## 1 Introduction  
Ascorbic acid (AsA), also known as vitamin C, is a vital eukaryotic antioxidant widely found in plants. Humans and other animal groups that have lost the terminal enzyme (L-gallono-1,4-lactone oxidase, or GulLO) catalyzing the last step of the AsA biosynthetic pathway cannot synthesize it, and for those, it has become a vitamin. All AsA auxotrophs, including humans, rely entirely on dietetic sources to meet their nutritional requirements of AsA [1]. For the majority of living species, AsA acts as a vital cofactor for numerous life-sustaining enzymes associated with a plethora of key processes, such as physiological and biochemical processes, including regulation of the cell cycle, photosynthetic pathways, and redox signaling [1,2]. In plants, AsA scavenges free radicals, thereby protecting DNA, proteins, and lipids from oxidative damage, which in turn increases yield through the improvement of cell health and resistance to stress [1,2]. The effectiveness of AsA in driving biochemical interactions is largely influenced by its concentration rather than its absolute content. Its higher concentration in specific tissues or cellular compartments improves molecular collision rates, allowing for efficient electron donation. However, diluted systemic distribution may limit bioavailability at critical sites, reducing therapeutic impact and highlighting that achieving sufficient concentration at action sites is crucial for efficacy [3,4]. The physiological significance of AsA in biotic/abiotic stress tolerance is well documented. In tomatoes, the exogenous application of AsA increased fruit ripening [5], while an AsA-deficient mutant of Arabidopsis showed earlier senescence than wild-type plants [6,7]. AsA applied together with drought stress-controlled ROS production and improved plant development by increasing the activity of antioxidant enzymes and stimulating drought stress tolerance in _Capsicum annuum_ L. plants [8]. As a major component of the ascorbate-glutathione (AsA-GSH) cycle, AsA plays a substantial role in ROS detoxification [9]. Concerning conditions, AsA-GSH was differentially activated during stress, germination, and tissue development depending on the cellular compartment/genotype, revealing that cellular AsA level fluctuations can profoundly affect plant growth [10]. In common beans, water stress reduced photosynthetic pigments, carbonic anhydrase activity, antioxidant activities, growth, and seed yield, while, in contrast, AsA foliar spray enhanced all studied traits, with a positive correlation to increases in AsA dose [11]. Upon priming with AsA, aged out seeds showed an effective alleviation of the detrimental effects of aging [12]. The above studies indicate that the harmful effects of abiotic or biotic stressors can be mitigated by improving the endogenous biosynthesis level of AsA or by its exogenous application. In plants, AsA is synthesized through well-defined biochemical pathways [13,14] (Figure 1). The principal pathway for AsA biosynthesis is D-mannose/L-galactose (also called the Smirnoff-Wheeler pathway), which was proposed about 25 years ago and continues to be the major route to AsA synthesis in plant species [13]. This pathway is primarily regulated by eight key enzymes: Phosphomannose isomerase (PMI), Phosphomannomutase (PMM), GDP-mannose pyrophosphorylase (GMP), GDP-mannose 3',5'-epimerase (GME), GDP-glucose phosphorylase (GGP), Gulose-1-phosphate phosphatase (GPP), Galactose dehydrogenase (GDH), and Galactonate dehydrogenase (GLDH) [15]. Furthermore, it has been observed that these genes exhibit light-responsive promoter motifs (such as the ATC, I-box, GT1 motif, etc.) [16]. Furthermore, it was shown that the GGP gene's 5\({}^{\prime}\) untranslated region contains a conserved cis-acting upstream open reading frame (5\({}^{\prime}\)-uORF), which promotes the post-transcriptional regulation of the gene [17]. D-galacturonate is another alternative pathway, derived from D-galacturonic acid, a component of pectin, a part of the plant's primary cell wall [18].