**1. Introduction**

The different cellular compartments such as chloroplasts, mitochondria, and the nucleus require a tightly orchestrated coordination of metabolic activity within each compartment by anterograde and retrograde signaling pathways [1,2]. Anterograde signaling is essentially a top-down regulatory pathway originated in the nucleus and sent to the organelles. Retrograde signaling, on the other hand, is the ability of organelles to coordinate, via signaling molecules, the expression of nuclear genes [3,4].

During the past years, significant advances in uncovering specific signals from plastids and their mechanisms of action have been made [4,5]. Several classes of factors including organelle gene expression, redox status, accumulation of pigment precursors like tetrapyrroles, reactive oxygen species (ROS), and metabolites have been proposed to act as plastidial signals [1]. They are sensed by plastidial factors including Executer1, Executer2, Genomes Uncoupled 1 (GUN1), and a thylakoid protein kinase (STN7), to initiate signaling cascades [1,4]. Some nuclear factors such as the transcription factor abscisic acid insensitive 4 (ABI4) have additionally been identified to be involved in retrograde signaling [6].

Much less is known about plant mitochondrial retrograde regulation/signaling [7,8]. Current research focuses on the response to a dysfunctional mitochondrial electron transport chain (ETR) and activation of genes encoding enzymes associated with the recovery of mitochondrial function, such as alternative oxidase (AOX) and alternative NAD(P)H dehydrogenases. It is also known that genes encoding proteins associated with the maintenance of the redox homeostasis, such as glutathione reductase, catalases, ascorbate peroxidases, and superoxide dismutases, are affected [9]. Recently an additional key player in the coordination between chloroplast and mitochondrial signaling pathways has been identified by Shapiguzov and coworkers (2019); their results suggest that the nuclear protein radical-induced cell death1 (RCD1) combines the signaling from both organelles in order to govern transcriptional and metabolic process within each organelle [10,11]. RCD1 mediates this regulation by suppressing the abscisic-acid-responsive NAC (ANAC) transcription factors ANAC013 and ANAC017, known as regulators of the mitochondrial dysfunction stimulon (MDS) genes, and also by receiving the ROS signals from the chloroplast underging protein modifications [10,11].

Many studies have emphasized the high degree of interrelationship between photosynthesis and respiration, the major energy production pathways that are confined to the chloroplast and mitochondria, respectively [12–14]. Metabolite signals are now frequently proposed as potential signals for inter-organellar communication and possible modulators to support photosynthesis during acclimation to fluctuating environments [3,15,16].

The focus of this review is on two vitamins, vitamin C (L-ascorbate) and vitamin B1 (thiamin), as potential metabolic signals within the plant cell, and to summarize recent advances on their roles in plant acclimation responses.

Vitamins in general are essential for plant metabolism, because many of them display important redox chemistry and antioxidant potential or are used as cofactors in several enzymatic reactions. Cartenoids (Pro-vitamin A), ascorbate, vitamin E (both tocopherols and tocotrienols), and vitamin B compounds (such as thiamin) are known to have predominant antioxidant roles in plants under oxidative stresses. Plastids are organelles highly exposed to oxidative stress because of oxygenic photosynthesis, and thus are protected by antioxidant vitamins, as reviewed in Asensi-Fabado and Munne-Bosch, 2010 [17].

Ascorbate is known as the most abundant and ubiquitous cellular antioxidant and is present in most cellular compartments [18]. The antioxidant function of ascorbate is mainly attributed to its action as a substrate for the ascorbate–glutathione cycle in scavenging hydrogen peroxide [19]. Ascorbate is also used as a cofactor for the violaxanthin de-epoxidase (VDE) enzyme, a critical component of the non-photochemical quenching (NPQ) [18]. Having the profound antioxidant functions to scavenge ROS renders ascorbate an important metabolite in the plant acclimation responses to changing environments [20,21]. For instance, ascorbate has been demonstrated to accumulate in Arabidopsis leaves during the acclimation process following the transition from low to high light conditions [21], as well as in the leaves of highland species and pea acclimated to high light and low temperature [22]. Ascorbate is connected to the mitochondria and the respiration processes, because the last enzyme of the pathway is located in the inner membrane of the mitochondria; however, it is found to be almost ubiquitously scattered in all cellular compartments, including chloroplast [23]. Further, ascorbate is known as a key component of the redox hub in balancing redox homeostasis in cellular compartments [19], and owing to the fact that redox equivalents can also be transferred between cellular compartments, ascorbate is, therefore, assumed as part of the inter-organellar communication.

Thiamin (or thiamine), also known as vitamin B1, is one of the water-soluble B-complex vitamins. The term refers to the three vitamers forms, free thiamin; thiamin monophosphate (TMP); and thiamin pyrophosphate (TPP, or thiamin diphosphate, TDP), which is the active form. TPP works as an essential coenzyme for enzymes involved in photosynthesis in chloroplasts, in ATP synthesis in the participation in oxidative decarboxylation of pyruvate, and in the tricarboxylic acid cycle in mitochondrial central metabolism, as well as in the pentose phosphate pathway and alcoholic fermentation in cytoplasm [24–27]. Thiamin has also been shown to be involved in the acclimation responses to abiotic stresses and photoperiod [28–32]. It plays important roles, working directly as an antioxidant, scavenging ROS, and protection molecule, and indirectly by contributing to the cell energy poll, conferring the cell the necessary metabolic flexibility to acclimate to new conditions [17,32,33].

In the next sections, a detailed description of each pathway and their roles in plant acclimation responses to environmental cues, in particular high light and photoperiod acclimation, are discussed.
