physiology of Cuscuta spp. differs ∗ Correspondence to: Dale L Shaner, USDA-ARS, Water Management Research from non-parasitic, higher plants and the relative importance of Unit, 2150 Center Avenue Building D, Suite 320, Fort Collins, CO 80526, USA. the parasite’s own biosynthetic pathways have not been clearly E-mail: [email protected] 9 defined. Although Cuscuta spp. have functional chloroplasts, the Weed Science Lab, Biological Science and Pest Management, Colorado State 811 plastids contain only small amounts of chlorophyll and other University, Fort Collins, CO 80527, USA Pest Manag Sci 2009; 65: 811–816 www.soci.org This article is a US Government work and is in the public domain in the USA www.soci.org T Nadler-Hassar et al. compare the efficacy of glyphosate, imazamox and glufosinate Horticultural Products Co., USA). At 5 DAS, approximately 30 acid- for control of C. campestris while attached to herbicide-resistant scarified C. campestris seeds were sown close to the developing oilseed rape. In addition, the response of C. campestris, C. gronovii oilseed rape seedlings. No Cuscuta seeds were sown in the ◦ Willd. and C. subinclusa Durand & Hilg. to these herbicides as control pots. Pots were kept in the greenhouse at 21–27/14–18 C unattached seedlings was examined. (day/night), watered as needed and fertilized with 20-20-20 Peter’s fertilizer (The Scott’s Company, USA) once a week. Herbicides (commercial formulations) were applied to Cuscuta-infested and 2 MATERIALS AND METHODS non-infested oilseed rape with a laboratory chain-driven sprayer − 2.1 Plant material calibrated to deliver 200 L ha 1 of glyphosate (0, 188, 315, − − Seeds of Cuscuta campestris (collected in Colorado, USA), Cuscuta 750, 1500 g AE ha 1), imazamox (0, 10, 20, 40, 80 g AE ha 1) − gronovii (collected in Massachusetts, USA) and Cuscuta subinclusa or glufosinate (0, 50, 100, 200, 400 g AE ha 1). Pots containing (collected in California, USA) were grown and propagated on C. campestris-infested WT oilseed rape and parasite-infested oilseed rape in the greenhouse to maintain a homogeneous imidazolinone-resistant, glyphosate-resistant and glufosinate- supply of viable seeds. All Cuscuta seeds were cleaned and acid resistant oilseed rape were treated with imazamox, glyphosate scarified in phosphoric acid for 20 min to improve germination. and glufosinate respectively. Host and parasite development were Seeds of oilseed rape (cv. Hyola 420, Advanta Seeds, The monitored and graded before and after treatment. At the final Netherlands) with no herbicide resistance (WT) were used as assessment time, 25 days after treatment (DAT) for glufosinate, 34 herbicide-susceptible controls for imazamox and glufosinate DAT for imazamox and 41 DAT for glyphosate, the host plant and treatments, and seeds of a sorghum sudangrass [Sorghum bicolor parasite were scored from 0 (dead) to 5 (full vigor). The host plant (L.) Moench] hybrid were used as herbicide-susceptible controls and Cuscuta tissue were harvested and separated for fresh and dry for glyphosate and imazamox. Two imidazolinone-resistant (Hylite weight determinations. 289 CF, Advanta Seeds, The Netherlands, and 46A76 Pioneer HB, Canada), a glyphosate-resistant (RR-DKL-35-85 Dekalb, USA) and a glufosinate-resistant (InVigor 2373, Bayer CropScience, 2.4.2 Large pot assays Germany) oilseed rape were used as hosts for C. campestris in Rape seeds were sown in 4.8 L pots containing Metro Mix 200 and the herbicide efficacy assays and as herbicide-resistant references incubated under the same conditions as described above. Three for the seedling bioassay. weeks after sowing, the oilseed rape was parasitized by coiling twodetachedCuscutashootsegments(10 cmlong)aroundoilseed rape plants. Two weeks later, successfully parasitized plants were 2.2 Herbicides −1 −1 −1 sprayed with imazamox (20 g AE ha ), glyphosate (750 g AE ha ) Glyphosate-potassium 550 g AE L SL (Roundup WeatherMAX ; − or glufosinate (400 g AE ha 1). Treated pots were monitored to Monsanto, USA), imazamox-ammonium 120 g AE L−1 SL (Raptor; detect signs of recovery until 90 DAT. BASF, USA) and glufosinate-ammonium 120 g AE L−1 SL (Rely; Bayer CropScience, USA) were evaluated. A non-ionic surfactant − (Activator 90; Loveland Products, Inc., USA) was added at 2.5 g L 1 2.5 Statistical analysis to spray solution of all herbicides; in addition, ammonium sulphate Seedling dose–response assays were conducted in a completely −1 at 20 g L was added to all glufosinate applications. In order to randomized design with three replications. Using Sigma Plot 9.0 improve glufosinate absorption, pots were watered 1 h before software (Systate Software Inc., USA), the shoot and root elon- 15 treatment to increase humidity levels. gation data were analyzed using a sigmoidal logistic regression equation model.17 The calculated R2 values indicate the goodness 2.3 Seedling bioassay of fit of each calculated curve from which I50 (the rate causing 50% Dose–response assays were performed on the basis of the method reduction in tissue elongation) was predicted for each herbicide of Tal et al.16 Seeds were placed on sheets of germination paper and seed variety. (20 × 25 cm; Seedburo Equipment Co., USA) soaked in glyphosate Tissue fresh weight data collected for pot assays after treatment (0, 0.01, 0.05, 0.1, 0.5, 1.0, 5.0, 10, 50, 100 mM), imazamox (0, 0.01, with herbicides were analyzed by ANOVA, and means were 0.1, 1.0, 10, 100, 1000, 10 000, 100 000 µM) or glufosinate (0, 0.01, separated at the P = 0.05 level by Tukey’s multiple comparison 0.05, 0.1, 0.5, 1.0, 5.0, 10, 50, 100 mM) dilutions made up in 10.0 mM procedure using JMPIN 5.1 software (SAS Institute, Inc., USA).