Disease relevance of VTE1

• Expression of VTE1 in Escherichia coli resulted in the production of a protein with high tocopherol cyclase and tocotrienol cyclase activity [1].
• The Sucrose export defective1 (Sxd1) gene of maize was cloned and shown to encode a novel protein conserved between plants and cyanobacteria [2].
• In addition, expression of maize SXD1 in a Synechocystis sp. PCC6803 slr1737 knockout mutant restored tocopherol synthesis, indicating that TC activity is evolutionarily conserved between plants and cyanobacteria [3].

High impact information on VTE1

• Two tocopherol-deficient mutant loci in Arabidopsis thaliana were used to examine the functions of tocopherols in seedlings: vitamin e1 (vte1), which accumulates the pathway intermediate 2,3-dimethyl-5-phytyl-1,4-benzoquinone (DMPBQ); and vte2, which lacks all tocopherols and pathway intermediates [4].
• However, vte2, and to a lesser extent vte1, exhibited dramatic phenotypes under low temperature (i.e., increased anthocyanin levels and reduced growth and seed production) [5].
• Together, these results establish that tocopherols in wild-type plants or DMPBQ in vte1 plants limit nonenzymatic lipid peroxidation during germination and early seedling development, thereby preventing the inappropriate activation of transcriptional and biochemical defense responses [4].
• In the absence of pathogens, the phytoalexin camalexin accumulated in vte2 seedlings to levels 100-fold higher than in wild-type or vte1 seedlings [4].
• Compared with the wild type, vte1 and vte2 had reduced rates of photoassimilate export as early as 6 h into low-temperature treatment, increased soluble sugar levels by 60 h, and increased starch and reduced photosynthetic electron transport rate by 14 d [5].

Biological context of VTE1

• Despite these increased sugar concentrations, photosynthetic gene expression was not significantly downregulated in affected areas of sxd1 leaf blades [2].
• During photo-oxidative stress, chlorophyll content and photosynthetic quantum yield were slightly reduced in vte1 as compared with wild type indicating a potential role for tocopherol in maintaining an optimal photosynthesis rate under high-light stress [1].
• Whereas growth, chlorophyll content, and photosynthetic quantum yield were very similar to wild type in vte1, vtc1, cad2, or vte1vtc1, they were reduced in vte1cad2, indicating that the simultaneous loss of tocopherol and glutathione results in moderate oxidative stress that affects the stability and the efficiency of the photosynthetic apparatus [6].
• The defective export of Suc in sxd1 suggests that in addition to presumed antioxidant activities, tocopherols or tocopherol breakdown products also function as signal transduction molecules, or, alternatively, the DMPBQ that accumulates in sxd1 disrupts signaling required for efficient Suc export in maize [3].
• By using vte1 mutants of Arabidopsis thaliana, with an insertion in the promoter region of the gene encoding tocopherol cyclase, we demonstrate here for the first time that tocopherol deficiency may alter endogenous phytohormone levels in plants, thereby reducing plant growth and triggering anthocyanin accumulation in leaves [7].

Anatomical context of VTE1

• We demonstrate specific plastoglobule association of members of the plastid lipid-associated proteins/fibrillin family as well as known metabolic enzymes, including the tocopherol cyclase (VTE1), a key enzyme of tocopherol (vitamin E) synthesis [8].
• A model for SXD1 function is proposed in which the protein is involved in a chloroplast-to-nucleus signaling pathway necessary for proper late-stage differentiation of maize bundle sheath cells, including the developmentally regulated modification of plasmodesmata [2].

Associations of VTE1 with chemical compounds

• The VTE1 sequence shows no similarities to genes with known function, but is similar to that of SXD1, a gene that was recently isolated based on the availability of the sucrose export defective1 maize mutant (sxd1) [1].
• Expression studies demonstrated that indeed VTE1 is a major limiting factor of tocopherol synthesis in leaves [6].
• Overexpression of VTE1 resulted in an increase in total tocopherol of at least 7-fold in leaves, and a dramatic shift from alpha-tocopherol to gamma-tocopherol [6].
• To study vitamin E deficiency in plants, a high-throughput biochemical screen for vitamin E quantification in Arabidopsis mutants has been developed, which has led to the identification of VTE1-encoding tocopherol cyclase [9].
• Levels of sucrose, glucose, and fructose were compared between wild-type and sxd1 plants [2].

Analytical, diagnostic and therapeutic context of VTE1

• In situ hybridization studies revealed high levels of Sxd1 mRNA in bundle sheath cells, with lower levels within the mesophyll [2].

References