Overexpression of SOD gene derived from Milk thistle increases drought tolerance in Tobacco through enhancement of photosynthetic functionality and photo-protective mechanism

Superoxide dismutase (SOD) is involved in the plant's primary defense mechanism against the adverse effects of reactive oxygen species (ROS), maintaining ROS homeostasis within cells. Therefore, it is essential to investigate and compare the stress tolerance mechanism in both tolerant and sensitive plants. This study investigates the drought stress tolerance of Tobacco following transferring of a SOD gene obtained from a drought-tolerant plant. The SOD gene was isolated from the Milk thistle ( Silybum marianum L.) plant, cloned into the pBI121 (Binary Agrobacterium plasmid) expression vector, and subsequently transformed into Agrobacterium tumefaciens . Pre-cultivated Tobacco ( Nicotiana tabacum L.) plants were inoculated with the recombinant bacteria. Following validation of transgene integration via PCR, the plants were subjected to drought stress conditions (five days of water withholding). Results demonstrated that the expression of SOD and its enzyme activity in drought-imposed transgenic plants were respectively 5 and 1.8 times higher than its expression and activity in non-transgenic plants. Imaging of chlorophyll fluorescence showed that drought-imposed transgenic plants had a higher maximum quantum yield of photosystem II (F _v /F _m ) and Non-photochemical quenching (NPQ) compared with wild-type plants. Stomatal density in drought-imposed plants was lower than in control plants, consistent across both transgenic and non-transgenic groups. Stomatal width decreased under stress in both plant types, with transgenic plants showing smaller width than non-transgenic plants. Stomatal length showed no significant differences between transgenic and non-transgenic plants or between drought-stressed and control conditions. In conclusion, SOD gene transfer from the Milk thistle to Tobacco plants increased the drought tolerance of the transgenic plants; this shed light on the path toward reaching drought-tolerant crops.