Characteristics Of Transgenic Bt Rice In Nitrogen Utilization And Resistance Expression

Bt(Bacillus Thuringiensis)transgenic crops,mainly Bt cotton and Bt maize,have been widely applied and planted all over the word in recent decades because of its obsivious advantage in restance to the target pests.And this has created great positive economic and environmental effects.In recent years,scitientists start to study and develop transgenic Bt rice,and a lot of new Bt rice lines were successfully transformed and bred.Among these lines,rice lines with Cry1C*、Cry2A* or Cry1Ab/Ac Bt genes are chosed as important candidate lines for breeding and applied.However,a lot of studies found changes in agronomic performance and physiological traits of some transgenic Bt crops when compared with their non-Bt counterparts,which usually led to yield reduction in crops.The reasons for the changes in transgenic Bt crops are still unclear.Bt protein synthesis in Bt crops needs nitrogen(N);therefore,scientists have become interested in determining whether transgenic Bt crops change relative to N utilization and metabolism.On the other hand,N will affect the Bt protein expression in Bt crops,so it is necessary to study the Bt protein expression in Bt crops under different N application.Meanwhile,investigation in the sustainability and stability in resitance under field environment is also a important step to estimate the new Bt rice lines.Yield and agronomic performance,nitrogen utilization and metabolism characteristics,Bt protein expression and resitance performance of Bt rice lines with Cry1C*、Cry2A* or Cry1Ab/Ac genes were studied under different N applications(N level,N proportion and N sort)and different pest control modes(control the pests and not control the pests),and then the reasons for the changes in the yield of Bt rice lines was discussed.The mian results of the study are as follows.(1)Grain yields of MH63(Cry1C*)were16.9%-31.0% lower than those of MH63 under different N applications.An analysis of the yield components revealed that decrease in the setting rate was the reason for reduced grain yield in MH63(Cry1C*).Grain yields of MH63(Cry2A*)were 12.3%-26.0% lower than those of MH63 only under zero N and early N applications.The decreased setting rate was also found to be the reason for the reduction in grain yield of MH63(Cry2A*).Futher investigation revealed that MH63(Cry2A*)had accelerated leaf senescence compared with MH63 under zero N and early N applications,which would be the reason for the decreased setting rate in MH63(Cry2A*).Reductions in grain yields of the 3 Bt rice lines were often observed under different N proportion and N sort applications.There were no differences in ―source‖ agronomic traits and grain quality between Bt-MH63 and MH63 under recommend N application.(2)No differences in the total N uptake levels were found between Bt-MH63 and MH63.However,MH63(Cry1C*)had lower partial factor productivity(PFPN)and internal N use efficiency(IEN)than MH63.MH63(Cry2A*)had lower IEN than MH63 under zero N application.Diffferences in leaf N metabolism characteristics at filling stage were also observed between MH63(Cry2A*)and MH63.The main difference is the lower souble protein content(SPC)in MH63(Cry2A*)leaf.(3)Bt protein content(BTC)in leaves of the 3 Bt rice lines increased with the increase of amount of N application.Responses of the 3 Bt rice lines to N sort were different.MH63(Cry1C*)and MH63(Cry1Ab/Ac)had higher BTC when applied NH4+-NO3-balanced N sort(NH4NO3).However,MH63(Cry2A*)had higher BTC when applied NH4+ N sort((NH4)2SO4).BTC in leaves of MH63(Cry2A*)and MH63(Cry1Ab/Ac)were about 10 and 8 times higher than that of MH63(Cry1C*),respectively.BTC in leaves of the 3 Bt rice lines first increased with growth of the plant,and then decreased with senescence of the plant.The decline in BTC was much slower in MH63(Cry2A*)leaves in the late growth stage.The fully expanded green leaf had higher BTC than the senescent leaf and tender leaf.(4)Grain yields of Bt-MH63 and Bt hybrid rice lines with Cry1C*、Cry2A* or Cry1Ab/Ac genes were 9.1%-38.0% higher than those of their respective non-Bt control lines.The variations in yield advantage of Bt rice lines were affected by both the different pest occurrences and different yield potentials of the rice lines.All the Bt rice lines with Cry1C*、Cry2A* or Cry1Ab/Ac genes had shown above 90% resitance to the taget pests under the field environments.Grain yield of MH63(Cry2A*)was lower than that of MH63 when only control for the target pests,which was probably due to the more severe infestation of non-target pests in the MH63(Cry2A*)field.The Cry1Ab/Ac Bt gene has brought stable and effective resistance to the inbred and hybrid rice lines without negative non-targeted effects on the other traits.(5)The reductions in grain yields of MH63(Cry1C*)and MH63(Cry2A*)under zero N application were directly cause by the decreased setting rates.The physiological reason for the the decreased setting rate in MH63(Cry1C*)was the lower ratio of assimilation translocation to grain,reflected by the lower ratio of dry matter translocation to grain(RDM),apparent ratio of transferred non-structural carbohydrates from stems to grain(ARNSC)and the reduced small vascular bundles(SVB)in panicle neck.The main reason for the the decreased setting rate in MH63(Cry2A*)was the accelerated leaf senescence during the the late growth stage.And this accelerated leaf senescence could be eliminated by increasing the N application level.As one of the methods to present the BTC,BTC/SPC was more sensitive to the N application.Under low N application level,BTC/SPC was as high as 4.3‰ in MH63(Cry2A*)leaf at the late growth stage.What’s more,the BTC/SPC was found to be correlated to the leaf senescence traits.All the Bt rice lines with Cry1C*、Cry2A* or Cry1Ab/Ac genes had gained stable and effective resistance,and they had obvious yield advantages under pesticide-free environment.However,lower setting rate was observed in MH63(Cry1C*)and MH63(Cry2A*).The reason for the the decreased setting rate in MH63(Cry1C*)was the weaker ―flow‖ traits,but in MH63(Cry2A*)was the accelerated senescence ―source‖ traits.And the accelerated senescence traits in MH63(Cry2A*)could be eliminated by increasing the N application level.N use efficiency(NUE)of MH63(Cry2A*)and MH63(Cry1C*)were changed because of the different responses of grain yield of MH63(Cry2A*)and MH63(Cry1C*)to N application compared with MH63.What’s more,we found some changes in N metabolism characteristics of MH63(Cry2A*).BTC varied among the 3 Bt rice lines,and it had spatio-temportal differences.BTC in the leaves was significantly affected by the N applications.BTC/SPC can be an important indicator to present the Bt protein experession under different N applications.To exert the yield and resitance potential of Bt rice,we need to adjust the breeding procedure and N management in order to increase the yield and NUE of Bt rice lines as well as timely and limitedly express Bt protein in the plant.