Insecticidal Protein Content Research Articles

Effect of Low Temperature on Insecticidal Protein Contents of Cotton (Gossypium herbaceum L.) in the Boll Shell and Its Physiological Mechanism.

Low temperature is the main factor for global natural disasters affecting the growth and distribution of plants, and cotton may be affected by low temperature and cold damage at all growth stages. In addition, the insecticidal resistance of cultivars has been reported to perform poorly or unstably due to adverse environments. The present study aimed to investigate the impact of low temperature on the levels of insecticidal protein in Bacillus thuringiensis (Bt) transgenic cotton plants during the peak boll stage. To achieve this, two Bt cotton cultivars, Sikang1 (SK1) and Sikang3 (SK3), were subjected to different temperature regimes and durations. The findings of the study demonstrated that the expression of insecticidal protein in the boll shell of Bt transgenic cotton plants was significantly inhibited under low-temperature stress. Specifically, in 2020, compared to the CK (27 °C), the insecticidal protein content in the boll shell of SK3 decreased by 28.19% after a 48 h of a 16 °C temperature. These results suggest that low-temperature stress can negatively impact the expression of insecticidal protein in Bt transgenic cotton, highlighting the need for appropriate measures to minimize its adverse effects on cotton production. In addition, the threshold temperature that leads to a significant decrease in the content of insecticidal proteins symbolizes an upward trend as the duration of stress prolongs. Decreased Bt protein content at low temperatures is associated with changes in the N metabolism. The present study revealed a significant positive correlation between the levels of glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT) activities, as well as in the soluble protein levels in the boll shell and the content of the Bt protein. On the other hand, a significant negative correlation was observed between the levels of free amino acids, peptidase, and protease activities, as well as of Bt protein content. These findings suggest that, in Bt cotton production, it is crucial to remain vigilant of prolonged low-temperature disasters, which last for over 12 h and drop below 17-20 °C during the peak boll stage. Such conditions may reduce insecticidal resistance, leading to substantial economic losses.

Enhancing boll protein synthesis and carbohydrate conversion by the application of exogenous amino acids at the peak flowering stage increased the boll Bt toxin concentration and lint yield in cotton

In Bacillus thuringenesis (Bt) transgenic cotton, the cotton boll has the lowest insecticidal protein content when compared to the other organs. The present study investigated the effects of amino acid spray application at the peak flowering stage on the cotton boll Bt toxin concentration and yield formation. Boll protein synthesis and carbohydrate conversion were also studied to reveal the fundamental mechanism. Three treatments (i.e., CK, the untreated control; LA1, five amino acids; LA2, 21 amino acids) were applied to two Bt cultivars of G. hirsutum (i.e., the hybrid Sikang 3 and the conventional Sikang 1) in the cotton-growing seasons during 2017 and 2018. Amino acid spray application at the peak flowering stage resulted in an increase of 5.2–16.4% in the boll Bt protein concentration and an increase of 5.5–11.3% in the seed cotton yield, but there was no difference between the two amino acid treatments. In addition, amino acid applications led to increases in the amino acid content, soluble protein content, glutamate pyruvate transaminase (GPT) activity, glutamate oxaloacetate transaminase (GOT) activity, glucose content, fructose content and soluble acid invertase (SAI) activity. This study also found that Bt protein content, enhanced boll number and the weight of opened bolls were closely related to carbon and nitrogen metabolism. The Bt protein content had significant linear positive correlations with amino acid and soluble protein contents. Enhanced boll number had significant linear positive correlations with the GPT and GOT activities from 15–25 days after flowering (DAF). The weight of opened bolls from 55–65 DAF had a significant linear positive correlation with the SAI activity. These results indicate that the enhancement of boll protein synthesis and carbohydrate conversion by amino acid application resulted in a simultaneous increase in the boll Bt protein concentration and cotton lint yield.

