Plant Molecular Biology 40: 343–354, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
ZmOCL1, an HDGL2 family homeobox gene, is expressed in the outer cell
layer throughout maize development
Gwyneth C. Ingram, Jean-Louis Magnard
, Philippe Vergne, Christian Dumas and Peter M.
R´eproduction & D´eveloppement des Plantes, UMR 9938 CNRS/INRA/ENS, Ecole Normale Sup´erieure de Lyon,
author for correspondence);
Present address: Plant Gene Expression Centre, USDA/ARS, University of
California, Berkeley, USA
Received 13 August 1998; accepted in revised form 5 February 1999
Key words: maize, protoderm, epidermis, homeobox, HDGL2
The formation of a morphologically distinct outer cell layer or protoderm is one of the ﬁrst and probably one
of the most important steps in patterning of the plant embryo. Here we report the isolation of ZmOCL1 (OCL
for outer cell layer), a member of the HDGL2 (also known as HD-ZIP IV) subclass of plant-speciﬁc HD-ZIP
homeodomain proteins from maize. ZmOCL1 transcripts are detected very early in embryo development, before
a morphologically distinct protoderm is visible, and expression then becomes localised to the protoderm of the
embryo as it develops. Subsequently, expression is observed in the L1 cell layer of both the developing primary
root and shoot meristems, and is maintained in developing leaves and ﬂoral organs. We propose that ZMOCL1
may play a role in the speciﬁcation of protoderm identity within the embryo, the organisation of the primary
root primordium or in the maintenance of the L1 cell layer in the shoot apical meristem. We also show that the
expression of ZmOCL1 is different from that of another epidermal marker gene, LTP2 (lipid transfer protein) and,
in meristems, is complementary to that of Kn1 (Knotted) which is transcribed only in underlying cell layers.
The development of maize embryos is a complex
process during which a seemingly disorganised globu-
lar mass of cells is transformed into a miniature plant.
The mature embryo contains a shoot, composed of an
apical meristem and several leaf primordia surrounded
by the coleoptile, a main root primordium, several ad-
ventitious root primordia and a single cotyledon (the
scutellum). The process of early pattern formation in
the maize embryo is poorly understood compared to
that in other model systems such as Arabidopsis.This
is largely due to the fact that early Arabidopsis embryo
development proceeds as a series of well characterised
cell divisions whereas early divisions observed in the
The nucleotide sequence data reported will appear in the
EMBL, GenBank and DDBJ Nucleotide Sequence Databases under
the accession number Y17898.
maize embryo follow no readily deﬁnable pattern after
the two-cell stage. The basic functions of the early
embryogenic process are, however, identical in the
two species, namely to produce a structure with a
speciﬁed ‘top’ (apical pole) and ‘bottom’ (basal pole),
‘outside’ (protoderm) and ‘inside’ (ground tissue and
procambial tissue) which is capable of differentiating
the meristems from which the organs of the mature
plant will develop.
The process of apical/basal and inside/outside pat-
tern formation in the plant embryo has been the subject
of considerable speculation and many reviews (West
and Harada, 1993; Goldberg et al., 1994; Jurgens,
1995; Meinke, 1995; Torres-Ruiz et al., 1996). Since
the egg cell is already highly polarised before fer-
tilisation, it is likely that the apical/basal polarity
of the zygote is determined very early, possibly by
the localisation of maternal factors within the egg