15th International Conference on Arabidopsis Research - TAIR

T02-119
AtRaptor and meristem activity
Garrett H. Anderson(1), Maureen R. Hanson(1)
1-Cornell University
Plant development depends on the regulated growth and division of root and
shoot apical meristem cells. We describe the disruption and characterization
of the Arabidopsis Raptor homologues, two genes whose protein products
play a critical role in regulating meristem activity. Raptor is a ~1,300 residue
protein comprised of multiple HEAT and WD-40 protein interaction domains.
In yeast and mammalian cells, Raptor forms a nutrient-sensitive complex with
the protein kinase TOR, a major regulator of protein synthesis and cell growth
(1,2). Disruption of the Arabidopsis homologue AtTOR is recessive lethal;
embryos undergo unstructured cell division but do not gain volume (3).
We have disrupted AtRaptor1A and AtRaptor1B, the two Arabidopsis Raptor
homologues. AtRaptor1B knockout plants show a broad range of subtle
shoot and root developmental phenotypes. Root growth is slower than
wild-type. Roots are often branched or curled, and show a defect in the
ability to penetrate agar. Leaf initiation is slower than wild type, bolting and
senescence are delayed, and there is a reduction in primary shoot apical
dominance, resulting in a highly branched shoot architecture. AtRaptor1A
insertion plants show no conspicuous phenotype. AtRaptor1A1B double mutants
arrest development as seedlings after minimal shoot apical meristem
activity.
Raptor is thought to act by recruiting substrates to TOR for phosphorylation;
known substrates include S6K, 4EBP, and E2F. We show that AtRaptor1B
interacts with AML1, a homologue of the fission yeast meiotic differentiation
regulator mei2 (4), suggesting that Arabidopsis mei2-like proteins act
downstream of an AtTOR signaling cascade (see T02-072).
Collectively, these results point to a role for AtRaptor / AtTOR signaling in the
regulation of cellular differentiation in the meristem.
1 Kim (2002) Cell110:163
2 Hara (2002) Cell110:177
3 Menand (2002) PNAS99:6422
4 Shinozaki-Yabana (2000) MCB20:1234
T02 Development 2 (Shoot, Root)
T02-120
Profiling Primary Auxin Responses and
Transcriptional Regulation Mediated by AXR1 and
SCFTIR1 Functions
Keithanne Mockaitis(1), Sunethra Dharmasiri(1), Nihal Dharmasiri(1), Mark
Estelle(1)
1-Dept. of Biology, Indiana University Bloomington, Indiana, 47405 USA
Rapid changes in gene expression occur in response to auxin when members
of the Aux/IAA family of transcriptional repressors are modified by ubiquitin
attachment and subsequently degraded. Root development and other
auxin-mediated processes are influenced critically by AXR1 modulation of
activities that initiate this selective elimination of repressors. AXR1-dependent
modification of ubiquitin-protein ligase SCF complexes influences the stability
of a variety of substrates, and it is the variable F-box protein component of
the SCF that determines recognition specificity.
As a starting point for examining a broader set of early auxin responses than
characterized previously, we profiled the root transcriptome after 30 minute
treatments with IAA. Microarray analyses using Affymetrix oligonucleotide
arrays representing >22,000 genes allowed the identification of numerous
early response genes beyond those previously known to be derepressed
by auxin-induced Aux/IAA protein degradation. We expanded this study
to examine influences of AXR1 function on the auxin-affected expression
profiles. axr1 plants exhibit defects in all known auxin-mediated processes,
suggesting that comparative transcriptional profiles would reveal genes involved
in each aspect of root growth and development, including cell elongation,
gravitropism, lateral root and root hair formation, and auxin-regulated metabolic
processes that influence total plant development. Transcriptional profiles
of axr1 roots allowed us furthermore to identify sets of genes regulated by
AXR1 function in the absence of auxin.
To deliniate specificities of function among SCF complexes downstream
of AXR1, we continued similar microarray studies with mutants in F-box
proteins involved in auxin response. SCFTIR1 acts downstream of AXR1 to
promote the degradation of Aux/IAA proteins. Mutations in TIR1 confer auxin
resistance that is less severe than observed in axr1, consistent with a smaller
set of effectors downstream of TIR1. Within the TIR1 subclade of the F-box
protein family are two proteins that share 84% identity and are 59% identical
to TIR1. These appear to act additively in auxin-sensitive processes in the
root (Dharmasiri et al., in preparation). Transcriptional profiles of tir1 and of
the triple mutant were analyzed and integrated into the above datasets.
Datasets presented here are of high statistical significance and detail the
extensive influence auxin exerts on the root transcriptome.
del Pozo JC, et al., 2002. Plant Cell, 14: 421-433.
Gray WM, et al., 2001. Nature, 414: 271-276.
15 th <strong>International</strong> <strong>Conference</strong> on Arabidopsis Research 2004 · Berlin