High plant density increases seed Bt endotoxin content in Bt transgenic cotton

Plant density is the cultivation practice usually employed to manipulate boll distribution, boll setting and yield in cotton production. In order to determine the effect of plant density on the insecticidal protein content of Bacillus thuringiensis (Bt) cotton plants, a study was conducted in Yangzhou University of China in 2015 and 2016. Five plant densities (PD1-PD5, representing 15000, 30000, 45000, 60000, and 75000 plants ha−1) were imposed on two Bt cotton cultivars, Sikang 1 (the conventional cultivar, SK-1) and Sikang 3 (the hybrid cultivar, SK-3). The boll number per plant, boll weight and boll volume all decreased as plant density increased. As plant density increased from 15000 to 75000 plants ha-1, seed Bt protein content increased, with increases of 66.5% in SK-1 and 53.4% in SK-3 at 40 days after flowering (DAF) in 2015, and 36.8% in SK-1 and 38.6% in SK-3 in 2016. Nitrogen (N) metabolism was investigated to uncover the potential mechanism. The analysis of N metabolism showed enhanced soluble protein content, glutamic-pyruvic transaminase (GPT) and glutamate oxaloacetate transaminase (GOT) activities, but reduced free amino acid content, and protease and peptidase activities with increasing plant density. At 20 DAF, the seed Bt toxin amount was positively correlated with soluble protein level, with correlation coefficients of 0.825** in SK-1 and 0.926** in SK-3 in 2015, and 0.955** in SK-1 and 0.965** in SK-3 in 2016. In contrast, the seed Bt protein level was negatively correlated with free amino acid content, with correlation coefficients of –0.983** in SK-1 and –0.974** in SK-3 in 2015, and –0.996** in SK-1 and –0.986** in SK-3 in 2016. To further confirm the relationship of Bt protein content and N metabolism, the Bt protein content was found to be positively correlated with the activities of GPT and GOT, but negatively correlated with the activities of protease and peptidase. In conclusion, our present study indicated that high plant density elevated the amount of seed Bt protein, and this increase was associated with decreased boll number per plant, boll weight and boll volume. In addition, altered N metabolism also contributed to the increased Bt protein content under high plant density.

Effect of Urea Spray on Boll Shell Insecticidal Protein Content in Bt Cotton.

Reproductive organs of Bacillus thuringiensis transgenic cotton, which contribute to cotton final yield, have low insect resistant efficacy, so it is important to improve their insect resistance. This study was conducted to find out the impact of different urea spray doses on the expression of Cry1A protein in boll shell of Bt cotton (Sikang 1 and Sikang 3), and nitrogen metabolism in this process was also studied to uncover the physiological mechanism. The experiment with six urea doses was organized during peak boll stage in 2017 and 2018. The results showed that urea spray could significantly increase boll shell insecticidal protein contents in both cultivars, with the highest Bt protein content observed at 28–32 kg ha−1 urea dose. In addition, urea spray increased the contents of soluble protein and free amino acid and the activities of GS, GOGAT, GOT, and GPT, but decreased the activities of peptidase and protease in boll shell. Correlation analysis showed that the amount of boll shell Bt protein was positively correlated with levels of soluble protein and amino acid, and activities of GS, GOGAT, GOT, and GPT, but negatively correlated with peptidase and protease activities. Thus, this study demonstrated that higher protein synthesis ability and lower proteolysis ability were related to increased Bt protein content in urea-sprayed boll shell.

Reduced square Bacillus thuringiensis insecticidal protein content of transgenic cotton under N deficit

To clarify the effect of the N deficit on the amount of square Bt insecticidal protein, different N application rates (0, 75, 150, 225, and 300 kg ha–1) were imposed on the conventional cultivar Sikang 1 (SK-1) and hybrid cultivar Sikang 3 (SK-3) during 2015–2016 cotton growth seasons. Under different N application rates, the square number per plant, square volume and square dry weight reduced when the N rates decreased from conventional rate (300 kg ha–1) to 0 kg ha–1. And the square Bt protein content decreased accordingly. The analysis of N metabolism showed that soluble protein content, GPT and GOT activities decreased, free amino acid, peptidase and protease activities increased under N deficit. Correlation analysis indicated that the reduced Bt protein content under N deficit was related to altered N metabolism. In conclusion, square development and the amount of square Bt toxin both decreased under N deficit, indicating that promoting the square development under appropriate N application rate would also promote the insect resistance during squaring stage.

