Constitutive Overexpression Of Glutamine Synthetase 1 Gene Or Glutamine Synthetase 2 Gene Affects Growth And Nitrogen Metabolism In Transgenic Tobacco

Glutamine synthetase(GS)plays a key role in nitrogen metabolism in higher plants.Here,two types of tobacco transformants,overexpressing wheat(Triticum aestivum L.)GS1(Ta GS1)or GS2(Ta GS2)driven by the constitutive super-promoter,were established and used to analyze the influences of nitrogen deficiency on transgenic tobacco growth and nitrogen metabolism.We found that overexpression of Ta GS1 and Ta GS2 stimulates lateral root formation in transgenic tobacco seedlings.When grown under optimum nitrogen conditions,the leaves of the Ta GS2-overexpressing transgenic tobacco showed high accumulation of the Ta GS2 transcript,but Ta GS1-overexpressing plants showed a very low level of the Ta GS1 transcript.However,no increase in GS activity or polypeptide levels was detected in the leaves of the two types of transgenic tobacco.While under nitrogen starvation,the level of the Ta GS1 transcript increased significantly in the middle and lower leaves,but Ta GS2 showed a significant decrease in the upper and middle leaves.In addition,GS activity increased significantly only in the middle and lower leaves of the two transformants when compared with WT.These results show that despite being driven by the super-promoter,Ta GS1 and Ta GS2 expression was regulated by nitrogen supply and leaf age.Under optimum nitrogen conditions,there was no marked difference in growth or nitrogen metabolism.However,under nitrogen starvation,the senescent leaves of the Ta GS1 transformants had lower free NH4+ and greater amino acid contents,but the upper leaves had greater amino acid,soluble protein,and chlorophyll contents.For the Ta GS2 transformants,the leaves of different strata had lower NH4+ and greater amino acid,soluble protein,and chlorophyll contents,and the transgenic plants had greater leaf area and higher biomass.These results suggest that,under nitrogen starvation,Ta GS1 overexpression can improve nitrogen transport from senescent leaves to developing ones,and Ta GS2 overexpression can improve the ability to assimilate nitrogen.