|VAH HORN, CHRISTOPER - Colorado State University
|WESTRA, PHIL - Colorado State University
Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2015
Publication Date: 8/27/2015
Citation: Nandula, V.K., Wright, A.A., Vah Horn, C., Molin, W.T., Westra, P., Reddy, K.N. 2015. Glyphosate resistance in giant ragweed (Ambrosia trifida L.) from Mississippi is partly due to reduced translocation. American Journal of Plant Sciences. 6:2104-2113.
Interpretive Summary: The rapid and widespread adoption of glyphosate-resistant crops, associated with the intense use of glyphosate and lack of rotation with non-glyphosate-resistant crops has resulted in evolution of several glyphosate-resistant weed species, including giant ragweed in Mississippi and several other states in the US. Glyphosate-resistant giant ragweed populations could reduce yield and quality of row crops such as corn, cotton, and soybean. Scientists from the Crop Production Systems Research Unit (USDA-ARS) and Colorado State University conducted studies to verify resistance to glyphosate and to understand why giant ragweed in Mississippi is resistant to glyphosate. A giant ragweed population in a glyphosate-resistant soybean field, Tunica County, Missisippi, was 1.5-fold more resistant to glyphosate than a sensitive population. The glyphosate-resistant giant ragweed population translocated less glyphosate compared to a susceptible population. No change in the DNA of an enzyme on which glyphosate is active was detected in the glyphosate-resistant population. These results greatly add to the knowledge of glyphosate resistance mechanisms in giant ragweed of which very less is understood.
Technical Abstract: A giant ragweed population from a glyphosate-resistant (GR) soybean field in Mississippi was suspected to be resistant to glyphosate. Greenhouse and laboratory studies were conducted to confirm and quantify the magnitude of glyphosate resistance in the giant ragweed population and to elucidate the physiological and molecular mechanisms of glyphosate resistance. Glyphosate dose response studies indicated that ED50 (effective dose required to reduce plant growth by 50%) values for glyphosate-resistant (GR-MS) and glyphosate-susceptible (GS-MS) biotypes, based on percent injury, were 0.52 and 0.34 kg ae ha-1 glyphosate, respectively, indicating a 1.5-fold level of resistance in GR-MS. The absorption pattern of 14C-glyphosate in the two giant ragweed biotypes was similar throughout the measured time course of 168 h after treatment (HAT). The amount of 14C-glyphosate that translocated out of the treated leaves of the GR-MS and GS-MS plants was similar up to 24 HAT. Thereafter, the GS-MS biotype translocated more (71% and 76% of absorbed at 48 and 96 HAT, respectively) 14C-glyphosate than the GR-MS biotype (44% and 66% of absorbed at 48 and 96 HAT, respectively) out of the treated leaf. No target site mutation was identified at the Pro106 location of the epsps gene of the GR-MS biotype. The mechanism of resistance to glyphosate in the giant ragweed population from Mississippi, at the least, is due to reduced glyphosate translocation.