Agriculture Reference
In-Depth Information
the activation of
AP3
and
AG
in
ap1 ML1::AP1
plants was restricted to the L1 layer,
indicating that
AP1
acts cell autonomously. These observations suggest that LFY,
but not AP1, triggers signaling between cell layers in the developing flower.
Non-cell autonomy in floral development is not restricted to floral meristem
identity genes that act early in floral development, such as
AP1
and
LFY
,but is also
shown by genes that specify the identity of individual organs. An example of this
comes from studies with the
DEF
and
GLOBOSA
(
GLO
) genes of
Antirrhinum
,
which are required for normal petal and stamen development (Sommer
et al.
, 1990;
Troebner
et al.
, 1992). Transposon-induced reversion of
def
mutations generated
somatic sectors in which activity of the gene occurred only in the L1 (Perbal
et al.
,
1996). This was sufficient to cause expansion of second whorl organs, similar to the
expansion shown by petal lobes, and accumulation of pigment in L1 cells similar
to the epidermis of petals. However, L2 and L3 cells accumulated chlorophyll and
did not show petal identity. Expression of
DEF
mRNA in the L1 is therefore not
sufficient to confer full petal identity on inner layers. These observations were
confirmed using transgenic
Antirrhinum
plants expressing
DEF
only in the L1 from
the
ANTIRRHINUM FIDDLEHEAD
(
AFI
) promoter (Efremova
et al.
, 2001). In
contrast, sectors in which
DEF
mRNA was expressed in the L2 and L3 were sufficient
to confer petal identity on the L1 layer, but not to promote petal expansion. This
observation indicates that
DEF
can act non-cell autonomously in the inner layers
to confer petal identity on the epidermis. Further examples of the analysis of cell
autonomy in floral organ identity gene function are those performed in
Arabidopsis
on
AG
, which is required for stamen and carpel expression, and on the
PISTILLATA
(
PI
) and
APETALA
3(
AP3
) genes that are required for petal and stamen identity
(Bouhidel & Irish, 1996; Sieburth
et al.
, 1998; Jenik & Irish, 2001).
7.3.2 Movement of transcription factors between cells defines one
mechanism for short-distance signaling in the developing flower
Signaling between animal cells typically involves activation of a receptor located
in the membrane of one cell by a ligand formed in a nearby cell (Pires-daSilva &
Sommer, 2003). Until recently short-distance signaling between plant cells during
flower development was assumed to be exclusively based on similar processes.
However, analysis of KNOTTED (KN), a homeobox transcription factor of maize,
demonstrated that transcription factors can move between plant cells (Jackson
et al.
,
1994) and that this may provide an alternative to membrane-bound receptor-based
systems in plant cells. The original observation was that although
KN
mRNA is
detected only in the L2 and L3 layers of maize SAM, the protein is found in the
L1 (Jackson
et al.
, 1994). The authors proposed that although KN is a nuclear
transcription factor, it may nevertheless move between plant cells. This was later
confirmed using fluorescently labeled forms of the KN protein (Lucas
et al.
, 1995;
Kim
et al.
, 2002, 2003).
The mechanism by which KN moves between plant cells has not been established,
butislikely to involve movement through plasmodesmata. These are channels that
