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The Arabidopsis LEAFY COTYLEDON2
(LEC2) gene is a central embryonic regulator that serves
critical roles both early and late during embryo development.
LEC2 is required for the maintenance of suspensor
morphology, specification of cotyledon identity, progression through
the maturation phase, and suppression of premature germination. We
cloned the LEC2 gene on the basis of its chromosomal
position and showed that the predicted polypeptide contains a B3
domain, a DNA-binding motif unique to plants that is characteristic of
several transcription factors. We showed that LEC2 RNA
accumulates primarily during seed development, consistent with our
finding that LEC2 shares greatest similarity with the B3 domain
transcription factors that act primarily in developing seeds,
VIVIPAROUS1/ABA INSENSITIVE3 and FUSCA3. Ectopic, postembryonic
expression of LEC2 in transgenic plants induces the
formation of somatic embryos and other organ-like structures and often
confers embryonic characteristics to seedlings. Together, these results
suggest that LEC2 is a transcriptional regulator that establishes a
cellular environment sufficient to initiate embryo development.
Embryogenesis in flowering
plants begins with the double fertilization event in which the zygote
and endosperm are formed after fusion of sperm cells with the egg cell
and central cell of the female gametophyte, respectively. The endosperm
initially undergoes syncytial development with formation of
nuclear-cytoplasmic domains, but later cellularizes (1). The developing
embryo is nourished by the endosperm and, in many plants, only the
peripheral layer of the endosperm remains in the mature seed.
Development of the zygote into the mature embryo can be divided
conceptually into two distinct phases. During the early morphogenesis
phase, the basic body plan of the plant is established with expression
of polarity as a shoot–root axis, specification of morphological
domains within the embryo, and formation of embryonic tissue and organ
systems (2–4). The morphogenesis phase is followed by a period of
maturation in which processes critical for seed formation occur (5, 6).
During this late phase, reserves such as storage proteins and lipids
are synthesized at high rates and accumulate in the seed. It is also
during the maturation phase that the embryo acquires the ability to
withstand desiccation at the final stage of seed development. At the
end of embryogenesis, the seed consists of a mature, desiccated embryo
that is quiescent metabolically. Although many aspects of embryogenesis
have been characterized extensively, little is known at a mechanistic
level of the processes that initiate embryo development.
The Arabidopsis LEAFY COTYLEDON (LEC) genes,
LEC1, LEC2, and FUSCA3 (FUS3), play
key roles in controlling embryo development (7). Unlike most other
embryonic regulators that function during specific stages of
embryogenesis (8–11), LEC genes are unique in that they are
required for normal development during both the morphogenesis and
maturation phases. Early in embryogenesis, LEC genes are
required to specify suspensor cell fate and cotyledon identity
(12–16). Late in embryogenesis, LEC genes are needed during
the maturation phase for the acquisition of desiccation tolerance and
the expression of many maturation-specific genes (13–17). Consistent
with the finding that conditions that promote maturation suppress
germination (6), lec mutant embryos prematurely activate the
postgermination program (13, 15, 16, 18). Thus, LEC genes
play a central role in controlling many aspects of embryogenesis, and
they are candidates as regulators that coordinate the morphogenesis and
maturation phases.
Identification and analysis of two LEC genes confirmed their
regulatory role in embryogenesis and provided insight into their
functions. LEC1 shares extensive sequence similarity with the HAP3
subunit of CCAAT-binding transcription factor, implicating LEC1 as a
transcriptional regulator (12). Ectopic expression of LEC1
confers embryonic characteristics to seedlings and results in the
formation of embryo-like structures on the surfaces of leaves,
suggesting that the gene plays a role in conferring embryogenic
competence to cells (12). Thus, we hypothesized that LEC1 establishes a
cellular environment that promotes embryo development and that this
environment coordinates the morphogenesis and maturation phases.
FUS3 also encodes a regulatory protein: a B3 domain
transcription factor that accumulates primarily during seed development
(19). Transient assays showed that FUS3 is sufficient to activate genes
usually expressed during maturation (20). Thus, two LEC
genes seem to be involved in controlling embryo development by
regulating transcription of other genes.
In this article, we focus on the LEC2 gene to determine its
role in embryo development. Because genetic studies suggest that
LEC1 and LEC2 may have partially redundant
functions (12, 15), it is possible that LEC2 also functions in the
initiation and coordination of embryo development. We cloned the
LEC2 gene and showed that it is expressed preferentially
during embryogenesis and encodes a protein with similarity to other
seed-specific transcription factors. Significant insight into the role
of the gene was obtained by showing that transgenic plants expressing
the LEC2 gene ectopically form somatic embryos. Together,
these results indicate that LEC2 is sufficient to induce
embryogenic competence.
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