Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 189

factors involved in the development of floral organs, can act non-cell autonomously
to influence floral organ development (Wu et al. , 2003). In this section, I review
these data and describe the mechanisms thought to underlie the non-cell autonomy.
7.3.1 Some of the transcription factors that control floral meristem or organ
identity act non-cell autonomously in the developing flower
The first molecular evidence for non-cell autonomy of genes involved in floral
development came from studying floricaula ( flo ) mutants in Antirrhinum majus .
The FLO and LFY genes are orthologues and have similar functions in Antirrhinum
and Arabidopsis , respectively (Coen et al. , 1990; Weigel et al. , 1992). FLO was
originally cloned based on the insertion of a transposon so that excision of the
transposon restores FLO activity (Coen et al. , 1990). Excision of the transposon can
generate somatic sectors so that a single plant contains both mutant and wild-type
cells. The shoot meristem and flowers that form on the flanks of the meristem consist
of three cell layers, the L1, L2 and L3, which represent separate clones formed
during embryogenesis (Huala & Sussex, 1993; Wu et al. , 2003). FLO mRNA is
expressed in all three layers, but a transposon excision in the flo mutant can generate
a wild-type clone of cells that restores gene function in a single layer. Restoration
of FLO gene function in the L1 allowed the formation of phenotypically wild-type
flowers, whereas FLO function in the L2 or L3 caused the development of flowers
that still showed some aspects of the mutant phenotype (Carpenter & Coen, 1995;
Hantke et al. , 1995). In flo mutants, the expression of the MADS box transcription
factors PLE, which is involved in stamen and carpel development, and DEF, which
is involved in petal stamen and development, is reduced. Analysis of expression of
these genes in flowers in which FLO mRNA is expressed in only one layer showed
that FLO can induce signaling events that affect other layers. For example, in plants
expressing FLO mRNA only in the L1, DEF mRNA expression occurred in all
layers in a similar pattern to wild-type plants, although its expression occurred later
and at lower levels (Carpenter & Coen, 1995; Hantke et al. , 1995).
In Arabidopsis , LFY acts non-cell autonomously in a similar way to FLO in An-
tirrhinum ,but in contrast another floral meristem identity gene, AP1 , acts almost
exclusively cell autonomously. Expression of LFY in the L1 layer using the ML1
epidermal-specific promoter was sufficient to fully complement the lfy mutant phe-
notype (Sessions et al. , 2000), and some plants showed a phenotype similar to that
of plants overexpressing LFY from the CaMV 35S promoter (Weigel & Nilsson,
1995). Also, as shown in Antirrhinum for FLO ,expression of LFY in the L1 layer
was sufficient to activate expression of the gene encoding the MADS box transcrip-
tion factors APETALA3 (AP3), required for petal and stamen development, and
AGAMOUS (AG), required for stamen and carpel development, in all three layers.
However, a quite different effect was observed when AP1 wasexpressed in the L1
layer in ap1 mutants (Sessions et al. , 2000). In these plants rescue of sepal and petal
identity occurred in the first and second whorls, but only in the L1 layer. The L2
and L3 layers of the sepals of these plants were more typical of bracts. Similarly,