Transcriptional framework for root patterning
SCARECROW/SHORTROOT and PLETHORA genes
The specification of cell types by transcription factors is a common theme in
developmental biology. We focus on transciption factors that specify the root
stem cells.SCARECROW/SHORTROOT
The SCR and SHR genes have been cloned in the laboratory of our long-standing
collaborator Philip Benfey.
They encode GRAS-type transcription factors long known to be required for the
asymmetric division of the ground tissue stem cell and for
endodermis
specification. Accordingly, SHORTROOT and SCARECROW protein reside in the nuclei
of a single cell layer surrounding the stele. SHR is transcribed in the stele
and the protein moves outward, whereas SCR is expressed in the same layer where
the protein is found.
SHORTROOT and SCARECROW in the root
Both SHR and SCR proteins are also present in the QC. We discovered that both
genes are also required for QC and stem cell specification. By putting SCARECROW
protein back into the QC region of a scr mutant we showed that at least SCR is
required in the QC itself to specify QC identity and the identity of the
surrounding stem cells.
WT, scr mutant and SCR expressed in the QC region of a scr
mutant
The JKD gene family
The restriction of the activity of SHR to the quiescent centre and endodermal
layer is a neat example of the definition of a single layer by transcription
factor movement. We have shown by mosaic analysis that this process requires
SCR. Zinc finger transcription factors that are in part under control of SHR and
in part expressed independently play an important role in the restriction of SHR
action. The JACKDAW gene encodes the founding member of this dedicated set of
proteins that delimit SHR activity.
JKD gene expression at tissue boundary restricts SHR activity
We also found transcription factors whose expression depends on auxin accumulation (and on
auxin responsive transcription factors) which are expressed in the QC and stem
cell region. These are homologous to the well-known floral homeotic gene
APETALA2. Double plethora1 plethora2 mutants mis-specify the quiescent center
and cannot maintain stem cells.
left panel: PLT genes expressed in stem cells region of root tip
right panel: plt1plt2 double mutants do not maintain the root meristem
When ectopically expressed, these genes specify new QC and root stem cells in
any position where SCARECROW and SHORTROOT are expressed.
ubiquitous expression of PLT2 leads to ectopic QC (dark-blue) and stem cells
Together with SCR and SHR, the PLT genes form a combinatorial code to specify
stem cells.
A combinatorial code of SCR, SHR and the auxin-dependent PLT genes specifies
the root stem cell region
Having at hand the major players in root stem cell patterning, and understanding
that their activation is a dynamic process, we have explored the interplay
between transcription factors and the polar auxin transport machinery that gives
rise to the amazing capacity of the root apex to regenerate its stem cells and
organizing centre. The re-deployment of patterning machinery that we observe
suggests that the dynamic nature of plant stem cell patterning can explain plant
regeneration potential.
Auxin re-accumulation (DR5 response) and PLT re-distribution after laser
ablation
- From stem cell to organ: Anatomy and Fate map
- Evolution: roots and shoots
- Auxins and root patterning: Auxin accumulation conveys positional information
- The digital plant: towards understanding Biocomplexity
- Gradients in root development
- Pattern and polarity connect: the PIN network in root development
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- Cell division control in the root:
- The HOBBIT gene and root meristem formation
- A receptor kinase pathway in root development?
- Genes involved in asymmetric cell division: SHR, SCR, JKD and MGP
- Methods
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