Figures (3)  Tables (1)

    • Figure 1. 

      Tocochromanols molecular structure: A chromanol head (with the differences of α, β, γ, and δ corresponding to the number or position of methyl groups in R1 and R2) and a tail of prenyl (non-saturated and with three unsaturation –3', 7', and 11'- for the cases of tocopherols and tocotrienols, respectively). Figure adapted from PubChem.

    • Figure 2. 

      Tocochromanol biosynthetic pathway. Both tocopherol and tocotrienols are synthesized by the combination of homogentisate and phytyl diphosphate (phytyl 2-P)(tocopherols) or geranyl-geranyl diphosphate (geranyl-geranyl 2-P) (tocotrienols). Violet, blue, yellow, and green boxes depict the tocopherol-core, shikimate (SK), methylerythritol-phosphate (MEP), and chlorophyll degradation pathways, respectively. Filled and dotted arrows mean direct and two or more intermediate enzymatic steps, respectively.

    • Figure 3. 

      Tocochromanols influence phytohormone signalling through several interconnected mechanisms. Tocopherol binding protein(s) (TBP) might be one of the carriers for the interactions hypothesized in this scheme. (1) Prevention of ROS accumulation and damage (membranes and cytoplasm). (2) Retrograde signalling pathway. (3) ABA, SA, JA, Aux, and GAs up-regulate the expression of several VTE genes. Aux, GA3, SA, and JA induce GGDR gene expression. (4) Feedback regulation of tocochromanols-ABA. (5) α-tocopherol modulates JA levels. (6) Regulation of ethylene perception by tocopherols. (7) VTE gene transcripts respond to ethylene signalling via CTR1/EIN1. (8) Tococrhomanols act upstream of CKs for activation of antifungal defense. (9) GA3 may influence tocopherol levels through multiple mechanisms. PG: plastoglobuli. SG: stacked grana.

