Little evidence exists to indicate enhanced compartmentalisation (Figure 12.4) as a resistance mechanism and what is known suggests it may act in conjunction with enhanced metabolism mechanisms. An example of this is the identification of an ATP-binding cassette (ABC) transporter which moves xenobiotic substances from the cytoplasm into the cell vacuole once they have been conjugated to glutathione. One such ATP-dependent glutathione ^-conjugate pump has been identified in barley (Hordeum sativum) (Martinola et al., 1993). Conjugation of the herbicide metolachlor to glutathione in this system results it its movement across the tonoplast into the vacuole. Similar ABC transporters have been reported to be responsible for the movement of chloroacetanilide and triazine herbicides into vacuoles (Ishikawa et al., 1997). Where conjugation to glutathione forms part of the metabolism of a herbicide, then enhanced activity or the presence of such an ABC transporter would remove herbicides from their site of action quicker, perhaps resulting in plant survival. Alternatively, an increase in the number of ABC transporters might also result in resistance. Similar transport systems for substances that have been conjugated to sugars have also been postulated as a means by which herbicides are removed from the cytosol. Yuan et al. (2006) have recently reviewed the role of enhanced compartmentalisation as a mechanism of herbicide resistance.
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