TOXICITY OF Bacillus thuringiensis (BERLINER), CRYSTAL TOXINS AGAINST RICE LEAF FOLDER, Cnaphalocrocis medinalis (GUENEE)
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Date
2012
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ACHARYA N. G. RANGA AGRICULTURAL UNIVERSITY
Abstract
Toxicity of Bacillus thuringiensis (Berliner) Cry toxins were studied against Ist
instar larvae (0-24 hour) of rice leaf folder, Cnaphalocrocis medinalis (Guenee) at
Division of Crop Protection, Directorate of Rice Research, Rajendranagar. Of the eight
Cry toxins viz., Cry1Aa, Cry1Ab, Cry1Ac, Cry2Aa, Cry1Ba, Cry1Ca, Cry1Da and
Cry1Ea evaluated, Cry1Ba (LC50 8.50 ppm) was most toxic followed by Cry1Ab (LC50
8.73 ppm) at 48 hours after treatment, whereas Cry1Aa, Cry1Ac, Cry2Aa, Cry1Ca,
Cry1Da and Cry1Ea did not result in significant mortality in the bioassay at 48 hours
after treatment. Larval mortality data at 72 hours revealed Cry1Ab as highly toxic (LC50
0.50 ppm) followed by Cry1Aa (LC50 4.07 ppm), Cry1Ac (LC50 4.84 ppm) and Cry1Ba
(LC50 6.42 ppm) and was used for establishing baseline susceptibility against C.
medinalis. Bioassay with toxins Cry2Aa, Cry1Ca, Cry1Da and Cry1Ea resulted in less
than 50% larval mortality, at 72 hours after treatment.
Baseline susceptibility estimation with Cry1Ab toxin against seven C.
medinalis populations ranged from LC50 0.37-16.25 ppm at 48 hours after treatment and
LC50 0.50 – 6.49 ppm, respectively. Resistance ratios for the populations surveyed with
regards to LC50 of Cry1Ab at 48 and 72 hours after treatment were worked out and
found to be 44 fold and 13 fold, respectively, in comparison to most susceptible
population of C. medinalis.
Temperature, pH and incubation time based on azocaesinolytic digestion method
for estimating the proteolytic activity of midgut proteases were optimum at 50oC, 11
and 13 pH and incubation time of 50 minutes, respectively. Inhibitory studies of midgut
protease and Cry protoxin (Cry1Aa and Cry1Ab) at various concentration ratios (1:50
and 1: 100) proved excellent ability of midgut proteases in converting protoxin to active
toxin even at short duration of one to ten minutes time interval for Cry1Ab over
Cry1Aa.
Genetic variation studies with regards to mitochondrial and genomic DNA
isolated from different populations of C. medinalis were studied with primers specific to
cytochrome oxidase I (COI) of mitochondrial DNA and inter specific sequence repeats
(ISSR) markers. Results showed the formation of two clades (each clade having a pair
of populations) with 50 per cent similarity among them and a single population out
stretched as a separate group from five populations with inter specific sequence repeats
markers alone. Genetic variation for these five populations was in the range of 50-90
per cent for the populations studied.
Carboxylesterase and Glutathione-S-Transferase enzymes present in the midgut
homogenates of two field populations(ICRISAT, Patancheru and DRR, Rajendranagar)
were studied of which ICRISAT population had 1.35 and 2.24 fold greater
carboxylesterase (155μmoles/min./mg protein) and glutathione-s-transferase
(12.59μmoles/min./mg protein) titres in its midgut homogenates over the DRR
population. Zymogram analysis for isolation of esterase isozymes of the ICRISAT
population under reduced conditions of native–PAGE with whole body and midgut
extracts resulted in two and three isozyme bands, respectively. Inhibition studies of
esterase isozymes of ICRISAT, C. medinalis which were isolated in native–PAGE
under reduced conditions when incubated with different substrates at concentrations of
10-4 M dichlorvos, 10-6 and 10-4 M eserine characterized the carboxylesterase isozymes
to be ‘B’ type esterases.
Description
Keywords
enzymes, animal developmental stages, toxins, biological phenomena, toxicity, rice, genetics, proteins, concentrates, insecticides, TOXICITY, Bacillus thuringiensis, BERLINER, CRYSTAL TOXINS, RICE LEAF FOLDER, Cnaphalocrocis medinalis, GUENEE