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

197 Cytokinin Receptors are Required for Normal Development of Auxin-transporting Vascular
Tissues in the Hypocotyl but not in Adventitious Roots
Takeshi Kuroha 1, 2 , Chiharu Ueguchi 3 , Hitoshi Sakakibara 2 , Shinobu Satoh 1
1
Institute of Biological Sciences, University of Tsukuba, Tsukuba 305-8572, Japan, 2 RIKEN Plant Science
Center, Tsurumi, Yokohama 230-0045, Japan, 3 Bioscience and Biotechnology Center, Nagoya University,
Nagoya 464-8601, Japan
Plants alter the architecture of their root systems to adapt to the environment by modulating post-embryonic (lateral
and adventitious) root formation and growth. To better understand the genetic basis of this regulation, we screened
ethyl-methane sulfonate-mutagenized lines of Arabidopsis thaliana for adventitious rooting mutants. One mutant
showed retardation of the primary root growth, no production of lateral roots, and enhanced formation of adventitious
roots. Mapping and genetic complementation revealed that this mutant named wooden leg-3 (wol-3) was an allele of
ARABIDOPSIS HISTIDINE KINASE 4 (AHK4), a locus known to encode a cytokinin receptor. Although the vascular
system of the primary root and hypocotyl in the wol-3 mutant was aborted, that of the adventitious roots was normally
developed. To investigate the relationship between the auxin response and the wol-3 phenotype, we introduced the auxin
responsive promoter construct DR5::GUS into wild-type and the wol-3 mutant. In the hypocotyl of the wol-3 mutant,
strong DR5::GUS expression was observed around the aborted vascular system. The wol-3 primary root shows no DR5::
GUS expression except in the root tip. When the wol-3 primary root was cut off, both the expression of DR5::GUS and
lateral root formation were inhibited. The application of auxin to the wol-3 primary root resulted in the induction of
DR5::GUS expression and subsequent lateral root formation, suggesting that the auxin response in the wol-3 primary
root is normal. Furthermore, basipetal movement of radiolabeled IAA transport from the hypocotyl to the primary root
in the wol-3 mutant was significantly inhibited compared to the wild type. Although only a single amino acid alteration
had occurred in AHK4, the root morphology in the wol-3 mutant was quite similar to that in the ahk2 ahk3 ahk4 triple
mutant, which is a loss-of-function mutant of the three cytokinin receptors. This implies that the functional disturbance
of AHK4 affects the function of the other receptors. Our results suggest that cytokinin receptors are necessary for the
formation of auxin-transporting vascular tissues in the hypocotyl, but not in adventitious roots.
198 SCRAMBLED Mediates Post-embryonic Positional Signaling For Epidermal Patterning In
Arabidopsis Roots
Su-Hwan Kwak, John Schiefelbein
Department of Molecular, Cellular and Developmental Biology, University of Michigan
In Arabidopsis roots, epidermal cells have two fates; to be a root hair cell or a non-hair cell. The fate decision is
influenced by the position of the epidermal cells. Developing epidermal cells overlying two cortical cells differentiate
as hair cells, whereas non-hair cells arise over a single cortical cell. SCRAMBLED (SCM), a leucine-rich repeat
receptor-like kinase (LRR-RLK) is required for interpreting the position of developing epidermal cells so that they adopt
appropriate fates. It is known that the position-dependent pattern of epidermal cells is initiated during embryogenesis,
because GLABRA2 (GL2) expression begins to exhibit a position-dependent pattern within protodermal cells at the heart
stage and is maintained throughout the remainder of embryogenesis. We examined the role of SCM in the embryonic
patterning. Although a SCM-GFP fusion protein expressed under the SCM promoter is localized in the plasma membrane
of embryonic cells, we find that scm mutants do not alter the embryonic GL2 expression pattern in protoderm cells. The
two closest homologues of SCM, SRE1 and SRE2, in the LRR-V family RLKs, were found to be dispensable for the
embryonic patterning. Embryos of sre1-1 sre2-1 double mutant and scm-2 sre1-1 sre2-1 triple mutant still possess the
position-dependent GL2 expression pattern. Thus, it appears that SCM and its close homologues SRE1 and SRE2 are
not involved in embryonic protodermal patterning, but SCM is involved in the position-dependent epidermal cell fate
determination in developing roots at the post-embryonic stage. This may mean that two position-dependent cell fate
determination mechanisms may exist; one for the embryonic protoderm and one for the epidermis of post-embryonic
developing roots.