Enhancing the resistance of wheat to water deficit stress through genetic engineering

Scope of study. Genetic engineering is a novel alternative to classical breeding for improving resistance to water deficit stress. Genetic engineering is a faster and precise means of achieving improved resistance because it avoids the transfer of unwanted chromosomal regions that are also introduced during breeding. As part of a scheme to improve resistance of wheat to water deficit stress, we have transformed wheat with the mtlD gene of E. coli for accumulation of the osmoprotectant mannitol. Mannitol was targeted into the cytosol or plastids. Transgenic plants were evaluated to determine if mannitol accumulation affects growth and if mannitol protects wheat from water stress and salinity.; Findings and conclusions. Fifty transgenic plants were recovered with a transformation frequency of 0.4%. In transformants expressing mannitol in the cytosol, mannitol content reached 0.3 to 2.0 μmoles gfw −1 in calli and 0.4 to 1.6 μmoles gfw−1 in plants. However, no detectable levels of mannitol accumulated in plastids. Half of the transgenic plants recovered were infertile, stunted in growth, and had twisted leaves and heads. Abnormal phenotypes correlated with increased mannitol (>1.0 μmoles gfw−1) and reduced sucrose content in the flag leaf. It is hypothesized that stunted growth and infertility of transgenic plants were due to reduced sucrose supply to apical meristem and developing floral organs. Response to stress was evaluated at the tissue and whole plant level using calli and T₂ plants that expressed mannitol in the cytosol. Calli were exposed to −1.0 MPa PEG and 100 mM NaCl for 60 days. Plants were stressed by watering with 1/3 (50–75 ml) of the unstressed level (150–225 ml) and by growing in nutrient solution containing 150 mM NaCl for 30 days. Accumulation of mannitol significantly improved growth under stress. However, the amount of mannitol accumulated was too small to have a major effect on osmotic adjustment. It is concluded that the improved performance of mannitol accumulating plants could be due to other stress protective functions of mannitol.