Plant Molecular Biology 40: 37–44, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
The IRT1 protein from Arabidopsis thaliana is a metal transporter with a
broad substrate range
Yulia O. Korshunova
, Mary Lou Guerinot
and Himadri B.
Department of Biology, Box 1137, One Brookings Drive, Washington University, St. Louis, MO 63130, USA
author for correspondence);
Nutritional Science Program, University of Missouri, Columbia, MO 65211, USA;
Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA
Received 14 September 1998; accepted in revised form 10 December 1998
Key words: copper, iron, manganese, metal transport, plant root, zinc
The molecular basis for the transport of manganese across membranes in plant cells is poorly understood. We
have found that IRT1,anArabidopsis thaliana metal ion transporter, can complement a mutant Saccharomyces
cerevisiae strain defective in high-afﬁnity manganese uptake (smf1). The IRT1 protein has previously been
identiﬁed as an irontransporter. The current studies demonstrated that IRT1, when expressed in yeast, can transport
manganese as well. This manganese uptake activity was inhibited by cadmium, iron(II) and zinc, suggesting that
IRT1 can transport these metals. The IRT1 cDNA also complements a zinc uptake-deﬁcient yeast mutant strain
(zrt1zrt2), and IRT1-dependent zinc transport in yeast cells is inhibited by cadmium, copper, cobalt and iron(III).
However, IRT1 did not complement a copper uptake-deﬁcient yeast mutant (ctr1), implying that this transporter is
not involved in the uptake of copper in plant cells. The expression of IRT1 is enhanced in A. thaliana plants grown
under iron deﬁciency. Under these conditions, there were increased levels of root-associated manganese, zinc and
cobalt, suggesting that, in addition to iron, IRT1 mediates uptake of these metals into plant cells. Taken together,
these data indicate that the IRT1 protein is a broad-range metal ion transporter in plants.
Manganese (Mn) is an essential element required in
trace amounts by virtually all organisms. A number
of metabolic and protein-processing enzymes require
Mnas a cofactor[18,19]. As a redox-activecofactorin
Mn-superoxidedismutase, Mn has animportantrolein
the detoxiﬁcation of free radical forms of oxygen .
This element also plays a crucial role in the life-cycle
of plants and other oxygenic photosynthetic organ-
isms, because it is required for light-induced evolution
of oxygen from water .
We are studying the transport of Mn across the
membranesin plant and bacterial cells [3, 4]. In plants,
Mn is taken up mainly as Mn(II) and is translocated
from the rootsto the shoot through the xylem . Mn
is accumulated in high concentrations in cells, imply-
ing that the uptake of Mn must be carried out by some
energy-requiring transport process . However, the
molecular mechanisms of Mn uptake in plants are not
To identify genes and proteins that are involved
in Mn transport in plants, we have used functional
complementation of yeast mutants. This approach has
been successful in the identiﬁcation and character-
ization of a number of plant transporters, such as
potassium channels [2, 32], and potassium , cop-
per  and iron  transporters. The Smf1 protein,
a member of the Nramp family of metal transporters,
functions as a Mn transporter in the yeast Saccha-
romyces cerevisiae . An smf1 deletion mutant is
viable, but cannot grow on medium containing EGTA.
However, the growth of this mutant can be rescued
by the addition of Mn . In this communication,
we describe the identiﬁcation of the IRT1 gene from