Experimental Methods for Estimation of Plant Fitness Costs Associated with Herbicide-Resistance Genes

Since the beginning of agriculture, crops have been exposed to recurrent invasion by weeds that can impose severe reductions in crop quality and yield. There have been continuing efforts to reduce the impacts of weeds on production. More than 40 yr ago, overreliance on herbicide technology to reduce weed infestations resulted in the selection of adaptive traits that enabled weed survival and reproduction under herbicide treatments (Délye et al. 2007; Powles and Yu 2010; Vila-Aiub et al. 2008). As a result, herbicide resistance in . 200 weed species has evolved worldwide (Heap 2013; Powles 2008). Resistant weeds are able to withstand the toxicity of herbicides because of the presence of resistance alleles originating from random DNA mutations (Powles and Yu 2010). These resistance alleles regulate a number of highly efficient, constitutive defense mechanisms that prevent herbicides from inhibiting key metabolic pathways. A set of defense mechanisms are involved in a reduction in herbicide that reaches the herbicide target protein (nontargetsite resistance) (Powles and Yu 2010). Mechanisms that (1) impair herbicide leaf uptake or translocation within plants via vacuolar sequestration or reduced cellular uptake, or (2) change the chemical properties of herbicides via herbicide-enhanced metabolism (detoxification) are included in this group (Ge et al. 2012; Nandula et al. 2008; Preston and Wakelin 2008; Preston et al. 1996; Sammons et al. 2010; Vila-Aiub et al. 2012; Wakelin et al. 2004). Another type of defense mechanism against herbicides involves a structural modification, via changes in the amino acid sequence, of the herbicide target protein, which minimizes herbicide binding (target-site resistance) (Powles and Yu 2010). Gene overexpression, resulting in the synthesis of excessive herbicide sensitive target protein because of promoter changes or gene amplification, is also regarded a target-site resistance defense mechanism (Dinelli et al. 2006; Gaines et al. 2010). Target and nontarget-site herbicide resistance alleles protect plants from fatal damage caused by herbicides. In other words, the presence of resistance alleles minimizes the plant fitness reduction expected from herbicide activity. Fitness can be defined as the average success in producing offspring contributing to the next generation by a particular phenotype relative to another phenotype (Crawley 1997; Primack and Hyesoon 1989; Scott et al. 2006). A simple way to represent plant fitness (W ) is given in Futuyma (2013):