Plant Molecular Biology 50: 237–248, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Molecular characterization of AtNAM: a member of the Arabidopsis NAC
, Tzung-Fu Hsieh
, Soo Young Kim
and Terry L. Thomas
Department of Biology, Texas A&M University, College Station, TX 77843, USA (
author for correspondence;
Kumho Life and Environmental Science Laboratory, 572 Ssang-Am-Dong, Kwang-
San-Gu, Kwangju 506-712, Korea
Received 14 March 2001; accepted in revised form 1 February 2002
Key words: DNA-binding domain, shoot meristem, transcriptional activation
The petunia NAM and Arabidopsis ATAF1 and CUC2 genes deﬁne the conserved NAC domain. In petunia, loss-of-
function nam mutants result in embryos that fail to elaborate shoot apical meristems (SAM), and nam seedlings do
not develop shoots and leaves. We have isolated a NAC domain gene, AtNAM, from an Arabidopsis developing seed
cDNA library. Expression of AtNAM mRNA is restricted primarily to the region of the embryo including the SAM.
The AtNAM gene contains three exons and is located on Chromosome 1. In vivo assays in yeast demonstrate that
AtNAM encodes a transcription factor and that the NAC domain includes a speciﬁc DNA binding domain (DBD).
The AtNAM DBD is contained within a 60 amino acid region which potentially folds into a helix-turn-helix motif
that speciﬁcally binds to the CaMV 35S promoter. The putative transcriptional activation domain is located in the
C-terminal region of the protein, a highly divergent region among NAC domain-containing genes. The Arabidopsis
genome contains 90 predicted NAC domain genes; we refer to these collectively as the AtNAC superfamily. The
ﬁrst two exons of all members of this superfamily encode the NAC domain. Most AtNAC genes contain three
exons with the last exon encoding an activation domain. A subfamily of AtNAC genes contains additional terminal
exons coding for protein domains whose functions are unknown.
Abbreviations: SAM, shoot apical meristem; NAC, NAM, ATAF1, and CUC2, DBD, DNA-binding domain
Plant morphogenesis requires precise regulation of the
rate of cell division, the orientation and position of the
cell division plane and cell expansion. For example,
the ﬁrst cell division that follows egg fertilization in
Arabidopsis is always asymmetric and occurs perpen-
dicularly with respect to the longitudinal axis of the
ovule. After the 2-cell stage, cell division conforms to
a strict pattern and indeed determines the shape and the
size of the embryo. Embryogenesis speciﬁes the three
polarity axes as well as the main organs and tissues of
The nucleotide sequence reported will appear in the GenBank
database under the accession number AF123311(AtNAM).
Postembryonic development and growth is fueled
by cell division occurring in specialized tissues re-
ferred to as meristems. At the shoot apex lies the shoot
apical meristem (SAM) which is speciﬁed as the em-
bryo enters the early maturation stage. The SAM is a
group of undifferentiated cells that generates the aerial
organs of the plant. These cells keep their pluripotency
and represent the reservoir of cells for the continu-
ous growth of the plant during its life cycle. In the
shoot apical meristem, the rate of cell division is bal-
anced with the rate of cell differentiation to achieve a
steady state with respect to the number of cells that
constitutes the SAM.
Homologues of the petunia NO APICAL MERIS-
TEM gene have been shown to be involved in the
development and maintenance of the shoot apical