Plant Molecular Biology 50: 743–756, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Post-phloem protein trafﬁcking in the maize caryopsis: ZmTRXh1,a
thioredoxin speciﬁcally expressed in the pedicel parenchyma of Zea mays
L., is found predominantly in the placentochalaza
, Timothy O’Connell
and Richard D. Thompson
Max-Planck-Institut für Züchtungsforschung, Carl-von-Linn´e Weg 10, D-50829 KÖLN Germany;
dresses: G.S.; MPI for Medical Research, Jahnstrasse 29, D-69120 Heidelberg Y.G.; Department of Molecular
and Cellular Biology, University of Arizona, Tucson, AZ, USA, T. O’C.; Sloning Bio Technology GmbH,
Neumünster, Germany (Author for correspondence, Legume Laboratory, 21110 Breteni`eres, France) (
correspondence: email: firstname.lastname@example.org)
Received 31 January 2002; accepted in revised form 15 April 2002
Key words: endosperm, maize, pedicel, phloem, solute, thioredoxin
The pedicel of the maize (Zea mays L.) caryopsis is a complex structure consisting of layers of specialized cell-
types involved with solute transfer into the developing kernel. A molecular marker for one of these cell layers, the
phloem parenchyma, has been obtained by differential screening of a cDNA library from 7 days after pollination
(DAP) kernels. The clone encodes a novel processed type of thioredoxin, ZmTRXh1, with a variant active site
sequence. The transcript is exclusively present in phloem parenchyma cells of the pedicel. The protein accumulates
predominantly in the adjacent placentochalazal layer up to 21 DAP, declining thereafter until, at 31 DAP, only
traces remain. ZmTRXh1, which is catalytically active, is present in the cytosol, and is restricted to a fraction of
the placentochalazal cells at 12 DAP, where it accumulates to high levels. ZmTRXh1 represents the ﬁrst example of
post-phloem protein trafﬁcking in the pedicel. The reasons for this transfer and possible functions for the protein
in the placentochalaza are discussed.
Abbreviations: DAP, days after pollination, PP, pedicel parenchyma, PCR, polymerase chain reaction, MES, 2-
(N-morpholino)ethanesulphonic acid, NTR, NADPH-thioredoxin reductases, PVDF, polyvinylidene diﬂuoride,
EDTA, Ethyl diamine tetraacetic acid, SDS, Sodium Dodecyl Sulphate, DIG, digoxigenin.
Solute transfer into the developing cereal grain takes
place via a specialized structure, the maternal pedicel.
Phloem sap is down-loaded at the phloem terminals,
and passes symplastically from the sieve elements into
adjacent companion cells, and from there into the thin-
walled and highly vacuolated pedicel parenchyma.
The cells of the pedicel parenchyma (PP) zone are
connected by numerous plasmodesmata, allowing for
further symplastic transport. The cells accumulate tan-
nin inclusions in vacuoles, which on maturity are
released and incorporated in secondary cell wall de-
positions. The pedicel layer between the PP and the
basal endosperm has quite a different structure to that
of the parenchyma. This specialized layer, termed the
placentochalaza, consists of rectangular, thick-walled
cells oriented perpendicular to the direction of solute
transport. Placentochalazal cells lose their cytoplas-
mic contents and become enucleate early in caryopsis
development. Upon maturing they form the black or
closing layer, the site of abscission of the kernel from
the peduncle (Daynard and Duncan, 1969).
Solute exchange across the pedicel/basal en-
dosperm junction is exclusively apoplastic, due to the
lack of plasmodesmata (Thorne, 1985). The heavy
suberization of the placentochalazal cell walls may
play a role in pathogen resistance or provide a route