Analysis of differentially expressed proteins affecting insecticidal protein content in Bt cotton under high‐temperature and water deficit stress using label‐free quantitation

AbstractDifferently expression proteins under high‐temperature and drought stress were screened to investigate how stress affected the insecticidal protein content. These stresses decrease insecticidal protein, so the goal of this study was to provide a theoretical reference for maintaining the insect resistance of Bt cotton. The effect of high‐temperature and drought stress on differently expression protein in Bt cotton cultivar Sikang 3 was investigated using a label‐free quantitation method. Two treatments [40% field capacity and 38°C (HD) and 70% field capacity and 32°C (CK)] were imposed in a controlled environment. The results showed that high‐temperature and drought significantly degraded insecticidal protein content in bolls, with a decrease of 65.2 ng/g fresh weight (FW) compared to CK. The analysis of differently expression proteins with a label‐free quantitative proteomic approach showed that 83 proteins were significantly up‐regulated, but 104 proteins were significantly down‐regulated (Ratio > ±2, p value < 0.05) compared with CK. In addition, 122 new proteins were detected and 167 proteins were not detected. The analysis of Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways showed that 14 KEGG pathways, in which the difference expression proteins were enriched, were significantly affected and three KEGG pathways (carbohydrate digestion and absorption, protein export and protein processing in endoplasmic reticulum) were related to protein synthesis. In the carbohydrate digestion and absorption pathway, the capacity of starch hydrolysis for HD increased, while the capacity to phosphorylate the hexoses, fructose and glucose decreased. In protein export pathway, the synthesis of peptide for HD was not significant affected, while the process of peptide transferred into the endoplasmic reticulum was confined. In the protein processing in endoplasmic reticulum pathway, the capacity of ubiquitin‐mediated proteolysis increased.

Recovery characteristics of Bt insecticidal protein and relative physiological mechanisms after high temperature stress termination in square of Bt cotton

The experiments were conducted at Key Laboratory of Genetics and Physiology of Yangzhou University. The conventional cultivar Sikang-1 and hybrid cultivar Sikang-3 were used as the experimental materials. The potted cotton at squaring stage was moved to artificial climatic chamber with 38°C for 72 h , and 38°C for 96 h respectively in 2017 and 2018 cotton growth season, and moved to natural conditions, then the squares were collected to measure the Bt protein concentration and nitrogen metabolic physiology at 0, 12, 24, 48, 72, 96 hours respectively after the high temperature termination. The square Bt insecticidal protein contents were able to recover to the corresponding control level, the time for the recovery was in need of longer as the stressed high temperature period increased. In 72 h high temperature treatment, the square Bt insecticidal protein content could recover to the corresponding control level at 72 h for Sikang-1 and at 48 h for Sikang-3. In 96 h high temperature treatment, the square Bt insecticidal protein content could recover to the corresponding control level at 96 h for Sikang-1 and at 72 h for Sikang-3. There were significantly positive correlations of Bt toxin content with soluble protein content, glutamate pyruvate transaminase activity and glutamate oxaloacetate transaminase activity (the correlation coefficients were 0.964**, 0.981**, and 0.971**, respectively), and significantly negative correlations of Bt toxin content with free amino acid contents, and activitied of protease and peptidase (the correlation coefficients were -0.894**, -0.912**, and -0.834**, respectively). Therefore, the recovery degree of Bt insecticidal protein content of cotton square can be predicted according to the duration of high temperature stress, reasonably controlling Helicoverpa armigera and related pests in production.

Amino acids application enhances flowers insecticidal protein content in Bt cotton

BackgroundLow insecticidal protein expression at reproductive organs affect insect resistance in Bt transgenic cotton. In order to enhance flower insecticidal protein expression, the conventional cultivar Sikang1 (S1) and the hybrid cultivar Sikang3 (S3) were used as experimental materials; the applications of selected 5 types of amino acids and 21 types of amino acids were sprayed on the flowers in 2016 and 2017 cotton growing seasons.ResultsThe flower Bt protein contents increased significantly under the two amino acid treatments in both cultivars, the Bt protein concentration increased by 15.2 to 25.8% compared with the control. However, no significant differences were detected between the two treatments of amino acid application. Increased amino acid and soluble protein contents, enhanced GPT, GOT, protease,and peptidase activities were observed under the amino acid application at the flowering stage.ConclusionsThese results suggest that exterior application of the amino acids treatments could bolster the flower insecticidal protein expression.

