15th International Conference on Arabidopsis Research - TAIR

T02-073
RHD6-like transcription factors involved in root hair
development
Benoît Menand(1), Stéphane Jouannic(1), Eoin Ryan(1), Paul Linstead(1), Liam
Dolan(1)
1-John Innes Centre, Norwich, NR47UH, UK
The Arabidopsis root epidermis is a well-characterised model system for
analysing cellular differentiation using genetic approaches. It is composed of
two cell types: trichoblasts that produce root hairs by polarised outgrowth,
and atrichoblasts that do not produce hairs. Once trichoblast/atrichoblast cell
fate has been determined under the control of transcriptions factors like WER,
GL2 and CPC, another point of regulation occurs at the initiation of root hair
growth controlled by RHD6. The rhd6-1 mutant shows signs of trichoblast
differentiation but produces almost no root hairs.
A screen of enhancer trap lines for root hair phenotypes resulted in the
identification of a new rhd6 allele. RHD6 was cloned and encodes a member
of the basic-Helix-loop-Helix (bHLH) transcription factor family. The gene is
expressed specifically in trichoblast cells. Five other rhd6 alleles with similar
phenotypes have been isolated.
A phenotypic analysis of available mutants for the five closest homologues of
RHD6 has been undertaken. This revealed that at least one homologue is also
involved in root hair development because mutants have shorter hairs than
wild type. Expression analysis shows that 4 of the homologues are specifically
expressed in roots and that 3 of them are strongly downregulated in the
rhd6-1 mutant. This indicates that they are positively controlled by RHD6.
The results suggest that the RHD6 subfamily of transcription factors form a
network to regulate the initiation of root hair growth.
15 th <strong>International</strong> <strong>Conference</strong> on Arabidopsis Research 2004 · Berlin
T02-074
Isolation and characterization of gulliver mutants
that are defective in the light and brassinosteroid
signaling pathways
Mi Kwon(1), Su Youn Jang(1), Jun Ho Ko(1), Sungwha Choe(1)
1-Seoul National University
Brassinosteroids are plant steroidal hormones that play essential roles during
plant growth and development. Although understanding of brassinosteroid
signaling pathway has been greatly improved via isolation and characterization
of the signaling mutants such as bri1, bzr1, bes1, dwf12/bin2, and
bsu1, signaling cascade still harbors the questions needed to be answered.
In order to uncover new components in brassinosteroid signaling pathways,
we used brassinazole (Brz) as a screening reagent: Brz is a brassinosteroid
biosynthetic inhibitor that had been successively employed to isolate bzr1
and bes1 mutants. We have identified three loci from either EMS mutagenized
or T-DNA activation population, and named gulliver mutants since they all
display the characteristic phenotypes of elongated hypocotyls and petioles.
Since gulliver mutations suppress the bri1-5 mutant phenotype as revealed
by double mutant analysis, these mutants are believed to be associated with
brassinosteroid signaling pathway. Interestingly, gul2 respond abnormally to
red light but not to the far-red and blue light, however, gul3 and gul4 respond
normally to the all light regimes tested. In addition, gul3 seems to be involved
in feedback regulation of the brassinosteroid biosynthesis at the transcriptional
level by altering the expression level of the brassinosteroid biosynthetic
enzyme such as DWARF4. Here we report the progress of the map-based
cloning of the gulliver mutants and discuss their possible roles in plant
growth and development.
T02 Development 2 (Shoot, Root)