**11. Concluding Remarks**

In this review, we addressed the current knowledge on the roles of ascorbate and thiamin in plant metabolism with the emphasis on plant acclimation responses, specifically to high light and photoperiod acclimation, regarding ascorbate and thiamin, respectively.

In brief, ascorbate has important roles in modulating the energy systems between the chloroplast and mitochondria during high light acclimation. Incorporation of GLDH, the ultimate enzyme of the pathway, into the mitochondrial electron transport chain is a rationale for considering the tight association of ascorbate biosynthesis and mitochondria metabolism. This relationship is found to be bidirectional as mETC has a positive regulatory role on ascorbate biosynthesis through both AOX and cytC respiratory pathways. The existence of such a relationship might guarantee the balance of the electron flow under environmental stresses [39]. Likewise, photosynthesis regulates ascorbate pool size under the light. However, carbohydrates are direct substrates for ascorbate biosynthesis; their role in light regulation of ascorbate remains ambiguous and appears to be species-specific. The relationship between ascorbate, respiration, and photosynthesis is bidirectional. In effect, ascorbate elevates the rate of photosynthesis by a variety of mechanisms in response to acclimation to high light, such as through the water–water cycle, by scavenging ROS, dissipating excess energy through the xanthophyll cycle, donating electrons to PSII, guard cell signaling, and stomatal movement. It also regulates the nuclear and chloroplastic encoded genes of the photosynthesis.

Furthermore, in this review, we addressed the complexity of thiamin biosynthesis regulation, the universality of thiamin-requiring enzymes across kingdoms, and their association with core metabolic pathways. Therefore, a general role of TPP in the regulation of cellular metabolism and the acclimation process can be considered. A schematic description of the organellar communication and the involvement of ascorbate and thiamin is depicted in Figure 5.

**Figure 5.** Schematic illustration of plant intracellular communication. Anterograde (nucleus to organelle) and retrograde (organelle to nucleus) signaling pathways, as well as the main active site of ascorbate (Asc) and thiamin (B1) as signaling molecules, are shown. The ubiquitous existence of ascorbate and thiamin in cellular organelles, as well as the tight interconnection of the two vitamins between chloroplast and mitochondria, points to their important roles in the crosstalk between the two organelles.

Despite the thus far identified physiological dependencies of the two vitamins on the key players of metabolism, photosynthesis, and respiration and their roles in optimizing their activities, the underlying signaling and genetic factors in this process have remained a challenge for future research. QTL mapping and GWAS can be considered as alternatives to fill this gap.

**Author Contributions:** L.R.-S. and F.A. wrote the manuscript with supervision and contribution of A.R.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Collaborative Research Centers, SFB (Sonderforschungsbereich, Grant TRR 175/1) to A.R.F and F.A.

**Conflicts of Interest:** The authors declare no conflict of interest.
