Annual bluegrass (Poa annua) is considered a most problematic annual winter weed in managed turfgrass, such as golf courses (Cross et al., 2013). This weed has traditionally been controlled by herbicides; however, populations that are resistant to various herbicides have been identified and studied (Brosnan et al., 2015; Heap, 2017; Kelly et al., 1999; McElroy et al., 2013). In the United States, P. annua accounts for more than half of reported herbicide resistance in managed turfgrass system since the 1980s, including populations resistant to photosystem II inhibitors, microtubule inhibitors, 5-enolpyruvylshikimate-3-phosphate synthase inhibitors, and, most recently, ALS inhibitors (Heap, 2012). Since 2012, the number of biotypes of P. annua resistant to ALS inhibitors has been increasing faster than any other mechanisms of action, resulting in resistant populations in Tennessee, Alabama, South Carolina, and Georgia (Brosnan et al., 2015; Heap, 2012). Annual bluegrass biotypes resistant to ALS inhibitors have not been documented previously in Mississippi.
One Mississippi biotype, Reunion, collected on 14 Feb. 2014, from Reunion Golf and Country Club in Madison, MS, was reported initially to have a diminished response to ALS-inhibiting herbicides foramsulfuron, flazasulfuron, and trifloxysulfuron. ALS-inhibiting herbicides work by inhibiting ALS (EC 220.127.116.11), which is the first enzyme in the branched-chain amino acid synthesis pathway that produces leucine, isoleucine, and valine (Umbarger, 1978). Inhibiting this enzyme results in plant death resulting from a lack of these essential amino acids (Ray, 1984; Tranel and Wright, 2002). The most effective ALS-inhibiting herbicides for P. annua are in the sulfonylurea family, and they include herbicides such as foramsulfuron, trifloxysulfuron, and rimsulfuron (Toler et al., 2007). However, herbicides in the sulfonylurea family are also more susceptible to the development of herbicide-resistant biotypes (Tranel and Wright, 2002). One reason for the rapid development of P. annua biotypes resistant to ALS inhibitors on managed turfgrass is repeated use of herbicide without alternating the modes of action (Cross et al., 2013; McElroy et al., 2013). The most common mechanism of resistance to ALS inhibitors in plants is through a mutation of the ALS gene, usually through a single nucleotide point mutation that changes the amino acid sequence of the ALS protein (Cross et al., 2013; Tranel and Wright, 2002). There are six target sites in the ALS gene that, when mutated, commonly confer resistance to ALS inhibitors: Ala122, Pro197, Ala205, Asp376, Trp574, and Ser653 (Tranel and Wright, 2002). In 2011, the first P. annua biotype not controlled by ALS inhibitors was reported in Alabama. McElroy et al. (2013) reported that this biotype had an amino acid substitution of Trp574 to Leu in the ALS gene. This specific mutation has been correlated with the most resistant biotypes of P. annua (Cross et al., 2015).
Plant fitness can be defined as the capability of a phenotype to produce offspring successfully relative to another phenotype. The fitness cost associated with herbicide resistance is the reduction of plant fitness in a stress-free environment as a result of negative pleotropic effects of herbicide-resistant alleles (Powles and Yu, 2010). Parameters used most often to assess plant fitness associated with herbicide-resistant phenotypes are seed germination, longevity, dormancy, growth rate (plant height), competitive ability, biomass, and seed yield (Holt and Thill, 1994). ALS inhibitor-resistant Lactuca serriola plants containing the Pro197His allele decreased in frequency by as much as 86% over a 3-year period, primarily because of reduced shoot biomass compared with susceptible plants (Alcocer-Ruthling et al., 1992). Similarly, Amaranthus powellii, with the Trp574Leu allele, showed significantly smaller and distorted leaves compared with susceptible plants (Tardif et al., 2006). Roots and shoots of susceptible plants were more developed and had 34% greater biomass than resistant plants (Tardif et al., 2006). These studies indicate the adverse pleotropic effects of resistance-associated mutations on the growth and development of plants, thus reducing overall fitness.
Because certain resistant biotypes are found to have a fitness cost associated with them, the objectives of this study were 1) to confirm and determine the resistance level of the Mississippi biotype, Reunion, compared with a susceptible P. annua biotype; 2) to determine whether resistance is the result of to a target-site mutation in the ALS gene; and 3) and to determine whether resistance to ALS inhibitors in the resistant Reunion biotype is associated with any fitness penalty.
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