## 4 The Role of Ascorbic Acid to Confer Abiotic Stress Tolerance in Tomato Plants
### Transgenic Efforts of AsA Manipulation towards Abiotic Stress Tolerance  
Apart from the obvious effect on AsA accumulation, overexpression of a wide range of AsA-regulatory genes could also enhance the tolerance of transgenic tomato plants to various abiotic stresses. For example, overexpressing a chloroplastic _MDHAR_ in tomato plants simultaneously elevates leaf AsA levels and improves plant tolerance to temperature and methyl viologen-induced oxidative stresses [112]. The efficient regeneration of AsA seems to be able to remove the ROS and protect the photosynthetic apparatus by alleviating the photoinhibition of the photosystem under these stresses. In fact, transgenic plants showed a lower level of H2O2 generation, as well as a higher net photosynthetic rate and maximal photochemical efficiency, under low- and high-temperature stresses. In tomato plants, the _AdBiL_ gene exerts an essential role in maintaining cellular ROS and reactive nitrogen species [113]. Overexpression lines accumulate less H2O2 and O\({}^{-2}\) are coupled with lower NO and SNOs compared to untransformed plants under chilling stress. This implies a possible physiological role of _AdBiL_ in the activation of the key enzymes of the AsA-GSH pathway, which may have potential implications in developing chilling-tolerant crop varieties through genetic manipulation. However, in the same study, biosynthetic genes are not influenced by cold stress, except for _GPP_ whose transcript levels accumulated at high levels starting at 3 h after exposure to 4 \({}^{\circ}\)C. This may also indicate that when increased AsA levels are required, the cell compensates by increasing the _GPP_ transcript levels for more AsA production [13]. Transgenic tomato plants over-expressing _SIGME1_ and _SIGME2_ have enhanced stress tolerance based on less chlorophyll content loss and membrane-lipid peroxidation under methyl viologen (paraquat) stress, higher survival rate under cold stress, and significantly higher seed germination rate, fresh weight, and root length under salt stress [114]. Transgenic tomato plants overexpressing _GalUR_ contain higher levels of AsA and are more tolerant to abiotic stresses induced by methyl viologen, NaCl, or mannitol than non-transformed plants [115]. Under salt stress of less than 200 mM NaCl, transgenic plants can survive, in contrast with control plants being unable to survive under such conditions. Finally, when four key biosynthetic genes, _GME_, _GMP_, _GGP_, and _GPP_, were pyramided in tomato by conventional hybridization, transgenic lines exhibited increased  
Figure 2: Schematic diagram of postharvest abiotic stress of tomato fruit and oxidative stress response. Fruit exposure to postharvest abiotic stress conditions such as wounding, chilling temperature, CO2/O2 injury, and dehydration, provoke a significant increase in ethylene (C2H4) and reactive oxygen species (ROS), which in low concentration serve as signaling molecules to regulate biological and physiological processes, whereas in high concentration can cause important damage to molecules and cell structure. Fruits organize an elaborate antioxidant network system as a defense mechanism, with AsA playing an important role especially by the robust enhancement of the AsA-GSH pathway, accompanied by a slower response of AsA biosynthesis. AsA content, along with enhanced light response, stress tolerance after 75 uM methyl viologen application, and AsA transport capacity [116]. Rather than being the rate-limiting step in enhancing AsA accumulation through recycling, enhanced cytosolic or chloroplastic _MDHAR_ expression has been associated with improved tolerance to abiotic stress factors in tomatoes in a light-dependent mechanism [55; 112; 117]. Additionally, plants with reduced MDHAR activity also showed an arrest in growth and yield, as well as reduced fruit size and sugar content [43].