First published online January 9, 2003; 10.1104/pp.015222
Plant Physiol, February 2003, Vol. 131, pp. 632-642
Homogentisate Phytyltransferase Activity Is Limiting for
Tocopherol Biosynthesis in Arabidopsis
Eva
Collakova and
Dean
DellaPenna*
Department of Biochemistry and Molecular Biology, Michigan State
University, East Lansing, Michigan 48824
Tocopherols are essential components of the human diet and
are synthesized exclusively by photosynthetic organisms. These lipophilic antioxidants consist of a chromanol ring and a 15-carbon tail derived from homogentisate (HGA) and phytyl diphosphate, respectively. Condensation of HGA and phytyl diphosphate, the committed
step in tocopherol biosynthesis, is catalyzed by HGA phytyltransferase
(HPT). To investigate whether HPT activity is limiting for tocopherol
synthesis in plants, the gene encoding Arabidopsis HPT,
HPT1, was constitutively overexpressed in Arabidopsis. In leaves, HPT1 overexpression resulted in a 10-fold
increase in HPT specific activity and a 4.4-fold increase in total
tocopherol content relative to wild type. 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  -tocopherol content. This
enlarged pool of -tocopherol was almost entirely converted to
 -tocopherol by crossing HPT1 overexpressing plants
with lines constitutively overexpressing -tocopherol
methyltransferase. Seed of the resulting double overexpressing lines
had a 12-fold increase in vitamin E activity relative to wild type.
These results indicate that HPT activity is limiting in various
Arabidopsis tissues and that total tocopherol levels and vitamin E
activity can be elevated in leaves and seeds by combined overexpression
of the HPT1 and -tocopherol
methyltransferase genes.
*
Corresponding author; e-mail
dellapen{at}msu.edu; fax 517-353-9334.
© 2003 American Society of Plant Biologists
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