Enhanced metabolism resistance

Plants possess a host of enzymes for the metabolism of xenobiotics and unwanted substances. It is these detoxifying enzymes that modify or break down herbicides once they enter a plant cell. The rate at which these enzymes carry out this task will determine whether a plant lives or dies, and is the main contributor to herbicide selectivity between crops and weed species (see Chapter 4 for more information about the enzyme systems involved).

If an individual weed biotype within a population has the ability to metabolise a herbicide at an increased rate, then it may survive a herbicide treatment. Such biotypes are described as possessing enhanced metabolism resistance (Figure 12.3). This type of resistance has recently been reviewed by Yuan et al. (2006).

The main enzymes involved in metabolism of xenobiotics in plants are the cytochrome P450s and glutathione ^-transferases (GSTs). These enzyme families have both been implicated in herbicide metabolism in tolerant crop and weed species and also in biotypes possessing enhanced metabolism resistance (Table 12.3). Enhanced GST activity has been correlated with resistance to isoproturon, clodinafop-propargyl and fenoxaprop-p- ethyl in A. myosuroides populations and offers a possible screening test for resistance in this and other species (Reade and Cobb, 2002). Increased GST and P450 activity may be due to a more active enzyme, increased transcription of gene(s) coding for the enzyme, or presence of more copies of the gene(s).

Susceptible

Plant death

Resistant

Resistant

additional structure(s)

Figure 12.3 Diagrammatic representation of Enhanced Metabolism Resistance. TS, target site; H, herbicide.

additional structure(s)

Figure 12.3 Diagrammatic representation of Enhanced Metabolism Resistance. TS, target site; H, herbicide.

Cytochrome P450 monooxygenases are a large and ubiquitous family of enzymes that carry out a number of reactions (see Chapter 4). Enhanced P450 activity has been reported in some weed biotypes that demonstrate herbicide resistance, implying that increased herbicide metabolism by P450s is the mechanism of herbicide resistance (Katagi and Mikami, 2000; Mougin et al.: 2001). Inhibitors of P450 activity (e.g. 1-aminobenzotria-zole) have been shown to reduce herbicide metabolism and also levels of herbicide resistance in some resistant populations (e.g. Singh et al.: 1998). It appears that in most cases the P450 is not induced in resistant biotypes by herbicide treatment but is consti-tutively expressed. This raises the interesting possibility that the protection mechanisms usually induced by herbicide application - often too late in the case of susceptible biotypes - are already being expressed in the resistant biotypes. Where P450s are responsible for resistance, then wide-ranging and unpredictable levels of cross-resistance can be encountered. This will be determined by the means by which a herbicide is metabolised rather than by its mode of action.

Glutathione S-transferases (GSTs, E.C. 2.5.1.18) are a superfamily of enzymes which catalyse the conjugation of a wide range of substrates, including some herbicides and

Table 12.3 Weed species for which GSTs and/or P450s have been implicated in herbicide resistance (from Devine and Preston, 2000).

Weed species

Herbicide(s)a

Proposed enzymatic system

Alopecurus myosuroides

Chlorotoluron

Pendimethalin

Diclofop-methyl

Fenoxaprop-P-ethyl

Propaquizafop

Chlorsulfuron

Cytochrome P450 monooxygenases/ glutathione S-transferase

Abutilon theophrasti

Atrazine

Glutathione S-transferase

Avena sterilis

Diclofop-methyl

Cytochrome P450 monooxygenases

Avena fatua

Triallate

Cytochrome P450?

Digitaria sanguinalis

Fluazifop-P-butyl

Unknown

Echinochloa colona

Propanil

Aryl acylamidase

Echinochloa crus-galli

Propanil

Aryl acylamidase

Hordeum leporinum

Fluazifop-P-butyl

Unknown

Diclofop-methyl

Fluazifop-P-butyl

Tralkoxydim

Chlorsulfuron

Metribuzin

Chlorotoluron

Cytochrome P450 monooxygenases/ unknown

Phalaris minor

Isoproturon

Cytochrome P450 monooxygenases

Stellaria media

Mecoprop

Cytochrome P450 monooxygenases

a 'P' in this column refers to herbicidally active isomer.

a 'P' in this column refers to herbicidally active isomer.

xenobiotics, to the tripeptide glutathione (GSH, y-Glu-Cys-Gly; see Chapter 4). Researchers have suggested a role for GSTs in the metabolism of atrazine (Anderson and Gronwald, 1991; Gray et al., 1996), alachlor, metolachlor and fluorodifen (Hatton et al., 1996). GST activity in a herbicide-resistant black-grass biotype has been reported to be approximately double that of herbicide-susceptible biotypes (Reade et al., 1997). GSTs may also play a role in protecting plants from damage from active oxygen species that result from the action of certain herbicides. In black-grass a GST possessing glutathione peroxidase activity has been identified which may undertake this role in the metabolism of organic hydroperoxides (Cummins et al., 1999). This raises the possibility of GSTs not only aiding in the metabolism of certain herbicides, but also in protecting the plant from the damage resulting from herbicide treatment. The diverse roles of GSTs in toxin metabolism, stress responses and secondary metabolism have been reviewed by Marrs (1996) .

Resistance to propanil in a biotype of Echinocloa colona is due to raised activities of aryl acylamidase. Inhibition of this enzyme in resistant biotypes increases their susceptibility and, as some of these inhibitors are also herbicides, have proved useful in control of resistant E. colona populations.

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