## 4 The Role of Ascorbic Acid to Confer Abiotic Stress Tolerance in Tomato Plants
### The Role of Ethylene in Regulating AsA Pathway in Fruits and Plants Exposed to Stress Factors  
A close link between ethylene metabolism and AsA accumulation has been previously suggested in tomato fruit [84; 85]. Transcriptome studies revealed remarkable alterations in the transcript levels of ethylene-, cell-wall-, and pigment-related genes at the beginning of the ripening process [84; 86; 87] that could probably affect carbon flux to the AsA pool, depending on fruit demands. In this regard, the availability of a broad number of well-characterized mutants that fail to ripen normally enables the in-depth study of the putative role of ethylene in AsA accumulation of tomato fruits through the course of ripening [85; 86]. Among them, the role of the ethylene-insensitive never-ripe mutation that blocks the ethylene perception has been most widely studied. Fruits from nearly isogenic lines homozygous for the _Nr_ mutation generally show a delayed onset of ripening, accompanied by poor coloration and marginally softening as ripening progresses, as well as enhanced AsA accumulation [84]. This suggests a possible link between AsA accumulation and ethylene perception, probably mediated by the delayed up-regulation of cell-wall-related genes such as _PG_, even though the exact mode of action remains obscure. Our previous findings demonstrated that AsA accumulation at the later stages of ripening can be affected by an interruption in the signal transduction of ethylene-mediated ripening pathways through the _Nr_ locus [87]. Interestingly, the location of the _Nr_ locus (Solyc09g075440) on the bottom of chr9 gives further support for this association, as it co-locates within the stable QTLs for fruit AsA concentrations in tomato [50]. The AP2/ERF (APETALA2/ethylene-responsive element binding factors) transcription factors are a large group of factors, present mainly in plants. Placed last in the ethylene signaling pathway [88], they regulate many developmental and physiological processes and participate in responsive mechanisms to various stresses. The AP2/ERF family is divided into four major subfamilies: DREB (Dehydration Responsive Element-Binding protein), ERF (Ethylene-Resposive-Element-Binding protein), AP2 (APETALA2), and RAV (Related to ABI3/VP), whereas few unclassified factors, consist of the Soloists group [89]. Several transcription factors, and specifically those related to ethylene signaling or perception, have been proposed to modulate the AsA content of plants exposed to oxidative stress or during plant growth, through regulation of expression patterns of genes implicated in the AsA biosynthetic/recycling pathway. In tobacco seedlings over-expressing a tomato ERF protein namely _JERF3_, an ethylene-induced gene, as well as cytosolic _APX1_, chloroplastic _APX2_, and glutathione peroxidase (_GPX_), all considered to use H\({}_{2}\)O\({}_{2}\) as an electron acceptor, were up-regulated three to eight times compared to wild type plants [90]. Transcriptional activation of ROS-related genes by _JERF3_ resulted in a decrease in accumulation of ROS and induced tolerance to drought, salt, and freezing. Another tomato _ERF_ transcription factor, named _TERF1_, may also regulate ROS production or scavenging [91]. After being exposed to ethylene gas, the tobacco _TERF1_-expressing seedlings showed much lower superoxide and H\({}_{2}\)O\({}_{2}\) content in relation to wild type. Not only genes catalyzing oxidative reactions, such as _GPX_ but also _GMP_ from the biosynthetic pathway, seemed to be transcriptionally induced under stress conditions. Therefore, _TERF1_ renders stress tolerance of tobacco seedlings to H\({}_{2}\)O\({}_{2}\). Those results suggest that _TERF1_ is an ethylene inducible factor regulating ROS scavenging during stress responses. In _Arabidopsis_, it has been recently demonstrated that ethylene and ABA antagonistically orchestrate AsA biosynthesis and ROS accumulation in response to abiotic stress factors, via ETHYLENE-INSENSITIVE3 (EIN3) and ABA INSENSITIVE4 (ABI4) transcriptional cascade [92].