Square Insecticidal Protein Concentration Relate to its Biomass in Bt Cotton

Core Ideas Cultivar with high square biomass for individual and whole plant had higher insecticidal protein content. The increased square biomass by mepiquat chloride application enhaced Bt protein concentration. The decreased square biomass by nitrogen deficit reduced square Bt protein concentration. The square insecticidal protein concentration relates to its biomass. Bt transgenic cotton has been cultivated in China and many other cotton production areas in the world. However, the insecticidal efficacy is often low for the reproductive organs, especially for the cultivars with a big boll in China. The objective of the study focused on the effect of square biomass on the Bt protein concentration during the squaring period for Bt cotton. In the variety effect study in 2015 and 2016, two hybrid Bt cultivars (Sikang3 and Sumian29) and two conventional Bt cultivars (Sikang1 and Nucotn33B) were compared for square biomass and Bt toxin content from 15 to 25 d after square appearance (DASP) during the cotton growing seasons. Cultivars with high square biomass for individual and whole plant had higher insecticidal protein content. In the cultural practice effect study in 2017, the experiment of mepiquat chloride (MC) dose and different nitrogen deficit extents were designed to further investigate the relationship between square biomass and square Bt protein concentration. The MC application significantly increased square mass and square Bt protein content. In contrast, the nitrogen deficit decreased square mass together with square insecticidal protein concentration. The positive correlation between square biomass (the individual square biomass and the square biomass per plant) and square insecticidal protein content was highly significant in variety study and cultural practice study. The results indicated that increment of square development by variety selection and cultural practice could increase insect resistance during squaring period of Bt cotton.

Mechanism of reduced insecticidal protein expression in Bt cotton under high temperature and drought based on proteomics.

To provide theoretical basis for the safety of insecticidal efficiency in Bt cotton, the effects of high temperature and drought stress on insecticidal protein expression and protein diffe-rently expression profile was studied. In this study, the Bt cotton cultivar Sikang 3 was used as expe-rimental material, with two treatments (40% field capacity and 38 ℃, HD, and 60% field capacity and 32 ℃, CK). Differences in proteomics of Bt cotton between HD and CK were compared using label-free quantitative proteomics technology. The results showed that high temperature and drought caused a significant reduction of insecticidal protein content in bolls, with a decrease of 38.2 ng·g-1 FM. The analysis of differential protein expression by label-free quantitative proteomic approach showed that 83 proteins were significantly up-regulated, but 104 proteins were significantly down-regulated in HD stressed cotton plants compared with CK. 122 new proteins were detected and 167 proteins expression was not observed under stressed conditions. Results from the enrichment analysis of differently expressed protein between two treatments showed that 14 KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways were affected under stress. Three KEGG pathways were related to the Bt protein synthesis and degradation: carbohydrate digestion and absorption pathway, protein export pathway, and protein processing in endoplasmic reticulum pathway. In the carbohydrate digestion and absorption pathway, the starch hydrolysis ability of HD treated cotton plants increased, while the ability to phosphorylate the hexoses, fructose and glucose decreased. In protein export pathway, the peptide synthesis in HD treatment was not significantly affected, while the process of transferring peptides into the endoplasmic reticulum was prohibited. In the protein processing in endoplasmic reticulum pathway, the ability of ubiquitin mediated proteolysis was increased in HD treatment.

Nitrogen (N) Application Gradually Enhances Boll Development and Decreases Boll Shell Insecticidal Protein Content in N-Deficient Cotton.

