**10. The Role of Thiamin in Mediating Plant Fitness and Acclimation**

In addition to its role in central metabolism, studies in different plant species have shown that alterations in the levels of thiamin vitamers result in smaller plants, chlorosis of the leaves, growth retardation, delayed flowering, fitness cost, and an influence on yield penalty [25,26,105,111,133,134]. Some of these phenotypes, such as chlorosis and the delayed flowering, have also been shown to be dependent on the light regime [25,32].

The functions of thiamin in the regulation of the metabolic networks during photoperiod transition were deeply investigated in our group [32]. While control plants display changes in the amplitude of diurnal oscillation in the levels of metabolites, TPP riboswitch mutant plants with high levels of TPP do not show such metabolic flexibility. The results also indicate a close relationship between photorespiration and the TCA cycle as the mutant plants accumulate less photorespiratory intermediates such as glycine, serine, and glycerate [32].

Thiamin biosynthesis has also been well documented to be activated when plants are exposed to abiotic stresses [28,31,135]. Rapala-Kozik and co-authors (2008; 2012) demonstrated that thiamin biosynthesis is activated on the acclimation response of Arabidopsis to salt, osmotic, and oxidative stress [29,30]. These stresses induce the expression of genes for TPP biosynthesis and thiamin dependent-enzymes, resulting in increased levels of thiamin and TPP, those which can consequently be incorporated into the requiring enzymes associated with central metabolic pathways, as described previously. Continuous abiotic stress such as high salinity and sugar deprivation was also shown to increase thiamin biosynthesis gene expression [30,136]. Interestingly, flooding/hypoxic conditions also impact thiamin biosynthesis expression patterns in roots. Under low O2 supply, roots switch from respiration to pyruvate fermentation, and high levels of thiamin could be required in this alternative route, as TPP acts also as a cofactor for PDC present in the fermentative metabolism [136].

Proteomics and transcriptomics studies in other species have also detected significant changes in thiamin biosynthesis and thiamin dependent enzymes during heat, drought, and cold stress conditions [137,138]. Changes in the protein levels associated with thiamin are transient, with increased abundance at early stages of stress followed by a decrease in protein levels associated with thiazole synthase in the thiamin biosynthesis pathway [137]. These conclusions are in agreement with our results describing the importance of thiamin levels for metabolic flexibility during acclimation [32]. Thiamin is also known as a potent antioxidant and a crop protection molecule in plants, playing important roles in plant acclimation [17]. It has been shown that thiamin has the antioxidant capacity by O2−/OH<sup>−</sup> scavenging and also recycling of vitamin C through the synthesis of NADPH [17,33]. Further, it has been revealed that paraquat-treated Arabidopsis, supplied by thiamin had reduced oxidative stress compounds, protein carbonyls, and dichlorofluorescein. It has also been shown in this study that Arabidopsis plants accumulated higher levels of TMP and TPP after exposure to high light, low temperatures, and osmotic and salt stress [31]. Yet, further investigations of thiamin in plants are needed to clarify whether it functions as an antioxidant directly or indirectly by supplying NADH and NADPH [17].

Although more scarce, studies on the function of thiamin during biotic stress have shown that treating different species with thiamin activates the systemic acquired resistance (SAR) and resistance to pathogen attack, such as fungal, bacterial, and viral infections. The thiamin treatment activates pathogen-related genes (PR) and stress signalling hormones, abscisic acid (ABA), and jasmonates, enhancing pathogen resistance in plants [135,139].

Taken together, these studies implicate that thiamin and its vitamers, despite being present at a very low concentration, play a general role in central metabolism and in acclimation responses. Indeed, their low concentrations are even representative of known signaling compounds. These results also support the idea that signaling molecules not only coordinate the expression of nuclear and organelle genes, but also maintain cellular functions at optimal levels in response to changes in environmental conditions [15,140,141].
