Resistance to Herbicides in the Model Organisms Saccharomyces cerevisiae and Arabidopsis thaliana: the Involvement of Multidrug Resistance Transporters

Herbicides are agrochemicals that control the growth of undesired weeds, bringing about a significant overall increase in crop productivity. Herbicide resistance, the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type, is a weed physiological characteristic (Twonson, 1997). This trait has the potential to cause not only large economic losses in agricultural production, forestry and landscaping, but also deleterious effects on the environment and human health as a result of rising herbicide application rates (Hayes & Wolf, 1997). On the other hand, crops of agronomic relevance exhibiting chemical stress resistance are highly desirable and can be obtained through genetic manipulation, based on the knowledge gathered on the underlying mechanisms. Emergence of multidrug resistance (MDR), that is, the simultaneous acquisition of resistance to a wide range of structurally and functionally unrelated cytotoxic chemicals, is found in a wide variety of organisms, from bacteria to mammals. The widespread use of herbicides has led, since the early 1970s, to a growing number of resistant weed species and biotypes that possess multiple resistance to various classes of herbicides (Hayes & Wolf, 1997). Indeed, herbicide application is one of the most important selective forces acting on a weed community in an agroecosystem (Owen & Zelaya, 2005). To date, and particularly during the past two decades, numerous cases of herbicide resistant weed species have been described, in at least 60 different countries (www.weedscience.org). Table 1 shows the number of cases of single and multiple herbicide resistance development in weed species registered in the last 30 years. Among them there are several weed strains that have acquired simultaneous resistance to herbicides with different modes of action (Table 1). For example, in 1982 in South Australia a strain of Lolium rigidum (rigid ryegrass), a monocot weed that infests barley and wheat fields, was registered as having developed resistance to multiple herbicides, exhibiting seven diverse modes of action (from the