Agriculture Reference
In-Depth Information
HAIRY MERISTEM
(
HAM
)
A
Petunia
mutation that has a similar mutant phenotype to
wus
.
HAM
encodes a
GRAS
-like transcription factor that is predominantly expressed
in the L3 layer of lateral organs and in the provascular tissue of the stem.
HAM
therefore promotes a non-cell autonomous signal that keeps
meristem cells in an undifferentiated state. Absence of this signal prevents
meristem cells from responding to
WUS
or
STM
expression (Stuurman
et al.
, 2002)
MGOUN1
(
MGO1
)
MGOUN2
(
MGO2
)
Mutations in these two loci cause a reduction in the number of leaves and
floral organs and an enlargement of the meristem. Within the meristem,
the organisation of the layers is disrupted. Following the transition to
flowering
mgo
meristems often fasciate, although the structure of these
apices are different to the fasciated
clv
meristems, in that there appears to
be many individual meristems grouped together within the large dome.
The enlarged meristem may be a secondary affect arising from the
reduction in organ primordia formation and the consequential increase in
cells within the peripheral zone. The
MGO
s are therefore likely to
function primarily in the peripheral zone where they promote organ
formation (Laufs
et al.
, 1998a).
PINHEAD
(
PNH
)/
ZWILLE
(
ZWL
)
PNH
is required for efficient SAM formation during embryogenesis and for
its maintenance post-embryonically. Termination of the SAM is often
associated with the formation of central radialised organ.
pnh
mutant also
has fewer axillary meristems (McConnell & Barton, 1995). PNH belongs
to a family of proteins that include the translation factor eIF2C and
ARGONAUTE, and is expressed in the vasculature, the adaxial domain of
organs and the throughout the SAM (Moussian
et al.
, 1998; Lynn
et al.
,
1999). One function of
PNH
is to maintain
STM
expression in the SAM.
STRUWWELPETER
(
SWP
)
The
swp
mutant has fewer cells in all aerial organs and a SAM that gradually
becomes disorganised. SWP is similar to proteins found in the Mediator
complex and therefore may function in the recruitment of RNA
polymerase II to promoters (Autran
et al.
, 2002). The SAMs of
swp
mutants are frequently fasciated, a feature that is correlated with ectopic
WUS
expression. Interestingly,
STM
expression is also patchy within these
meristems (Autran
et al.
, 2002).
WIGGUM
(
WIG
)/
ERA1
wig
/
era1
mutants are pleiotropic, displaying alterations in seed germination,
flowering time, senescence, internode elongation, phyllotaxis as well as
having meristem defects similar to the
clv
mutants (Running
et al.
, 1998;
Yalovsky
et al.
, 2000).
WIG
/
ERA1
encodes an FTase b -subunit which is
involved in farnesylation, a process that increases the hydrophobicity of
target proteins, facilitating either membrane attachment or protein-protein
interactions (Cutler
et al.
, 1996; Ziegelhoffer
et al.
, 2000).
ULTRAPETALA
(
ULT
)
The inflorescence meristem of the
utl
mutant is larger than normal, and
produces more floral meristems.
ult
flowers have an increased number of
organs as well as more whorls of organs, indicating that the floral
meristems are also bigger than normal. One function of
ULT
is to restrict
CLV1
expression to the central zone, as loss of
ULT
activity results in
lateral expansion of
CLV1
.However, genetic analysis indicates that
ULT
has functions independent of
CLV1
. Thus the lateral expansion of
CLV1
in
the
ult
mutant may simply reflect an increased proliferation of L3 cells.
The precise function of
ULT
will become apparent only when the gene
has been identified (Fletcher, 2001).
