**Laise Rosado-Souza, Alisdair R. Fernie \* and Fayezeh Aarabi \***

Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, 14476 Potsdam-Golm, Germany; Rosado@mpimp-golm.mpg.de

**\*** Correspondence: Fernie@mpimp-golm.mpg.de (A.R.F.); Arabi@mpimp-golm.mpg.de (F.A.)

Received: 20 December 2019; Accepted: 10 January 2020; Published: 13 January 2020

**Abstract:** Cell compartmentalization allows incompatible chemical reactions and localised responses to occur simultaneously, however, it also requires a complex system of communication between compartments in order to maintain the functionality of vital processes. It is clear that multiple such signals must exist, yet little is known about the identity of the key players orchestrating these interactions or about the role in the coordination of other processes. Mitochondria and chloroplasts have a considerable number of metabolites in common and are interdependent at multiple levels. Therefore, metabolites represent strong candidates as communicators between these organelles. In this context, vitamins and similar small molecules emerge as possible linkers to mediate metabolic crosstalk between compartments. This review focuses on two vitamins as potential metabolic signals within the plant cell, vitamin C (L-ascorbate) and vitamin B1 (thiamin). These two vitamins demonstrate the importance of metabolites in shaping cellular processes working as metabolic signals during acclimation processes. Inferences based on the combined studies of environment, genotype, and metabolite, in order to unravel signaling functions, are also highlighted.

**Keywords:** metabolite signaling/acclimation; TCA cycle; Calvin–Benson cycle; photoperiodic changes; photosynthesis; redox-regulation; environmental adaptation
