Lessons from the vegetative shoot apex - Intercellular Communication in Plants - page 152

Table 6.1 Genes involved in Arabidopsis meristem formation and maintenance
BELLRINGER
( BLR )/ PENNYWISE
( PNY )

The blr / pny mutant phenotype is similar to the brevipedicellus mutant
(see text) having both short internodes resulting in a stunted appearance
and excessive development of axillary meristems (Byrne et al. , 2003;
Smith & Hake, 2003). Although blr / pny mutants have normal SAMs, in
different genetic backgrounds BLR / PNY is required for meristem
maintenance. For instance blr / pny enhances the weak stm mutant
phenotype and is required for meristem maintenance in an as1 stm mutant
background (Bryne et al. , 2003). BLR/PNY encodes a BELL1 -like
homeodomain transcription factor physically interacts with the STM and
KNAT1 proteins, resulting in high affinity binding to target sequences
(Byrne et al. , 2003; Smith & Hake, 2003).

CUP-SHAPED
COTYLEDON1
( CUC1 )
CUP-SHAPED
COTYLEDON2
( CUC2 )
CUP-SHAPED
COTYLEDON3
( CUC3 )

CUC1 , CUC2 and CUC3 encode plant-specific NAC domain proteins with
extensive homology to the NO APICAL MERISTEM ( NAM ) gene of
Petunia and the recently identified CUPILLIFORMIS ( CUP ) gene of
Antirrhinum (Souer et al. , 1996; Aida et al. , 1997; Takada et al. , 2001;
Vroemen et al. , 2003; Weir et al. , 2004). All three genes are required
redundantly to promote the formation of the shoot apical meristem and the
separation of the cotyledons during embryogenesis. In the developing
embryo all three CUC genes are expressed in cells that will form the SAM
and are then subsequently expressed at the boundary between organ
primordia and the meristem during post-embryonic development. Based
on the expression of these genes during embryogenesis and their
combined mutant phenotype, it has been proposed that they either
promote STM expression directly or provide signals that establish a
programme of SAM development (Aida et al. , 1997, 1999; Long &
Barton, 1998; Takada et al. , 2001; Vroemen et al. , 2003). Consistent
with this is the formation of ectopic meristems on cotyledons of plants
expressing CUC1 ubiquitiously (Takada et al. , 2001).

DORNROSCHEN
( DRN )/ ENHANCER
OF SHOOT
REGENERATION1
( ESR1 )

Identified as a gain-of-function mutation, DRN / ESR1 is expressed in lateral
organs and the meristem and encodes an AP2/EREB transcription factor.
Misexpression in cultured roots leads to accelerated regeneration of
shoots (Banno et al. , 2001), whereas increased DRN / ESR1 levels in shoots
causes an enlargement of the meristem and the successive radialisation of
leaves, before the arrest of the meristem. DRN/ESR1 regulates STM ,
CLV3 and WUS expression and functions to repress stem cell fate. The
role of DRN/ESR1 in organ formation is unclear, but appears to be
required non-cell autonomously (Kirch et al. , 2003).

FASCIATA1 ( FAS1 )
FASCIATA2 ( FAS2 )

Mutations in these genes severely disrupt cellular organisation of the SAM
and RAM, leading to fasciation (Leyser & Furner, 1992). In the SAM,
WUS expression expands both laterally and apically into all three layers.
FAS1/2 encode subunits of the Arabidopsis chromatin assembly factor-1
(CAF-1) and presumably functions in maintaining epigenetic states during
cell division within meristems (Kaya et al. , 2001).

FOREVER YOUNG
( FEY )

The fey mutant has an altered phyllotaxis, and produces radially symmetric
organs at a high frequency. In addition cells within the vegetative
meristem and leaf primordia are more vacuolated than normal. FEY
encodes a nodulin-like protein, although how this protein functions in
meristem and leaf development has yet to be determined (Callos et al. ,
1994)