Herbicide-Resistant Crops: Utilities and Limitations for Herbicide-Resistant Weed Management

Since 1996, genetically modified herbicide-resistant (HR) crops, particularly glyphosate-resistant (GR) crops, have transformed the tactics that corn, soybean, and cotton growers use to manage weeds. The use of GR crops continues to grow, but weeds are adapting to the common practice of using only glyphosate to control weeds. Growers using only a single mode of action to manage weeds need to change to a more diverse array of herbicidal, mechanical, and cultural practices to maintain the effectiveness of glyphosate. Unfortunately, the introduction of GR crops and the high initial efficacy of glyphosate often lead to a decline in the use of other herbicide options and less investment by industry to discover new herbicide active ingredients. With some exceptions, most growers can still manage their weed problems with currently available selective and HR crop-enabled herbicides. However, current crop management systems are in jeopardy given the pace at which weed populations are evolving glyphosate resistance. New HR crop technologies will expand the utility of currently available herbicides and enable new interim solutions for growers to manage HR weeds, but will not replace the long-term need to diversify weed management tactics and discover herbicides with new modes of action. This paper reviews the strengths and weaknesses of anticipated weed management options and the best management practices that growers need to implement in HR crops to maximize the long-term benefits of current technologies and reduce weed shifts to difficult-to-control and HR weeds

Purification and properties of an Arthrobacter oxydans P52 carbamate hydrolase specific for the herbicide phenmedipham and nucleotide sequence of the corresponding gene.

Arthrobacter oxydans P52 isolated from soil samples was found to degrade the phenylcarbamate herbicides phenmedipham and desmedipham cometabolically by hydrolyzing their central carbamate linkages. The phenylcarbamate hydrolase (phenmedipham hydrolase) responsible for the degradative reaction was purified to homogeneity. The enzyme was shown to be a monomer with a molecular weight of 55,000. A 41-kb wild-type plasmid (pHP52) was identified in A. oxydans P52, but not in a derivative of this strain that had spontaneously lost the ability to hydrolyze phenylcarbamates, indicating that the gene for phenylcarbamate degradation (pcd) is plasmid encoded. Determination of two partial amino acid sequences allowed the localization of the coding sequence of the pcd gene on a 3.3-kb PstI restriction fragment within pHP52 DNA by hybridization with synthetic oligonucleotides. The phenylcarbamate hydrolase was functionally expressed in Escherichia coli under control of the lacZ promoter after the 3.3-kb PstI fragment was subcloned into the vector pUC19. A stretch of 1,864 bases within the cloned Pst fragment was sequenced. Sequence analysis revealed an open reading frame of 1,479 bases containing the amino acid partial sequences determined for the purified enzyme. Sequence comparisons revealed significant homology between the pcd gene product and the amino acid sequences of esterases of eukaryotic origin. Subsequently, it was demonstrated that the esterase substrate p-nitrophenylbutyrate is hydrolyzed by phenmedipham hydrolase

Repression of Acetolactate Synthase Activity through Antisense Inhibition (Molecular and Biochemical Analysis of Transgenic Potato (Solanum tuberosum L. cv Desiree) Plants).

Acetolactate synthase (ALS), the first enzyme in the biosynthetic pathway of leucine, valine, and isoleucine, is the biochemical target of different herbicides. To investigate the effects of repression of ALS activity through antisense gene expression we cloned an ALS gene from potato (Solanum tuberosum L. cv Desiree), constructed a chimeric antisense gene under control of the cauliflower mosaic virus 35S promoter, and created transgenic potato plants through Agrobacterium tumefaciens-mediated gene transfer. Two regenerants revealed severe growth retardation and strong phenotypical effects resembling those caused by ALS-inhibiting herbicides. Antisense gene expression decreased the steady-state level of ALS mRNA in these plants and induced a corresponding decrease in ALS activity of up to 85%. This reduction was sufficient to generate plants almost inviable without amino acid supplementation. In both ALS antisense and herbicide-treated plants, we could exclude accumulation of 2-oxobutyrate and/or 2-aminobutyrate as the reason for the observed deleterious effects, but we detected elevated levels of free amino acids and imbalances in their relative proportions. Thus, antisense inhibition of ALS generated an in vivo model of herbicide action. Furthermore, expression of antisense RNA to the enzyme of interest provides a general method for validation of potential herbicide targets