Plant Architecture
Brevipedicellus:
The brevipedicellus (bp) mutant was originally discovered by Maarten Koornneef
and coworkers in the early 1980s, and
since that time it has been used as a mapping marker for chromosome 4. bp exhibits
multiple defects, including a reduction in height due to defects in internode
elongation, very short pedicels which point downward, bends at nodes, and unusual
stripes of tissue which spiral down internodes from the abaxial side of pedicels.
We found that there exist
defects in both cell division and cell elongation in stems and pedicels. The
stripe cells are achlorophyllous and they exist in files. Thus proper differentiation
of the epidermal cells does not occur where stripes exist, and interestingly,
stripes are always found over vascular bundles. We hypothesize the the BP gene
countermands the effect of a vasculature associated repressor of chlorenchyma
development, especially at nodes. We used map based cloning to identify the
BP gene, and found that BP encodes the well-known KNAT1 homeobox protein. This
work and other influences of BP and the ERECTA gene on inflorescence architecture
are reported in The Plant Cell (get PDF) and Developmental
Biology (get PDF)
Current and Future
Work
1. Characterization of suppressor mutants of bp.
Flasher
(fsh):Partially suppresses bp, resulting in the
elaboration of flowers and siliques which are perpendicular to the stem
axis. In addition, the buds open prematurely due to defects in sepal development,
exposing the reproductive whorls (hence flasher). Flasher produces very
little pollen. Map-based cloning and other analyses revealed that FSH is
an allele of the YABBY/FILAMENTAOUS FLOWER gene.
Other suppressors:
We have identified several other suppressors and are now engaged in their
characterization and gene identification. The severe pedicel phenotype makes
identification of enhancers impossible, and we have therefore generated
a bpER line the pedicels of which elongate somewhat and which are elaborated
perpendicular to the stem axis. This will serve as the genetic background
for a second screen for enhancers of bp.
2. Analysis of
BP function via transcription profiling.
Although
BP is expressed in the embryo, there is no embryonic phenotype. There are
defects in stem development, including bends at nodes and an altered positioning
of floral buds/siliques. In addition, the achlorophyllous stripe of tissue
which subtends each lateral branch indicates that BP acts to control internode
elongation and chlorenchyma development, particularly at nodes. We are currently
engaged in microarray analyses of a bp point mutant to identify genes which
are controlled by this putative transcription factor.
3. Analysis of
chromosome instability at the bp locus.
Five
independent bp alleles have been analysed at the molecular level to ascertain
the nature of the mutations, which were generated by EMS, T-DNA insertion,
or fast neutron mutagenesis. Four of the five are due to large deletions
of at least 150kbp. BP resides near the centromere of chromosome 4 in an
area of repetitive DNA. We postulate that this area is particularly succeptible
to DNA damage and we are examining the breakpoints of the deletions to determine
if there exist common features.
4. Analysis of
GFP gene trap lines with expression at nodes.
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To
identify genes involved in stem architecture and branching, we screened
a population of GFP gene trap lines developed at the University of Toronto.
The photograph illustrates WG335, one of several lines with GFP signatures
at nodes and in this case, in the floral receptacle. Suppression PCR and
sequencing have identified the 'trapped' gene as encoding a TCP domain protein.
Current and future work in this area centers on elucidating the role of
the TCP protein by both mutant analysis and overexpression studies. Interestingly,
in a bp background, the WG335 signature changes, with the expression domain
at nodes being translated into subtending internodes along the stripe (see
Dev. Biol. PDF). Thus, WG335 serves also serves as a useful marker
of nodal identity.