    • Tocochromanol/hormone changes cis-regualtory motifs in VTE genes Key VTE genes (*) Species Tissue Genetic context (evidence type) Ref.
      ABA
      ↑ α-Tocopherol, ↑ γ-Tocopherol VTE1, VTE4, HPPD ↑ Arabidopsis thaliana Leaves ABA treatment (transcriptomic, metabolomic) [22]
      ↑ ABA in vte1, vte4 mutants VTE1, VTE4 (loss) Arabidopsis thaliana Rosettes/roots VTE-deficient mutants + salt stress (genetic/mutant, biochemical) [32]
      ↑ α-Tocopherol VTE3 and VTE4 ↑ Medicago truncatula Seeds ABA-insensitive abi5 mutant (genetic/mutant) [34]
      ABI4 motifs in VTE gene promoters SlVTE4, SlVTE3, SlVTE1, SlHPPD, SlGGPS Solanum lycopersicum Fruits ABI4-binding motifs in VTE gene promoters (transcriptomic, promoter analysis) [28]
      ↑ β-Tocotrienol, ↑ VTE, ↓ ABA after MsHPPD overexpression MsHPPD Arabidopsis thaliana Seeds MsHPPD OE (genetic, transcriptomic) [30]
      ↑ α-Tocopherol ABA dependent/independent VTE2, HPPD ↑ Arabidopsis thaliana Leaves Drought/osmotic stress (metabolomic, transcriptomic) [24]
      ABRE and IIb motifs in VTE
      gene promoters      		 OsγTMT (VTE4), OsHPPD, OsMPBQMT1 (VTE3) Oryza sativa Multiple tissues Promoter analysis (promoter/
      cis-element analysis)      		 [27]
      ↑ α-Tocopherol ↑ ABA HPT/VTE2 Cistus creticus Leaves Drought stress (biochemical) [25]
      Hordeum vulgare; Triticum sp.;
      ABA-responsive elements in VTE2 promoter VTE2 (HPT) Thinopyrum intermedium; Avena sp.; Brachypodium distachyon Leaves/stems/
      inflorescences      		 Promoter analysis (promoter/cis-element analysis) [47]
      SA
      ↓ Tocopherols in vte2
      mutant → ↓ SA      		 VTE2 (loss) Arabidopsis thaliana Leaves vte2 mutant + P. syringae (genetic/mutant, biochemical) [37]
      ↑ SA in VTE-deficient mutants (low phosphate) VTE1, VTE4 (loss) Arabidopsis thaliana Seedlings vte1, vte4 mutants, low-P (genetic/mutant, metabolomic) [48]
      SA-responsive cis-element in VTE4 promoter GmVTE4 Glycine max Leaves Promoter analysis (promoter/cis-element analysis) [31]
      ↑ Tocopherols in heat-shock HPPD, VTE1, VTE2, VTE3, VTE4 Arabidopsis thaliana Seedlings Heat shock treatment (biochemical/metabolomic/genetic) [36]
      ↑ tocopherol (α, γ, δ, total) GGDR Arabidopsis thaliana; Glycine max Hordeum vulgare; Triticum sp.; Seeds and leaves GGDR OE (biochemical, physiological, expression analysis) [61]
      SA-responsive elements in VTE2 promoter VTE2 (HPT) Thinopyrum intermedium; Avena sp.; Brachypodium distachyon Leaves/stems/
      inflorescences      		 Promoter analysis (promoter/cis-element analysis) [47]
      JA
      HAHB4 motifs in VTE gene promoters SlVTE4, SlVTE3, SlVTE1, SlHPPD, SlGGPS Solanum lycopersicum Fruits HAHB4 binding motifs in VTE gene promoters (transcriptomic, promoter analysis) [28]
      ↑ JA in vte1, vte4 mutants VTE1, VTE4 (loss) Arabidopsis thaliana Rosettes/roots VTE-deficient mutants + salt stress (genetic/mutant, biochemical) [32]
      ↑ JA in VTE-deficient mutants (low phosphate) VTE1, VTE4 (loss) Arabidopsis thaliana Seedlings vte1, vte2, vte4 mutants, low-P (genetic/mutant, metabolomic) [48]
      ↓ α-Tocopherol ↑ JA levels VTE1 (loss) Arabidopsis thaliana Leaves vte1 mutant analysis (biochemical, genetic) [5]
      ↑ Tocopherols in JA-insensitive mutants; ↑ VTE4 VTE4 ↑ Solanum lycopersicum Fruits JA-insensitive tomato mutants (genetic/mutant, transcriptomic) [46]
      ↑ tocopherol (α, γ, δ, total) GGDR Arabidopsis thaliana;
      Glycine max      		 Seeds and leaves GGDR OE (biochemical, physiological, expression analysis) [61]
      JA-responsive elements in VTE2 promoter VTE2 (HPT) Hordeum vulgare; Triticum sp.;
      Thinopyrum intermedium; Avena sp.;
      Brachypodium distachyon      		 Leaves/stems/
      inflorescences      		 Promoter analysis (promoter/cis-element analysis) [47]
      Ethylene
      HAHB4 motifs in VTE gene promoters SlVTE4, SlVTE3, SlVTE1, SlHPPD, SlGGPS Solanum lycopersicum Fruits HAHB4 binding motifs in VTE gene promoters (transcriptomic, promoter analysis) [28]
      ↑ ethylene in vte1, vte4 mutants VTE1, VTE4 (loss) Arabidopsis thaliana Rosettes/roots VTE-deficient mutants + salt stress (genetic/mutant, biochemical) [32]
      ↑ α-Tocopherol in ein2-1 mutant (ethylene insensitive) VTE5 Arabidopsis thaliana Roots vte5 /ein2-1 mutants + phytol/nematode treatment (biochemical, genetic) [52]
      Auxin
      ↑ tocopherol (α, γ, δ, total) GGDR Arabidopsis thaliana;
      Glycine max      		 Seeds and leaves GGDR OE (biochemical, physiological, expression analysis) [61]
      Auxin-responsive elements in VTE2 promoter VTE2 (HPT) Hordeum vulgare; Triticum sp.;
      Thinopyrum intermedium; Avena sp.;
      Brachypodium distachyon      		 Leaves/Stems/Inflorescences Promoter analysis (Promoter/cis-element analysis) [47]
      Cytokinin
      ↓ Tocopherols in vte1/vte4 mutants → ↓
      cytokinin increment during fungal infection      		 VTE1/VTE4 (loss) Arabidopsis thaliana Leaves vte Arabidopsis mutants + Botrytis (Genetic/mutant, biochemical) [1]
      Gibberellin
      ↑ Tocopherols, altered α-/γ-ratio (genotype- dependent) VTE1/VTE4 Brassica napus L; Arabidopsis thaliana Seeds GA3 exogenous treatment (Metabolomic) [65]
      ↑ tocopherol (α, γ, δ, total) GGDR Arabidopsis thaliana;
      Glycine max      		 Seeds and leaves GGDR OE (biochemical, physiological, expression analysis) [61]
      GAs-responsive elements in VTE2 promoter VTE2 (HPT) Hordeum vulgare; Triticum sp.;
      Thinopyrum intermedium; Avena sp.;
      Brachypodium distachyon      		 Leaves/stems/inflorescences Promoter analysis (Promoter/cis-element analysis) [47]
      * Only research papers showing evidence about transcriptional control on VTE genes are listed in this table.

      Table 1. 

      Summary table on each hormone-tocochromanol cross-talk evidence.