Study On Drought Tolerance Of Transgenic Cotton With Betaine Synthesis Related Genes

Water shortage is one of the key factors limiting agricultural production,because drought stress can cause dehydration in plant cells and affect the growth and the yield of plants.Cotton is an important economic plant.In addition to providing fibers,seeds of cotton are also important sources of oil and protein feed.However,there exists significant difference in the drought resistance of different cotton varieties.Even the highly droughtresistant varieties available now are limited in yield when faced with significant drought stress.Therefore,through genetic engineering technology to further increase the drought resistance of cotton to meet the demand of cotton production for drought resistance,which is of great significance to the development and utilization of farmlands with limited water access.The materials used in this research were transgenic cotton with ApGSMT2gApDMT2g gene and transgenic cotton with GhPEAMT-betA gene.Overexpression of ApGSMT2g and ApDMT2g genes endowed cotton with the ability of glycine to synthesize betaine.Overexpression of GhPEAMP and betA genes endowed cotton with the ability of choline to synthesize betaine.The present study explored the drought tolerance of transgenic cotton using a combination of trials in rainproof shelters（or field trials）,and pot trials.The results showed that the transgenic cotton with ApGSMT2g-ApDMT2g gene had a 21.75%increase in seed cotton yield compared to the wild-type control for trials in rainproof shelters.Additionally,transgenic cotton with GhPEAMP-betA gene had a 39.86%increase in seed cotton yield compared to the wild-type control for trials in fields with natural drought condtions.These results demonstrate the superior performance of transgenetic cottons in terms of drought resistance.In this study,the cotton just growing to the bud stage in pot trials was treated under drought stress,and the physiological and biochemical indexes of cotton,such as betaine content,relative water content,reactive oxygen species level,antioxidant enzyme activity,net photosynthetic rate,PSII performance,were measured.Furthermore,the transcription levels of partial genes related to endogenous betaine synthesis,antioxidant enzymes synthesis,and PSII synthesis and degradation in cotton were determined to analyze the possible mechanism of enhanced drought tolerance in transgenic cotton.The result shows that the content of betaine in transgenic cotton was 37.23%higher than that of wild-type cotton under drought stress.Furthermore,transgenic cotton had higher RWC and lower saturation permeability than wild cotton under drought condition,and RWC increased by 32.05%compared with wild-type.Transgenic cotton displayed lower level of reactive oxygen species and higher photosynthetic performance than wild-type cotton under drought stress.In particular,the content of H2O2 in transgenic cotton leaves was 48.94%lower than that of the wild-type cotton,and the superoxide anion production rate in transgenic cotton leaves was 38.84%lower than that of the wild-type cotton.Additionally,the net photosynthetic rate of transgenic cotton leaves was 2.29 times of that of the same leaf location of the wild-type.In general,betaine can improve the transgenic cotton’s ability to remove reactive oxygen species of transgenic cotton under drought stress by promoting the activity of antioxidant enzymes and inducing an increase in the transcription level of antioxidant enzyme-related genes.The activities of APX,CAT,POD and SOD in transgenic cotton cells were 31.76%,37.77%,19.77%and 192.18%higher than those in wild-type cotton cells.Simultaneously,betaine can also promote the damage repair of PSII.In our study,under drought stress,there was a significant increase in the transcription levels of GhpsbA encoding D1 protein of the plant’s PSII reaction center.Additionally,the transcription levels of genes encoding degradation-related enzymes,such as GhFTSH,was higher when compared to wild-type cotton.These results suggest that at the transcriptional level,transgenic cotton have higher D1 protein of PSII turnover ability than wild-type cotton under drought stress,accompanied by enhanced ability to repair PSII damage.Additionally,enzymes such as Rubisco carboxylase in the Calvin cycle of transgenic cotton leaf cells had higher activity under drought stress.It can further improve the CO2 fixation ability of transgenic cotton under drought and hence enhance the drought resistance ability of transgenic cotton.To sum up,under drought stress,transgenic cottons carrying betaine biosynthesisrelated genes have higher betaine level in cells,which helps to increase the transcription level of antioxidant enzyme-related genes and to promote the activity of antioxidant enzymes.This allows the cottons in our study to possess improved capacity to remove reactive oxygen species and exhibit decreased level of reactive oxygen species.Additionally,as the cottons with higher betaine level in cells have improved ability to repair PSII damage and protect the Calvin cycles,their photosynthetic capacity under drought stress is also enhanced.All these factors contribute to the better drought resistance and the better yield of the transgenic cottons.