Ascorbate and Thiamin: Metabolic Modulators in Plant Acclimation Responses

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

Figure 1

D-Mannose/L-Galactose pathway of ascorbate biosynthesis in plants. The genes of the pathway are highlighted in purple and written in italics. The enzymes are highlighted in green. Phosphoglucose isomerase (PGI), phosphomannose isomerase (PMI), and phosphomannomutase (PMM) are responsible for the conversion of D-glucose-6-P to D-mannose-1-P, the direct precursor of GDP-D-mannose pyrophosphorylase (GMP), the first committed enzyme of the pathway encoded by VTC1. GDP-mannose-3′-5′-epimerase (GME), GDP-L-galactose transferase (GGP), L-galactose-1-phosphate phosphatase (GPP), L-galactose dehydrogenase (GDH), and L-galactono-1,4-lactone dehydrogenase (GLDH) are the next enzymes of the pathway. GGP is the key enzyme of the pathway encoded by VTC2 and VTC5 paralogs. This enzyme undergoes feedback regulation by ascorbate pool size. GLDH is located in the intermembrane of mitochondria and is connected to the mitochondria respiratory chain.

Figure 2

Overview of light regulation on ascorbate biosynthesis. Ascorbate biosynthesis is regulated transcriptionally by the ascorbic acid mannose pathway regulator 1 (AMR1), ethylene response factor 98 (ERF98), and Solanum lycopersicum basic helix–loop–helix (bHLH) transcription factor 59 (slbHLH59; tomato-specific). AMR1 has negative effects on GMP, GME, GGP, GPP, GDH, and GLDH in Arabidopsis. AMR1 expression decreases rapidly under light; thereafter, ascorbate levels increase. AtERF98 is the positive regulator of the ascorbate pathway by directly binding to the promoter of VTC1, and up-regulating the expression of VTC1, VTC2, GDH, and GLDH. The light-specific functionality of ERF98 has yet to be investigated. slbHLH59 activates the genes of the pathway in tomato fruits and increases ascorbate levels under the light. The close homolog of this transcription factor (TF) in Arabidopsis is unfertilized embryo sac 12 (UNE12), however, its role for ascorbate biosynthesis has yet to be investigated. Ascorbate undergoes post-translational regulation via constitutive photomorphogenic9-signalosome subunit 5B (CSN5B), VTC3, and the feedback regulation of VTC2. CSN5B binds to VTC1 and promotes its degradation under the dark. VTC3 is a putative kinase/phosphatase for light regulation of ascorbate. Feedback regulation of ascorbate is controlled by an unusual open reading frame (uORF), located upstream of the VTC2 gene. uORF functionality under the light needs further investigations. Ascorbate is also controlled via photosynthetic and mitochondria electron transport chains, designated as photosynthetic electron transport chain (pETC) and mitochondrial electron transport chain (mETC), respectively. This relationship is bidirectional.

Figure 3

Thiamin biosynthesis pathway in plants. Thiamin is synthesized from pyrimidine and a thiazole moiety, both of which are synthesized in the chloroplast. The detailed description of the pathway is presented in the text. Abbreviations of the substrates: 5-aminoimidazole ribonucleotide (AIR), S-adenosylmethionine (SAM), nicotinamide adenine dinucleotide (NAD⁺), 4-amino-2-methyl-5-hydroxymethylpyrimidine phosphate (HMP-P), adenylated thiazole intermediate (ADT), 4-methyl-5-b-hydroxyethylthiazole phosphate (HET-P), thiamin monophosphate (TMP), thiamin pyrophosphate (TPP). Abbreviations of the enzymes: thiamin C synthase (THIC), -methyl-5-b-hydroxyethylthiazole phosphate (HET-P) synthase (THI1), thiamin monophosphate pyrophosphorylase (TH1), haloacid dehalogenase (HAD) family phosphatase (TH2), TPP kinases (TDPK).

Figure 4

Cell localization of the main thiamin dependent-enzymes and its participation in metabolic pathways. Thiamin pyrophosphate (TPP), the active form of thiamin, works as an essential coenzyme for the enzymes involved in photosynthesis in chloroplasts, pentose phosphate pathway, and alcoholic fermentation in cytoplasm, as well as in ATP synthesis in the participation in oxidative decarboxylation of pyruvate and tricarboxylic acid cycle in mitochondrial central metabolism. Thiamin has also been shown to be involved in the acclimation responses to abiotic stresses; photoperiod; and working directly as an antioxidant, scavenging ROS; a protection molecule; and indirectly by contributing to the cell energy poll, conferring the cell the necessary metabolic flexibility to acclimate to new conditions. The thiamin-dependent enzymes shown are α-ketose transketolase (TK); 1-deoxy-D-xylulose-5-phosphate synthase (DXPS); acetohydroxyacid synthase (AHAS); pyruvate dehydrogenase (PDH); 2-oxoglutarate dehydrogenase (OGDH); and branched chain 2-oxoacid dehydrogenase (BCOADH). MEP, methylerythritol pathway.

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.