41
chlorohydrolase encapsulated in organically modified silica gel. They reported that
atrazine biodegradation is highly dependent on the adsorption.
2.2.2 Biodegradation of S-Triazine Herbicides by Fungus
Several soil fungi including Aspergillus fumigatus, A. flavipes, A. ustus, Fusarium
oxysporum, F. roseum, F. moniliforme, Rhizopus stolonifer, Penicillium decumbens,
P. luteum, P. janthinellum, P. rugulosum, and Trichoderma viride are reported to
degrade atrazine by N-dealkylation of either alkylamino groups. They were unable
to cleave the triazine ring. Although dealkylation is a pathway in majority of the
fungal strains, formation of hydroxyatrazine was also observed in few fungal species such as P. luteum (Kaufman and Blake 1970). Other atrazine-degrading fungal
strains include white rot fungus Phanerochaete chrysosporium (Mougin et al. 1994)
and Pleurotus pulmonarius (Masaphy et al. 1993). Donnelly et al. (1993) studied
the atrazine degradation efficiency of Hymenoscyphus ericae, Oidiodendron griseum, Trappea darkeri, and Rhizopogon vinicolor and reported that with increase in
nitrogen concentration results in increased herbicide degradation. Penicillium
steckii DS6F is the first simazine-degrading fungus ever reported (Kodama et al.
2001). Szewczyk et al. (2018) reported the degradation of the ametryn by entomopathogenic fungi. Metarhizium brunneum leads to formation of 2-hydroxy atrazine,
ethyl hydroxylated ametryn, S-demethylated ametryn, and deethylametryn.
2.2.3 Biodegradation of S-Triazine Herbicides by Plants
In plants, three metabolic pathways are involved in atrazine transformation. The
major pathway of atrazine detoxification in some resistant weeds is glutathione conjugation in which the glutathione S-transferase displaces chlorine atom at 2-carbon
atom of atrazine (Lamoureux et al. 1970). The second mechanism is hydrolysis
where the chlorine atom in atrazine is replaced with a hydroxyl group. Resistance
of corn to atrazine and simazine was primarily attributed to 2- hydroxylation pathway (Hamilton and Moreland 1962). The third pathway is N-dealkylation, in which
cytochrome P450 monooxygenases remove the ethylamino and isopropyl amino
side chains. In pea and resistant sorghum, only the N-dealkylation pathway was
performed in which atrazine is degraded to desethylatrazine and desisopropylatrazine. The first instance of atrazine uptake and degradation by aboveground plant
biomass was shown in poplar trees (Burken and Schnoor 1997). In poplar trees, corn
(Zea mays L.), and sorghum (Sorghum vulgare Pers.), atrazine metabolism occurs
via 2-hydroxylation and N-dealkylation pathways (Shimabukuro 1967). Plant root
exudates influence the atrazine degradation through the enhancement of microbial
activity. Atrazine-contaminated soils planted with Pennisetum clandestinum showed
faster atrazine degradation than in unplanted soil (Singh et al. 2004b). Rhizosphere
2 Biodegradation and Bioremediation of S-Triazine Herbicides
chlorohydrolase encapsulated in organically modified silica gel. They reported that
atrazine biodegradation is highly dependent on the adsorption.
2.2.2 Biodegradation of S-Triazine Herbicides by Fungus
Several soil fungi including Aspergillus fumigatus, A. flavipes, A. ustus, Fusarium
oxysporum, F. roseum, F. moniliforme, Rhizopus stolonifer, Penicillium decumbens,
P. luteum, P. janthinellum, P. rugulosum, and Trichoderma viride are reported to
degrade atrazine by N-dealkylation of either alkylamino groups. They were unable
to cleave the triazine ring. Although dealkylation is a pathway in majority of the
fungal strains, formation of hydroxyatrazine was also observed in few fungal species such as P. luteum (Kaufman and Blake 1970). Other atrazine-degrading fungal
strains include white rot fungus Phanerochaete chrysosporium (Mougin et al. 1994)
and Pleurotus pulmonarius (Masaphy et al. 1993). Donnelly et al. (1993) studied
the atrazine degradation efficiency of Hymenoscyphus ericae, Oidiodendron griseum, Trappea darkeri, and Rhizopogon vinicolor and reported that with increase in
nitrogen concentration results in increased herbicide degradation. Penicillium
steckii DS6F is the first simazine-degrading fungus ever reported (Kodama et al.
2001). Szewczyk et al. (2018) reported the degradation of the ametryn by entomopathogenic fungi. Metarhizium brunneum leads to formation of 2-hydroxy atrazine,
ethyl hydroxylated ametryn, S-demethylated ametryn, and deethylametryn.
2.2.3 Biodegradation of S-Triazine Herbicides by Plants
In plants, three metabolic pathways are involved in atrazine transformation. The
major pathway of atrazine detoxification in some resistant weeds is glutathione conjugation in which the glutathione S-transferase displaces chlorine atom at 2-carbon
atom of atrazine (Lamoureux et al. 1970). The second mechanism is hydrolysis
where the chlorine atom in atrazine is replaced with a hydroxyl group. Resistance
of corn to atrazine and simazine was primarily attributed to 2- hydroxylation pathway (Hamilton and Moreland 1962). The third pathway is N-dealkylation, in which
cytochrome P450 monooxygenases remove the ethylamino and isopropyl amino
side chains. In pea and resistant sorghum, only the N-dealkylation pathway was
performed in which atrazine is degraded to desethylatrazine and desisopropylatrazine. The first instance of atrazine uptake and degradation by aboveground plant
biomass was shown in poplar trees (Burken and Schnoor 1997). In poplar trees, corn
(Zea mays L.), and sorghum (Sorghum vulgare Pers.), atrazine metabolism occurs
via 2-hydroxylation and N-dealkylation pathways (Shimabukuro 1967). Plant root
exudates influence the atrazine degradation through the enhancement of microbial
activity. Atrazine-contaminated soils planted with Pennisetum clandestinum showed
faster atrazine degradation than in unplanted soil (Singh et al. 2004b). Rhizosphere
2 Biodegradation and Bioremediation of S-Triazine Herbicides