Agriculture Reference
In-Depth Information
(Xu
et al.
, 2003). Similar interactions between
KNOX
and
AS1
-like genes occur in
maize and
Antirrhinum
, showing the importance of this pathway in defining cell
identities within the shoot apex (Timmermans
et al.
, 1999; Tsiantis
et al.
, 1999).
In addition to
STM
, there are three other class I
KNOX
genes in
Arabidopsis
. These
genes -
BREVIPEDICELLUS
[
BP
, formerly
KNOTTED-like from ARABIDOPSIS
THALIANA1
(
KNAT1
)],
KNAT2
and
KNAT6
- are expressed in the shoot apical
meristem; however unlike
STM
, their expression is restricted to specific domains (for
example see Lincoln
et al.
, 1994).
BP/KNAT1
is expressed in the basal regions of the
meristem around the sites of incipient organ formation, where one of its functions
is to promote internode growth (Douglas
et al.
, 2002; Venglat
et al.
, 2002). The
function of
KNAT2
has yet to be determined, as
knat2
mutants lack a discernable
phenotype, possibly because of redundancy with the closely related
KNAT6
gene
(Byrne
et al.
, 2002).
The surprising finding that
as1 stm
plants have relatively normal vegetative meris-
tem raised the possibility that in the absence of
STM
, other
KNOX
genes might
acquire
STM
-like functions (Byrne
et al.
, 2000). Consistent with this hypothesis is
the finding that
BP/KNAT1
promotes meristem formation in
as1 stm
mutant plants
(Byrne
et al.
, 2002). Based on these observations, a model depicting the likely inter-
actions between
KNOX
genes and
AS1
/
AS2
has been proposed (see Fig. 6.3). While
the downregulation of
KNOX
genes is closely associated with organ formation and
Figure 6.3
Factors regulating meristem maintenance and the patterning of lateral organs. This model
shows the likely relationship between
KNOX
genes (
STM
,
KNAT1
),
AS1
,
AS2
and growth regulators
such as gibberellin (GA) and cytokinin (CK) in the meristems and organs of
Arabidopsis
.
STM
is
expressed throughout the meristem but excluded from organ founder cells and initiating primordia. The
function of
STM
is to keep meristems cell in an undifferentiated state, which is achieved in two ways.
STM
restricts
AS1
and
AS2
expression to organ initials and in doing so allows
KNAT1
expression in the
meristem. Both
KNOX
genes prevent an accumulation of GA in the meristem by directly repressing the
expression of GA biosynthetic genes. In contrast,
KNOX
genes promote the accumulation of CK, which
in turn promotes
KNOX
gene expression and meristem activity.
AS1
and
AS2
accumulate in organ
primordia, where they likely form a complex that represses
KNOX
gene expression and allows, either
directly or indirectly, accumulation of GA.
