75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

189 Arabidopsis REGULATOR OF AXILLARY MERISTEMS1 Controls a Leaf Axil Stem Cell Niche
and Modulates Vegetative Development
Peter Doerner, Thomas Keller
Edinburgh University
Shoot branching is a major determinant of variation in plant stature. Branches, which form secondary growth axes,
originate from stem cells activated in leaf axils. The initial steps by which axillary meristems (AM) are specified and
their stem cells organized are still poorly understood. We recently reported gain- and loss-of-function alleles at the
Arabidopsis REGULATOR OF AXILLARY MERISTEMS 1 (RAX1) locus (Keller et al. 2006). RAX1 is encoded by
the Myb-like transcription factor AtMyb37, and is an Arabidopsis homolog of the tomato blind gene. RAX1 is transiently
expressed in a small central domain within the boundary zone separating SAM and leaf primordia from early on in leaf
primordium development. RAX1 genetically interacts with CUP-SHAPED COTYLEDON (CUC) genes and is required
for the expression of CUC2 in the RAX1 expression domain, suggesting that RAX1 acts through CUC2. RAX1 also
interacts with other meristem genes, including CLV3. We propose that RAX1 functions to positionally specify a stem cell
niche for axillary meristem formation. RAX1 also affects the timing of developmental phase transitions by negatively
regulating gibberellic acid levels in the shoot apex. RAX1 thus defines a novel activity that links the specification of AM
formation with the modulation of the rate of progression through developmental phases.
Keller, T., J. Abbott, T. Moritz and P. Doerner (2006). Plant Cell 18: 598-611.
190 Class I TCP Genes Link Regulation of Growth and Cell Division Control
Chengxia Li 1 , Thomas Potuschak 1 , Adan Colon-Carmona 2 , Rodrigo Gutierrez 3 , Roxana Nadershahi 1 , Peter Doerner 1
1
Edinburgh University, 2 Salk Institute, San Diego, USA, 3 New York University, New York, USA
During postembryonic plant development, cell division is coupled to cell growth. In active shoot and root meristems
there is a stringent requirement to couple these processes, else cell size would be highly variable. In root meristems
and during shoot organogenesis cells transit through a zone with high rates of cell growth and proliferation, similar to
transient amplification cells in animals. The dynamics of this transition zone implies a need for coordinate regulation of
genes underpinning these two fundamental cell functions. We have recently identified a mechanism for co-regulation
of cell division control genes and cell growth effectors in proliferating cells (Li et al. 2005). We identified a GCCCR
motif necessary and sufficient for the high levels of gene expression normally observed in these target genes. This motif
is overrepresented in the promoters of many ribosomal protein genes required for cell growth. The GCCCR motif is
required for cyclin CYCB1;1 expression at G2/M and for high-level expression of the S27 and L24 ribosomal subunit
genes we examined. We found that class I TCP genes, exemplified by here by the Arabidopsis TCP20 gene, and its
product p33TCP20, bind to the GCCCR element in the promoters of cyclin CYCB1;1 and ribosomal protein genes
in vitro and in vivo. The expression patterns of the TCP genes we examined are consistent with their proposed role in
coordinating high-level gene expression in the transient-amplifying zone of the meristem. We propose a model in which
organ growth rates, and possibly shape in aerial organs, are regulated by the balance of positively and negatively acting
Teosinte-branched, Cycloidea, PCNA factor (TCP) genes in the proximal meristem boundary zone where cells become
mitotically quiescent before expansion and differentiation.
Li, C., T. Potuschak, A. Colon-Carmona, R. A. Gutierrez and P. Doerner (2005). Proc Natl Acad Sci U S A 102: 12978.