Expression of insecticidal protein in transgenic Bacillus thuringiensis (Bt) is lower in cotton reproduction organs, especially during boll development period. The current study investigated the effects of nitrogen fertilization under nitrogen deficit on boll development and Bt toxin content in boll shell, which is the first target of boll worm harm. The protein synthesis and degradation in this process was also studied to uncover the underlying mechanism. Five nitrogen levels (under nitrogen deficiency) were imposed on two cultivars, Sikang3 (hybrid) and Sikang1 (conventional), at the Yangzhou University Farm, Yangzhou, China during 2015 to 2016 cotton growth seasons. Under nitrogen deficiency, enhanced nitrogen dose increased the boll number per plant, boll volume, boll weight, boll shell amino acid content, protease, and peptidase activities, but reduced boll shell Bt protein content, soluble protein content, glutamic pyruvic transaminase (GPT) and glutamate oxaloacetate transaminase (GOT) activities. There was a significant negative correlation between boll growth and boll shell insecticidal protein content under nitrogen deficiency, which was a result of uneven distribution of nitrogen in cotton bolls. Under increased nitrogen application, most nitrogen was transported and assimilated in boll seed instead of boll shell in developing cotton bolls, which resulted in decreased protein synthesis and increased protein degradation, and thus declined Bt protein content in boll shell.

Effects of Soil Water Deficit on Insecticidal Protein Expression in Boll Shells of Transgenic Bt Cotton and the Mechanism.

This study was conducted to investigate the effects of soil water deficit on insecticidal protein expression in boll shells of cotton transgenic for a Bt gene. In 2014, Bt cotton cultivars Sikang 1 (a conventional cultivar) and Sikang 3 (a hybrid cultivar) were planted in pots and five soil water content treatments were imposed at peak boll stage: 15% (G1), 35% (G2), 40% (G3), 60% (G4), and 75% field capacity (CK), respectively. Four treatments (G2, G3, G4, and CK) were repeated in 2015 in the field. Results showed that the insecticidal protein content of boll shells decreased with increasing water deficit. Compared with CK, boll shell insecticidal protein content decreased significantly when soil water content was below 60% of maximum water holding capacity for Sikang 1 and Sikang 3. However, increased Bt gene expression was observed when boll shell insecticidal protein content was significantly reduced. Activity assays of key enzymes in nitrogen metabolism showed that boll shell protease and peptidase increased but nitrogen reductase and glutamic-pyruvic transaminase (GPT) decreased. Insecticidal protein content exhibited significant positive correlation with nitrogen reductase and GPT activities; and significant negative correlation with protease and peptidase activities. These findings suggest that the decrease of insecticidal protein content associated with increasing water deficit was a net result of decreased synthesis and increased decomposition.

Planting density and leaf-square regulation affected square size and number contributing to altered insecticidal protein content in Bt cotton

Lower insect resistant efficacy of reproductive organs has been observed widely during development in Bt cotton. The objective of this study was to investigate the effects of altered square development of Bt cotton cultivars on Bt toxin concentration, insecticidal efficacy and seed cotton yield, as well as its underlying mechanism by studying the nitrogen metabolism in this process. Three experiments were conducted at the Yangzhou University Farm, Yangzhou, China during 2014–2016 cotton growth seasons. Two cultivars including Sikang3 (hybrid) and Sikang1 (conventional) with five planting densities were compared for square number, square volume and Bt toxin content during square stage in 2014 and 2015. The planting densities had a significant effect on all the three phenotypes. Low planting density with higher square number per plant and square volume exhibited higher insecticidal protein contents in both cultivars. In addition, higher amino acid and soluble protein content, GPT and GOT activities, lower protease and peptidase activities were also detected in the low planting density treatment. In 2015, leaf cut (LC) and square spraying with GA3 treatments were applied to Sikang1 and Sikang3. The square number per plant, square volume, and insecticidal protein content were decreased by LC, but increased by GA3 application. In both experiments, the correlations between square growth and amino acid, soluble protein contents, GPT, GOT activities were positive, but the correlation between square growth and protease and peptidase activities were negative. In 2016, the insect harm and seed cotton yield were investigated under three planting density (1.50, 4.50 and 7.50 plants m−2), leaf cut and GA3 application. The lower boll worms number and hazard square rate with higher yield were observed at low planting density and GA3 application, and contrary effects were detected at high planting density and leaf cut treatment. Therefore, the results of our present study indicated that the square insecticidal protein content and insecticidal efficacy were significantly affected by square growth, and higher square growth ability benefit insect resistance of cotton plants during square stage.

