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Altered Zn Compartmentation in the Root Symplasm and Stimulated
Zn Absorption into the Leaf as Mechanisms Involved in Zn
Hyperaccumulation in Thlaspi caerulescens
Mitch M. Lasat,
Alan J.M. Baker, and
Leon V. Kochian*
United States Plant, Soil, and Nutrition Laboratory, United States
Department of Agriculture-Agricultural Research Station, Cornell
University, Ithaca, New York 14853 (M.M.L., L.V.K.); and Department of
Animal and Plant Sciences, University of Sheffield, Sheffield, United
Kingdom S10 2TN (A.J.M.B.)
We investigated Zn compartmentation
in the root, Zn transport into the xylem, and Zn absorption into leaf
cells in Thlaspi caerulescens, a Zn-hyperaccumulator
species, and compared them with those of a related nonaccumulator
species, Thlaspi arvense. Zn-compartmental
analysis conducted with roots of the two species indicated that a
significant fraction of symplasmic Zn was stored in the root vacuole of
T. arvense, and presumably became unavailable for
loading into the xylem and subsequent translocation to the shoot. In
T. caerulescens, however, a smaller fraction of the absorbed Zn was stored in the root vacuole and was readily transported back into the cytoplasm. We conclude that in T. caerulescens, Zn absorbed by roots is readily available for
loading into the xylem. This is supported by analysis of xylem exudate
collected from detopped Thlaspi species seedlings. When
seedlings of the two species were grown on either low (1 µM) or high (50 µM) Zn, xylem sap of
T. caerulescens contained approximately 5-fold more Zn
than that of T. arvense. This increase was not
correlated with a stimulated production of any particular organic or
amino acid. The capacity of Thlaspi species cells to
absorb 65Zn was studied in leaf sections and leaf
protoplasts. At low external Zn levels (10 and 100 µM),
there was no difference in leaf Zn uptake between the two
Thlaspi species. However, at 1 mM
Zn2+, 2.2-fold more Zn accumulated in leaf sections of
T. caerulescens. These findings indicate that altered
tonoplast Zn transport in root cells and stimulated Zn uptake in leaf
cells play a role in the dramatic Zn hyperaccumulation expressed in
T. caerulescens.
*
Corresponding author; e-mail lvk1{at}cornell.edu; fax
1-607-255-1132.
Plant Physiol. (1998) 118: 875-883
Copyright Clearance Center: 0032-0889/98/118//09
© 1998 American Society of Plant Physiologists
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