## 1 Introduction  
L-ascorbate, Asc, (Vitamin C) constitutes one of the most versatile metabolites in plants and animals [1, 2]. Some primates, including humans, have lost the ability to synthesize Asc due to a gene mutation that transformed the last enzyme of this pathway, L-gulone-lactone-oxidase (GulLO), into a non-functional one [3]. Thus, its supply depends on the ingestion of fruit and vegetables rich in this vitamin [4]. In animal cells, vitamin C plays a vital role, acting as a general antioxidant, cofactor of mono- and dioxygenases and absorption and cellular uptake of iron, and in the demethylation of DNA and histones [5; 6]. In plants, Asc acts as a major antioxidant and enzymatic cofactor, involved in several cellular and molecular processes, such as cell division, expansion, photosynthesis, respiration, and in tolerance to environmental stresses [1; 4]. Plants are sessile organisms constantly exposed to high temperatures, salinity, drought, flooding, pathogens attacks, and other environmental stressors, which challenge their survival. Under these conditions, plants trigger several biochemical and physiological responses, including increase in reactive oxygen species (ROS) production, photosynthesis unbalancing, increase in leaf temperature, photorespiration, and transpiration [7; 8], impairing normal plant development and even causing death. In response to oxidative stresses, plants trigger two antioxidant mechanisms: the enzymatic system composed mainly of superoxide dismutase, catalase, and ascorbate peroxidase, and the non-enzymatic system composed mainly of ascorbate and glutathione. As an antioxidant, Asc prevents the overproduction of ROS, thereby avoiding the oxidation of DNA, proteins, lipids, and other cell structures and functioning as an essential micronutrient for rapid and efficient plant response and recovery to the stressors [9]. Most of the current understanding for elucidation of Asc biosynthesis was built using _Arabidopsis_ and tomato models, species that possess very low Asc contents [10; 11; 12]. As a result, four pathways have been proposed for Asc biosynthesis in plants (Figure 1). Genetic and biochemical approaches have been employed to elucidate all steps of the D-mannose/L-galactose pathway and described as primary approaches for ascorbate synthesis [2; 10; 13]. Furthermore, emerging evidence indicated the presence of alternative pathways of D-galacturonate [14], L-glucose [15], and myo-inositol [11]. However, the meaningfulness of their contribution to the ascorbate pool is still quite controversial [2; 16; 17]. Furthermore, degradation, recycling, and transport processes are also determinants for fine-tuning of ascorbate reserves [12; 13; 18; 19]. Although plants possess different pathways to biosynthesize ascorbate, studies point out that their contribution to this process varies between species, organs, and developmental stages. Analysis of the camu-camu transcriptome revealed the presence of transcripts encoding all proposed pathways [20]. Investigations in jujube and orange fruits suggested the D-mannose/L-galactose pathway was the principal pathway, assisted by the myo-inositol pathway [17; 21; 22]. On the other hand, the D-mannose/L-galactose pathway predominated in immature strawberry fruit, while D-galacturonate and myo-inositol pathways showed a positive correlation with Asc accumulation in ripe fruit [23], whereas the transcriptome of pineapple and acerola (fruit, leaf and flower) pointed out the only contribution of the D-mannose/L-galactose pathway [24; 25], thus revealing that the L-galactose pathway is constitutively and ubiquitously operating in plants. Ascorbate biosynthesis reflects the genetic base of each species, being continuously adjusted according to cell compartment, plant organs, developmental stage, circadian cycle, and stress conditions [13; 23]. The relevance of Asc for plant biochemical and physiological responses to abiotic and biotic stresses is becoming clear.