Effect of High Temperature Stress on Bt Insecticidal Protein Content and Nitrogen Metabolism of Square in Bt Cotton

Effect of High Temperature Stress on Bt Insecticidal Protein Content and Nitrogen Metabolism of Square in Bt Cotton

Combined stress of low temperature and flooding affects physiological activities and insecticidal protein content in transgenic Bt cotton

Low temperature and flooding are severe stresses to cotton plant growth and development in some parts of the world. This study was conducted to determine the interactive effects of low temperature and flooding on physiological and growth parameters of cotton plants, and yield of seed cotton. Transgenic Bt cotton plants at the boll-setting stage were treated with low temperature, flooding, and their combinations. The stresses decreased photosynthetic rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration by up to 37%, 71%, 52%, and 60%, respectively. The content of soluble protein and insecticidal protein was also decreased by up to 65% and 53%, respectively. By contrast, malondialdehyde (MDA) content increased by up to 114%. The length of fruiting branch was shortened by combined low temperature and flooding stress. Boll number and weight were decreased by 3.2% and 28.1%, respectively. Seed cotton yield was decreased by up to 43.2%. Our results suggest that concurrent low temperature and flooding at the boll-setting stage negatively influenced leaf photosynthetic activities and insecticidal protein production, leading to poor protection and yield. We recommend that agronomic measures be designed to stabilise physiological activities and insecticidal protein production in Bt cotton plants under stress conditions.

Boll size affects the insecticidal protein content in Bacillus thuringiensis (Bt) cotton

Bacillus thuringiensis (Bt) transgenic cotton has been planting in large-scale for more than 10 years in China. It was observed that resistance to bollworm was reduced with the application of big-boll cultivars in China. The objective of the study was to investigate the effects of boll size on the contents of CryIA insecticidal protein in boll shell and cotton seed during the course of boll development. Two experiments were conducted at the Yangzhou University Farm, Yangzhou, China. In 2004, three cultivars including Sikang 1 (small-boll), Xiangza 3 (medium-boll) and Kemian 3 (big-boll) were compared for boll shell volume and 100-seed weight from 10 to 40 days after flowering (DAF), and insecticidal protein contents in boll shell and cotton seed from 10 to 50 DAF. As expected, the differences between cultivars were significant for all the four characteristics. Cultivars with bigger boll shell volume had higher 100-seed weight but lower insecticidal protein contents in both of the boll shell and cotton seed. The correlation between boll size and boll insecticidal protein content was negative (−0.653). The correlation between 100-seed weight and cotton seed insecticidal protein content was significantly negative (−0.645). In 2005, leaf cut (LC) and square removal (SR) treatments were applied to Sikang 1 and Sikang 3. The boll shell volume and 100-seed weight were decreased by LR, but increased by SR. LC significantly enhanced and SR significantly reduced the insecticidal protein contents of boll shell and cotton seed. The correlation between boll size and boll insecticidal protein content (−0.870) and the correlation between 100-seed weight and cotton seed insecticidal protein content were both highly significant and negative (−0.841). Therefore, the results of the study indicated that the boll insecticidal protein content was significantly affected by boll size. The implications of the observed results in breeding, cultural practices and pest management were discussed.

Physiological activities and expression of insecticidal protein in Bacillus thuringiensis transgenic cotton leaves subjected to high and low temperatures

