Comparison of transgenic Bt rice and their non-Bt counterpart in yield and physiological response to drought stress

High expression of Bacillus thuringiensis (Bt) protein in transgenic Bt crops has dramatically improved the resistance of crops to some insect pests. However, some Bt crops were reported to have poorer adaptabilities to abiotic stress environments compared to their non-Bt counterparts. The biological reasons for poorer adaptabilities were still unclear. A two-year field experiment was conducted to investigate yield and physiological response of newly bred Bt rice lines with different Bt protein expression levels [MH63 (Cry1C*), MH63 (Cry2A*) and MH63 (Cry1Ab/Ac)] and their non-Bt counterpart MH63 to drought stress. MH63 (Cry1C*), MH63 (Cry2A*) and MH63 (Cry1Ab/Ac) all showed lower grain yields and lower biomass under drought treatment compared to MH63. The lower grain yields and lower biomass of Bt-MH63 were mainly due to the earlier leaf senescence associated with disorder in protein metabolism under drought treatment. These variations in Bt-MH63 were found to be correlated to the BT/SP (ratio of Bt protein to soluble protein) in Bt-MH63 leaves. The impact of drought on traits of Bt-MH63 was aggravated with increase of the Bt protein expression level in the plant. These results suggested that Bt-MH63 had poorer adaptabilities to drought stress compared to their non-Bt counterpart MH63. “Bio-burden” caused by the high expression of Bt protein in the plant was proposed to be one possible reason for the poorer adaptability, and BT/SP was estimated to be an indicator for the assessment of the “bio-burden”.