15th International Conference on Arabidopsis Research - TAIR

T02-067
TYPE-B RESPONSE REGULATORS FUNCTIONALLY
OVERLAP IN THE REGULATION OF CYTOKININ
RESPONSES
Michael Mason(1), Dennis Matthews(2), Eric Schaller(1)
1-Biological Sciences Dept. Dartmouth College, NH. USA.
2-Plant Biology Dept. University of New Hampshire, NH. USA.
Two-component signaling systems involve histidine kinases, histidinecontaining
phosphotransfer proteins, and response regulators, and have
been implicated in plant responses to hormones and environmental factors.
Genomic analysis of Arabidopsis thaliana supports the existence of 22 response
regulators (ARRs) that can be divided into at least two distinct groups
designated type-A and type-B. Phylogenetic analysis indicates that the type-B
family is composed of one major and two minor subfamilies. The expression
of the type-B ARRs was examined by using both RT-PCR and GUS fusion
constructs. The major subfamily of type-B ARRs showed particularly high
expression in regions where cytokinins play a major role, including cells near
the apical meristem and in young leaves that would be undergoing cell division.
Type-B ARRs were also found near the root tip with highest expression in
the root elongation zone. Based upon the analysis of T-DNA insertion mutant
lines, we found that the members of the major subfamily of type-B ARRs not
only show similarities in expression but also appear to functionally overlap. Of
the single mutants, only arr1 plants displayed clearly reduced cytokinin sensitivity.
Progressively greater insensitivity to cytokinin was observed in higher
order mutants. Our data support a role for multiple type-B ARRs in modulating
cytokinin responses with ARR1 playing the most significant role.
T02 Development 2 (Shoot, Root)
T02-068
Analysis of TRANSPARENT TESTA GLABRA2 involved
in trichome differentiation
Tetsuya Ishida(1), Sayoko Hattori(1), Kiyotaka Okada(1, 2), Takuji Wada(1)
1-Plant Science Center, RIKEN
2-Graduate School of Science, Kyoto University
Trichomes and root hairs are specialized cells differentiated from the
epidermis. Several genes have been identified as regulators of epidermal
cell differentiation, playing a role in the development of these special cells.
The Arabidopsis TRANSPARENT TESTA GLABRA2 (TTG2) gene encodes a
WRKY transcription factor. The ttg2 mutant has fewer trichomes than the
wild type, its trichomes are less branched, and it has defects in tannin and
mucilage production in its seed coat. TTG2 is expressed in leaf primordia,
trichomes, seed coats, and hairless cells of developing roots. It is proposed
that TTG2 functions downstream of TRANSPARENT TESTA GLABRA1 (TTG1)
and GLABRA1 (GL1), and shares functions with GLABRA2 (GL2) in trichome
development (Johnson et al., 2002).
To further elucidate the relationship between TTG2 and other genes that
regulate epidermis differentiation, we analyzed TTG2 expression in various
trichome and root hair mutants. We found that TTG2 expression in roots was
suppressed in the werewolf-1 (wer-1) mutant, and that TTG2 is ectopically
expressed in root-hair cells in the caprice-1 (cpc-1) mutant. These results
suggest that TTG2 is positively regulated by WER and negatively regulated
by CPC. WER encodes a MYB protein homologous to GL1, and negatively
regulates root-hair cell differentiation. CPC encodes a small MYB protein and
promotes root-hair cell differentiation. There are several putative Myb binding
sites in the TTG2 promoter. We are analyzing transgenic plants carrying a
deletion series of TTG2 promoter::GUS to determine whether or not Myb
proteins directly regulate TTG2.
We also analyzed phenotypes of double mutants of ttg2-1 which had
some trichome mutations. In the glabra3-2 (gl3-2) mutant, the number of
trichomes is reduced, and its trichomes are less branched than those of the
wild type. In the ttg2-1 gl3-2 double mutant, the number of trichomes was
much reduced, and trichomes were detected as small pointed outgrowths.
In the triptychon-82 (try-82) mutant, trichomes are often clustered and more
branched than those of the wild type. In the ttg2-1 try-82 double mutant,
the number of trichomes was intermediate between those of its parental
mutants, and its trichomes were similar to those of the ttg2-1 mutant. We will
discuss the genetic interaction of TTG2 with other regulators of epidermal cell
differentiation.
Johnson, C. S. et al. (2002) Plant Cell 14: 1359-1375
15 th <strong>International</strong> <strong>Conference</strong> on Arabidopsis Research 2004 · Berlin