Agriculture Reference
In-Depth Information
of cells that attaches the proembryo to maternal tissue. At the early globular stage,
the uppermost cell is specified to become the hypophysis - the founder of the root
meristem. At the triangular stage, the early patterning is finished with the initiation
of two symmetrically positioned embryonic leaves - cotyledons (Jurgens, 2001).
A role for auxin in embryo patterning has been suspected for some time, not the
least because embryo defects can be induced by blocking PAT (Hadfi
et al
., 1998).
In addition, genetic disruption of auxin response in
Arabidopsis
mutants such as
monopteros
(
mp
) and
bodenlos
(
bdl
) led to defects in embryonic axis formation.
Molecular analysis of these mutants revealed that
MP
encodes a transcriptional ac-
tivator - the auxin response factor 5 (ARF5), and
BDL
encodes the corresponding
transcriptional repressor - IAA12, both components of auxin signaling (Hardtke &
Berleth, 1998; Hamann
et al
., 2002). To really pinpoint the role of auxin in embryo-
genesis, the distribution of auxin and its response was monitored using anti-IAA
antibodies and
DR5rev::GFP
. Immediately after the division of the zygote, when
the apical cell is specified, auxin accumulates in this cell and during subsequent
development persists in the proembryo. At around the 32-cell stage, when the basal
embryo pole is being specified, the gradient of auxin accumulation suddenly re-
verses and forms a new maximum in the uppermost suspensor cells, including the
hypophysis (Plate 1.1I). At later stages of embryogenesis, additional
DR5
reporter
gene signals appear in the tips of the developing cotyledons (Friml
et al
., 2003).
Both chemical (AEIs) and genetic (
gn
or multiple
pin
mutants) inhibition of PAT in-
terfere with this dynamic distribution of auxin during embryogenesis. Furthermore,
they cause identical developmental defects, ranging from cup-shaped embryos with
misspecified apical structures and a nonfunctional root pole (Plates 1.1F and 1.1G),
to ball-shaped embryos without any discernible apical-basal axis. These findings,
together with analysis of PIN expression and localization, completed the picture,
indicating a role for PIN-dependent auxin distribution in embryo patterning. At early
stages (Fig. 1.5A), auxin is actively provided to the apical cell from the adjacent
basal cell by the action of apically localized PIN7. This apical-basal auxin gradient
is required for the specification of the apical cell. At subsequent stages, the cells of
the suspensor continue to localize PIN7 at their apical side, while in the proembryo
another protein, PIN1, is expressed without apparent polarity. But after the 32-cell
stage (Fig. 1.5B), PIN1 becomes localized to the basal membranes of the provascu-
lar cells, suggesting downward transport toward the region of the future root pole.
Simultaneously, the asymmetric localization of PIN7 is reversed within the basal
cells, mediating auxin transport out of the embryo. Subsequently, PIN4 expression
starts at the basal pole of the embryo, supporting the action of both PIN1 and PIN7.
As a result of these changes in auxin flow, the auxin gradient reverses, displaying
its new maximum in the uppermost suspensor cell, which in response to auxin is
specified to become the hypophysis - the founder of the future root meristem. Thus,
developmentally regulated changes in polarity of PIN proteins result in the redirec-
tion of auxin fluxes for local auxin accumulation, which is then required first for
specification of the apical and later for the basal pole of the embryonic apical-basal
axis (Friml
et al
., 2003).
