Pesticide Metabolism in Plants
45
The formation of the N-glucoside and glycosyl ester of chloramben appears to
be a significant mechanism for chloramben selectivity (Frear, 1975; Frear et al.,
1978). Enzymatic hydrolysis of propanil (Frear and Still, 1968; Still and Kuzerian, 1967) in rice yielded 3,4-dichloroaniline (Fig. 2.18). This metabolite was
subsequently glucosylated to the N-(3,4-dichlorophenyl) glucosylamine (Still,
1968). The 3,4-dichloroaniline appears to be further metabolized to insoluble
lignin or ligninlike products (Balba and Still, 1977; Yih et al., 1968).
The formation of glucose esters of pesticides and plant growth regulators
appears to be an increasingly important mechanism for the bioregulation of plant
hormone levels and the selective toxicity of some herbicides. The glucosyl esters
of 2,4-0 (Thomas et al., 1964) and indole acetic acid (IAA) (Zenk, 1961) have
been identified (Fig. 2.19). Many phytotoxic forms of pesticides are acids or
readily hydrolyzable esters. The glucose ester of chloramben (Fig. 2.19) (Frear
et al., 1978), the complex glucosyl ester of flamprop-methyl (Dutton et al.,
1976), and the apparent glucose ester of diclofop-methyl (Fig. 2.19) (Shimabukuro et al., 1979) have been identified. The identification of the glycosyl
moiety of the diclofop-methyl metabolite is only tentative. It may be similar to
the complex glucosyl ester of flamprop-methyl (Fig. 2.20).
A large proportion of the IAA is bound as inactive glucose ester conjugates
(Bandurski and Schulze, 1977). Changes in free IAA and IAA-ester levels with
concomitant changes in the growth rate of com seedlings have been demonstrated
(Bandurski et al., 1977). The N-glucosyl chloramben was not hydrolyzed in
plant tissues, but the glucose ester of chloramben was hydrolyzed readily to
regenerate chloramben (Frear et al., 1978). Very little of either the methyl ester
or the acid of diclofop-methyl and chlorfenprop-methyl remained in resistant and
susceptible plants (Fedtke and Schmidt, 1977; Shimabukuro et al., 1979). However, a large concentration of an apparent glucose ester conjugate of diclofop
accumulated in susceptible wild oats, whereas resistant wheat formed predominantly phenolic conjugates of the aryl hydroxylated metabolite (Fig. 2.2) (Shimabukuro et al., 1979). The regeneration of the acid, diclofop, from the glucose
ester has not been demonstrated, but a bioregulation mechanism similar to that
of IAA and chloramben may also be an important selective mechanism for
diclofop-methyl. The glucose ester of chloramben was identified as the a-anomer
and not the (3-anomer as with most glucosides from plants (Frear et al., 1978).
The physiological significance of the a-configuration and the mechanism of its
formation have not been determined.
The gentiobioside of diphenamid (Hodgson et al., 1973) and several malonate hemiesters of (3-glucosides have been reported (Dutton et al., 1976; Hoffer
and Hodgson, 1978; Shimabukuro et al., 1975). The proposed biosynthetic
pathways for these complex glycosides include reactions catalyzed by UDPglucosyl transferase and malonyl CoA transferase (Frear, 1976). Diphenamid
was oxidized to its N-hydroxymethyl-N-methyl derivative (Fig. 2.9) and con-
45
The formation of the N-glucoside and glycosyl ester of chloramben appears to
be a significant mechanism for chloramben selectivity (Frear, 1975; Frear et al.,
1978). Enzymatic hydrolysis of propanil (Frear and Still, 1968; Still and Kuzerian, 1967) in rice yielded 3,4-dichloroaniline (Fig. 2.18). This metabolite was
subsequently glucosylated to the N-(3,4-dichlorophenyl) glucosylamine (Still,
1968). The 3,4-dichloroaniline appears to be further metabolized to insoluble
lignin or ligninlike products (Balba and Still, 1977; Yih et al., 1968).
The formation of glucose esters of pesticides and plant growth regulators
appears to be an increasingly important mechanism for the bioregulation of plant
hormone levels and the selective toxicity of some herbicides. The glucosyl esters
of 2,4-0 (Thomas et al., 1964) and indole acetic acid (IAA) (Zenk, 1961) have
been identified (Fig. 2.19). Many phytotoxic forms of pesticides are acids or
readily hydrolyzable esters. The glucose ester of chloramben (Fig. 2.19) (Frear
et al., 1978), the complex glucosyl ester of flamprop-methyl (Dutton et al.,
1976), and the apparent glucose ester of diclofop-methyl (Fig. 2.19) (Shimabukuro et al., 1979) have been identified. The identification of the glycosyl
moiety of the diclofop-methyl metabolite is only tentative. It may be similar to
the complex glucosyl ester of flamprop-methyl (Fig. 2.20).
A large proportion of the IAA is bound as inactive glucose ester conjugates
(Bandurski and Schulze, 1977). Changes in free IAA and IAA-ester levels with
concomitant changes in the growth rate of com seedlings have been demonstrated
(Bandurski et al., 1977). The N-glucosyl chloramben was not hydrolyzed in
plant tissues, but the glucose ester of chloramben was hydrolyzed readily to
regenerate chloramben (Frear et al., 1978). Very little of either the methyl ester
or the acid of diclofop-methyl and chlorfenprop-methyl remained in resistant and
susceptible plants (Fedtke and Schmidt, 1977; Shimabukuro et al., 1979). However, a large concentration of an apparent glucose ester conjugate of diclofop
accumulated in susceptible wild oats, whereas resistant wheat formed predominantly phenolic conjugates of the aryl hydroxylated metabolite (Fig. 2.2) (Shimabukuro et al., 1979). The regeneration of the acid, diclofop, from the glucose
ester has not been demonstrated, but a bioregulation mechanism similar to that
of IAA and chloramben may also be an important selective mechanism for
diclofop-methyl. The glucose ester of chloramben was identified as the a-anomer
and not the (3-anomer as with most glucosides from plants (Frear et al., 1978).
The physiological significance of the a-configuration and the mechanism of its
formation have not been determined.
The gentiobioside of diphenamid (Hodgson et al., 1973) and several malonate hemiesters of (3-glucosides have been reported (Dutton et al., 1976; Hoffer
and Hodgson, 1978; Shimabukuro et al., 1975). The proposed biosynthetic
pathways for these complex glycosides include reactions catalyzed by UDPglucosyl transferase and malonyl CoA transferase (Frear, 1976). Diphenamid
was oxidized to its N-hydroxymethyl-N-methyl derivative (Fig. 2.9) and con-