This research was to investigate physiological activities and the expression of insecticidal protein in cotton leaves of Kemian 1, a widely grown cotton cultivar in the lower reaches of the Yangtze River, treated under high (40°C and 35°C) and low (25°C and 20°C) temperatures for 48 h. The main results indicated that the contents of free amino acid and malondialdehyde (MDA) increased during the heat and cold stress with the biggest increments in the first 12 h; and the content of soluble protein, activities of superoxide dismutase (SOD), and glutamic-pyruvic transaminase (GPT) decreased during the stress with the biggest decrements also in the first 12 h; the activity of nitrate reductase (NR) decreased in the first 24 h, and during the remaining 24 h its activity followed a gradual uptrend. The content of insecticidal protein in cotton leaves followed a downtrend during the stress, and the biggest decrements occurred in the first 12 h, suggesting that the expression of insecticidal protein was immediately inhibited during the process of stress. The results also indicated that physiological activities and the expression of insecticidal protein in cotton leaves were closely related. Compared with other stress temperatures, 40°C produced more immediate effects on the physiological activities and insecticidal protein content. In contrast to the peak flowering period, the physiological activities and insecticidal protein expression at the peak boll-setting period were more susceptible to heat and cold stress.

Variability of Endotoxin Expression in Bt Transgenic Cotton

AbstractTransgenic cotton expressing Bt (Bacillus thuringiensis) toxins is currently cultivated on a large commercial scale in many countries, but observations have shown that it behaves variably in toxin efficacy against target insects under field conditions. Understanding of the temporal and spatial variation in efficacy and the resulting mechanisms is essential for cotton protection and production. In this review, we summarize current knowledge on variability in Bt cotton efficacy, in particular on the induced variability by environmental stresses. We also discuss the resulting mechanisms and the countermeasures for the inconsistence in efficacy in Bt cotton. It is indicated that insecticidal protein content in Bt cotton is variable with plant age, plant structure or under certain environmental stresses. Variability in Bt cotton efficacy against target insect pests is mainly attributed to the changes in Bt protein content, but physiological changes associated with the production of secondary compounds in plant tissues may also play an important role. Reduction of Bt protein content in late‐season cotton could be due to the overexpression of Bt gene at earlier stages, which leads to gene regulation at post‐transcription levels and consequently results in gene silencing at a later stage. Methylation of the promotor may be also involved in the declined expression of endotoxin proteins. As a part of total protein, the insecticidal protein in plant tissues changes its level through inhibited synthesis, degradation or translocation to developing plant parts, particularly under environmental stresses, thus being closely correlated to N metabolism. It can be concluded that developing new cotton varieties with more powerful resistance, applying certain plant growth regulators, enhancing intra‐plant defensive capability, and maintenance of general health of the transgenic crop are important in realizing the full transgenic potential in Bt cotton.

The effect of high temperature on the insecticidal properties of Bt Cotton

Bt transgenic cotton has not shown the same level of resistance to bollworm in China, as in other major Bt cotton growing areas of the world. The objective of this study was to investigate the effects of high temperature on the CryIA insecticidal protein content and nitrogen metabolism, in the leaf of Bt transgenic cotton. The study was undertaken on two transgenic cotton cultivars, one conventional (Xinyang 822) and the other a hybrid (Kumian No. 1), during the 2001 and 2002 growing seasons at the Yangzhou University Farm, Yangzhou, China. In the 2001 study, potted cotton plants were exposed to 37 °C for 24 h under glasshouse conditions at three growth stages – peak square, peak flowering and peak boll developing periods. Based on the 2001 results, in 2002 the same two cultivars were exposed to the same temperature for 48 h at two growth stages—peak flowering and boll developing periods. The results of the study indicated that the insecticidal protein content of the leaf was not significantly affected by the stress during the square and flowering periods. However, exposure to high temperature for 24 h during the boll period reduced the CryIA protein content by approximately 51% in the cultivar Kumian No 1, and 30% in Xinyang 822 in the 2001 study, and by approximately 73 and 63% for 48 h with the same cultivars, respectively, in the 2002 study. Glutamic–pyruvic transaminase (GPT) activity, total free amino acid and soluble protein content, and the activity of protease in the leaf, showed relatively little change in response to high temperature in the flowering period. However, exposure to high temperature in the boll period resulted in the following changes - a reduction of GPT activity, a sharp increase in free amino acid content, a significant decrease in soluble protein content, and significant increases in the activity of protease. The results suggest that high temperature may result in the degradation of soluble protein in the leaf, with a resulting decline in the level of the toxin CryIA. It is believed that this may be the cause of the reduced efficacy of Bt cotton in growing conditions in China, where temperatures during the boll period often reach 36–40 °C.