15th International Conference on Arabidopsis Research - TAIR

T04-021
Identification of a new ABA biosynthesis locus,
AtABA4, in Arabidopsis thaliana
Helen North(1), Aurélie De Almeida(1), Annie Marion-Poll(1)
1-IJPB-Seed Biology Laboratory, UMR204 INRA INA-PG, 78026 Versailles cedex, France
ABA is derived from xanthoxin by the cleavage of oxygenated carotenoids,
also called xanthophylls. The isolation of mutants impaired in ABA biosynthesis,
has proved to be particularly effective for cloning genes involved in most
steps of the pathway. The identification of mutants accumulating zeaxanthin
has previously led to the cloning of the genes encoding the enzyme zeaxanthin
epoxidase (ZEP) from several species. Zeaxanthin epoxidation leads to
the formation of all-trans-violaxanthin. However only cis-isomers of violaxanthin
and neoxanthin are cleaved into xanthoxin by a family of 9-cis-epoxycarotenoid
dioxygenases. Due to the absence of mutants impaired in these
catalytic steps, the genes involved in the conversion of all-trans-violaxanthin
to cis-xanthophylls remained to be characterized. We recently isolated a new
ABA-deficient mutant of Arabidopsis, Ataba4, which is unable to synthesize
cis and trans isomers of neoxanthin. The AtABA4 gene has been identified by
map-based cloning and its characterization is underway.
15 th <strong>International</strong> <strong>Conference</strong> on Arabidopsis Research 2004 · Berlin
T04-022
Ethylene responses in Arabidopsis seedlings' roots
require a local boost in auxin production.
Anna N. Stepanova(1), Joyce M. Hoyt(1), Alexandra A. Hamilton(1), Jose M.
Alonso(1)
1-Department of Genetics, North Carolina State University, Box 7614, Raleigh, NC 27695, USA
Exposure of dark-grown Arabidopsis seedlings to exogenous ethylene leads
to characteristic morphological changes collectively known as the triple response.
The phenotypic effects of ethylene in seedlings' roots require normal
levels of auxin signaling and response. We are interested in understanding
the mechanisms of the ethylene-mediated root shortening and, in particular,
the role of ethylene-auxin crosstalk in this process. Towards this goal, we
have isolated several mutants with reduced ethylene sensitivity, yet normal
auxin responsiveness, of seedlings' roots.
Two complementation groups, wei2 and wei7, represented
by three independent alleles each, were phenotypically indistinguishable
from each other. Positional cloning of wei2 and wei7
revealed that they harbor mutations in the alpha and beta subunits of the
ANTHRANILATE SYNTHASE (AS) genes ASA1 and ASB1,
respectively. AS is a tryptophan (Trp) biosynthetic enzyme that catalyzes
conversion of chorismate into anthranilate (Ant), the rate-limiting step of the
pathway. Trp and its biosynthetic intermediates, in turn, serve as precursors
to auxin. Consistent with this notion, the phenotypic defects of wei2
and wei7 in ethylene can be complemented by exogenous Ant, Trp,
or auxin.
Analysis of the transcriptional fusions of the WEI2 and WEI7
promoters with GUS revealed that expression of both genes was inducible
by ethylene in root tips of Arabidopsis seedlings. By increasing the levels of
the rate-limiting enzymes ASA1 and ASB1 in roots, ethylene may stimulate
production of tryptophan and, ultimately, auxin. In fact, expression of a
synthetic auxin reporter DR5-GUS in root tips is enhanced in the presence
of exogenous ethylene. Remarkably, this ethylene-mediated induction of the
auxin reporter is fully dependent on functional WEI2 and WEI7. Furthermore, the wei2 and wei7 mutations can suppress
the "high-auxin" phenotypes of the cyp83b1 knockout mutant,
consistent with the central role of the AS genes in auxin biosynthesis in the
situations of high auxin demand.
In summary, our data implicate WEI2 and WEI7 in the
ethylene response. By stimulating transcription of these enzymes, ethylene
enhances the rate of tryptophan and auxin biosynthesis in a tissue-specific
manner. Such boost in auxin production is a prerequisite for the full-scale
ethylene response of Arabidopsis seedlings' roots.
T04 Interaction with the Environment 1 (Abiotic)