Disease relevance of HPT1

• Isolation and functional analysis of homogentisate phytyltransferase from Synechocystis sp. PCC 6803 and Arabidopsis [1].

High impact information on HPT1

• Surprisingly subtle differences were observed between the tocopherol-deficient vte2 mutant and the wild type during high-light, salinity, and drought stresses [2].
• Both biochemical and transcriptional analyses of vte2 seedlings indicate that nonenzymatic lipid peroxidation plays a significant role in modulating plant defense responses [3].
• In the absence of pathogens, the phytoalexin camalexin accumulated in vte2 seedlings to levels 100-fold higher than in wild-type or vte1 seedlings [3].
• 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) [2].
• The rapid reduction in photoassimilate export in vte2 coincides with callose deposition exclusively in phloem parenchyma transfer cell walls adjacent to the companion cell/sieve element complex [2].

Chemical compound and disease context of HPT1

• HPTs from both organisms used homogentisic acid and PDP as their preferred substrates in vitro but only Synechocystis sp. PCC 6803 HPT was active with geranylgeranyl diphosphate as a substrate [1].
• In addition, disruption of Synechocystis sp. PCC 6803 HPT function causes an absence of tocopherols without affecting plastoquinone-9 levels, indicating that separate polyprenyltransferases exist for tocopherol and plastoquinone synthesis in Synechocystis sp. PCC 6803 [1].

Biological context of HPT1

• 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 [2].
• In this study, we describe the identification of conserved amino acid sequences within VTE2 and HGGT and the application of these conserved sequences for a motif analysis resulting in the discovery of a VTE2-paralog in the Arabidopsis genome [4].

Associations of HPT1 with chemical compounds

• To investigate whether HPT activity is limiting for tocopherol synthesis in plants, the gene encoding Arabidopsis HPT, HPT1, was constitutively overexpressed in Arabidopsis [5].
• This enlarged pool of gamma-tocopherol was almost entirely converted to alpha-tocopherol by crossing HPT1 overexpressing plants with lines constitutively overexpressing gamma-tocopherol methyltransferase [5].
• The committed step in tocopherol biosynthesis involves condensation of homogentisic acid and phytyl diphosphate (PDP) catalyzed by a membrane-bound homogentisate phytyltransferase (HPT) [1].
• In seeds, HPT1 overexpression resulted in a 4-fold increase in HPT specific activity and a total seed tocopherol content that was 40% higher than wild type, primarily because of an increase in gamma-tocopherol content [5].
• Leaf discs of two vitamin E mutants, a tocopherol cyclase mutant (vte1) and a homogentisate phytyl transferase mutant (vte2), were exposed to high light stress at low temperature, which resulted in bleaching and lipid photodestruction [6].

References