84
been investigated intensively and extensively for their widespread application in
field biocontrol.
Carboxylesterases are enzymes with very broad substrate specificity and are
remarkable candidates in the decomposition of organophosphate pesticides.
Pyrethroids and malathion are examples of this. This group of enzymes can catalyse
the breakage of ester, amide and thioester linkages. Singh et al. (2012a, b) carried
out genus-level identification of some bacteria which are able to degrade malathion
effectively in liquid culture with the enzyme carboxylesterase. Lysinibacillus sp.
KB1, Brevibacillus sp. KB2 and Bacillus cereus PU are these strains and
Lysinibacillus sp. KB1 could degrade 20% malathion and 47% malaoxon. For malaoxon 72.20% and for malathion 36.22% were utilised by Brevibacillus sp. strain
KB2. Bacillus cereus PU showed the highest degree of degradation with 87.40% of
malaoxon and 49.31% of malathion. The incubation time was 7 days for the experiments. Wu et al. (2006) reported that Klebsiella sp. strain ZD112 could produce an
esterase-degrading pyrethroid (EstP) and cloned the same gene in E. coli. They
found that the purified EstP could hydrolyse malathion, pesticides of the pyrethroid
group and rho-nitrophenyl esters of some fatty acids. Cloning of pyrethroidhydrolysing carboxylesterases gene was also reported by Wang et al. (2009) and
Zhai et al. (2012) from Sphingobium sp. strain JZ-1 and Ochrobactrum anthropi
YZ-1.
3.4
Enzymes in Biocontrol
Accelerated use of agrochemicals has adversely affected nature and has resulted in
the emergence of a more acceptable and environment-friendly approach such as
biocontrol strategy (Harman et al. 2010). Biocontrol agents include actinomycetes,
bacteria and fungi, and may act on pathogens through antagonism, parasitism or
antimicrobial compounds or by the induction of systemic resistance representing an
environment-friendly approach to reduce losses due to pests and diseases or as an
alternative to agrochemicals (Lugtenberg et al. 2002; Whipps 2001).
In the natural ecosystem antagonistic microorganisms have critical roles in keeping the balance of microbial habitats. This antagonistic property can be exploited
for biological control (Henis and Chet 1975). Fungal pathogens are effectively
restricted by degrading their cell walls containing chitin and β-1,3-glucans (Lam
and Gaffney 1993). There are reports about members of bacteria and fungi that host
enzymes that can hydrolyse glucans, protein, chitin, etc. Chitinases, glucanases and
proteases are the major hydrolytic enzymes involved in biological control mechanisms. Abraham (2015) reported that even the tough structures like zoospores,
conidia, chlamydospores, etc. could be destabilised by enzymes like chitinases.
Extracellular proteases and ß-1,3-glucanases were also able to degrade spores.
I.C. Nair and K. Jayachandran
been investigated intensively and extensively for their widespread application in
field biocontrol.
Carboxylesterases are enzymes with very broad substrate specificity and are
remarkable candidates in the decomposition of organophosphate pesticides.
Pyrethroids and malathion are examples of this. This group of enzymes can catalyse
the breakage of ester, amide and thioester linkages. Singh et al. (2012a, b) carried
out genus-level identification of some bacteria which are able to degrade malathion
effectively in liquid culture with the enzyme carboxylesterase. Lysinibacillus sp.
KB1, Brevibacillus sp. KB2 and Bacillus cereus PU are these strains and
Lysinibacillus sp. KB1 could degrade 20% malathion and 47% malaoxon. For malaoxon 72.20% and for malathion 36.22% were utilised by Brevibacillus sp. strain
KB2. Bacillus cereus PU showed the highest degree of degradation with 87.40% of
malaoxon and 49.31% of malathion. The incubation time was 7 days for the experiments. Wu et al. (2006) reported that Klebsiella sp. strain ZD112 could produce an
esterase-degrading pyrethroid (EstP) and cloned the same gene in E. coli. They
found that the purified EstP could hydrolyse malathion, pesticides of the pyrethroid
group and rho-nitrophenyl esters of some fatty acids. Cloning of pyrethroidhydrolysing carboxylesterases gene was also reported by Wang et al. (2009) and
Zhai et al. (2012) from Sphingobium sp. strain JZ-1 and Ochrobactrum anthropi
YZ-1.
3.4
Enzymes in Biocontrol
Accelerated use of agrochemicals has adversely affected nature and has resulted in
the emergence of a more acceptable and environment-friendly approach such as
biocontrol strategy (Harman et al. 2010). Biocontrol agents include actinomycetes,
bacteria and fungi, and may act on pathogens through antagonism, parasitism or
antimicrobial compounds or by the induction of systemic resistance representing an
environment-friendly approach to reduce losses due to pests and diseases or as an
alternative to agrochemicals (Lugtenberg et al. 2002; Whipps 2001).
In the natural ecosystem antagonistic microorganisms have critical roles in keeping the balance of microbial habitats. This antagonistic property can be exploited
for biological control (Henis and Chet 1975). Fungal pathogens are effectively
restricted by degrading their cell walls containing chitin and β-1,3-glucans (Lam
and Gaffney 1993). There are reports about members of bacteria and fungi that host
enzymes that can hydrolyse glucans, protein, chitin, etc. Chitinases, glucanases and
proteases are the major hydrolytic enzymes involved in biological control mechanisms. Abraham (2015) reported that even the tough structures like zoospores,
conidia, chlamydospores, etc. could be destabilised by enzymes like chitinases.
Extracellular proteases and ß-1,3-glucanases were also able to degrade spores.
I.C. Nair and K. Jayachandran