First published online October 3, 2002; 10.1104/pp.007062
Plant Physiol, October 2002, Vol. 130, pp. 649-656
L-Ascorbic Acid Is Accumulated in Source Leaf
Phloem and Transported to Sink Tissues in Plants1
Vincent R.
Franceschi* and
Nathan M.
Tarlyn
School of Biological Sciences, Washington State University,
Pullman, Washington 99164-4236
L-Ascorbic acid (AsA) was found to be loaded into
phloem of source leaves and transported to sink tissues. When
L-[14C]AsA was applied to leaves of intact
plants of three different species, autoradiographs and HPLC analysis
demonstrated that AsA was accumulated into phloem and transported to
root tips, shoots, and floral organs, but not to mature leaves. AsA was
also directly detected in Arabidopsis sieve tube sap collected from an
English green aphid (Sitobion avenae) stylet. Feeding a
single leaf of intact Arabidopsis or Medicago sativa
with 10 or 20 mM L-galactono-1,4-lactone (GAL-L), the immediate precursor of AsA, lead to a 7- to
8-fold increase in AsA in the treated leaf and a 2- to 3-fold increase of AsA in untreated sink tissues of the same plant. The amount of AsA
produced in treated leaves and accumulated in sink tissues was
proportional to the amount of GAL-L applied. Studies of the ability of organs to produce AsA from GAL-L showed mature
leaves have a 3- to 10-fold higher biosynthetic capacity and much lower AsA turnover rate than sink tissues. The results indicate AsA transporters reside in the phloem, and that AsA translocation is likely
required to meet AsA demands of rapidly growing non-photosynthetic tissues. This study also demonstrates that source leaf AsA biosynthesis is limited by substrate availability rather than biosynthetic capacity,
and sink AsA levels may be limited to some extent by source production.
Phloem translocation of AsA may be one factor regulating sink
development because AsA is critical to cell division/growth.
1
This work was supported in part by the National
Science Foundation (grant no. MCB-9904562 to V.R.F.).
*
Corresponding author; e-mail vfrances{at}mail.wsu.edu; fax
509-335-3184.
© 2002 American Society of Plant Physiologists
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