Transgenic poplar characterized by ectopic expression of a pine cytosolic glutamine synthetase gene exhibit enhanced tolerance to water stress

Previous studies have shown that overexpression of pine cytoslic glutamine synthetase (GS) in transgenic poplars increases the production of endogenous glutamine, and is concurrent with plant growth. Furthermore, the ability of cell cultures of Douglas-fir to withstand water stress is directly correlated with an increase in intercellular levels of glutamine. The present work centers on the hypothesis that overexpression of GS in poplars will result in an increase ability of GS1 transgenic plants to withstand water stress compared to controls at different development levels. Gas exchange measurements showed that transgenic poplar lines had higher net photosynthetic rates prior, during, and after water stress compared to control lines. Stomatal conductances were also higher in transgenic plants compared to controls especially before water stress. Chlorophyll fluorescence data suggested higher electron transport rate in GS1 transgenic plants during and after water stress. Prior to water stress, sudden exposure to lower photosynthetic photon flux densities showed that transgenics had higher photorespiratory activity as indicated by greater post-lower illumination burst (PLIB) of CO₂ than control plants. Enzyme linked immunosorbent assay showed that both transgenic and control poplars increased abscisic acid production during water stress, possibly to decrease stomatal conductance. Immunoblots showed that transgenic plants better maintained expression of fundamental enzymes (GS, GOGAT, and Rubisco) under severe water stress and during recovery, as well as high chlorophyll and total soluble protein contents. Furthermore, transgenic plants maintained higher amino acid and polyamine levels, especially during the recovery period suggesting higher osmotic adjustment in transgenics. Glutamine levels were higher and ammonium contents were lower in transgenic poplars compared to controls throughout the experiment. Transgenic poplars showed a stronger antioxidant defense system compared to controls suggesting stronger resistance to photooxidation. These findings indicate that overexpression of pine cytosolic GS1 enhanced sustained photosynthetic electron transport capacity under severe stomatal limitation. Furthermore, they suggest that ectopic expression of cytosolic GS increases photorespiratory activity, and that this serves as an effective protective energy sink to light-harvesting capacity. These data also suggest that transgenic poplars can alter their nitrogen metabolism more efficiently than control plants to tolerate and recover from water stress.