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

411 Using High-throughput Chemical Genetics to Understand Abscisic Acid Hormonal Action
Jignasha Patel, Frederic Delmas, Peter McCourt
University of Toronto, Department of Botany
Abscisic acid (ABA) is a phytohormone that regulates many agronomical important aspects of plant development as
well as mediating responses to stresses. The mechanisms by which ABA regulates these processes have yet to be fully
delineated. A variety of approaches have been used thus far including genetic, biochemistry and cell biological. We are
using a high-throughput chemical genetics approach to further understand ABA signaling. To begin, I have conducted two
screens for chemicals that alter (enhance or suppress) the response of seeds (at the level of germination) to exogenous
levels of ABA. To identify chemical enhancers of ABA, Arabidopsis seeds (Columbia ecotype) were screened on 4800
chemicals in the presence of 0.3 uM ABA for inhibition of germination. And to identify suppressors of ABA, seeds
were screened with the same libraries of chemicals (LOPAC and Spectrum) on 3 uM ABA for seeds that were able to
germinate. Chemicals with reproducible activities were further characterized as having ABA specific action by testing
their effects with ABA for root growth and stomatal responses. Two chemical suppressors of ABA and seven chemical
enhancers of ABA are currently being investigated.
In addition, low concentrations of ABA was found to have a protective effect against a group of chemicals that
cause Arabidopsis seedlings to otherwise bleach. We hypothesize that ABA is acting as a chemical safener against these
compounds and that it induces degradation or metabolism of the bleaching compound. HPLC analysis is currently being
used to determine metabolic differences of these compounds observed in plants in the absence and presence of ABA.
412 IAR4 Modulates Basal Auxin Response Through Regulating Auxin Homeostasis
Marcel Quint 1 , Lana Barkawi 2 , Jerry Cohen 2 , William Gray 1
1
Department of Plant Biology, University of Minnesota - Twin Cities, 2 Department of Horticultural Science,
University of Minnesota - Twin Cities
In a screen for enhancers of tir1-1 auxin resistance, we identified two novel alleles of the putative mitochondrial
pyruvate dehydrogenase E1alpha subunit, IAR4. The iar4-3 mutation interacts synergistically with tir1-1 in root growth
inhibition and lateral root development assays. Additionally, iar4 single mutants exhibit numerous auxin-related phenotypes
including auxin-resistant root growth and reduced lateral root development, as well as severe defects in primary root
growth, root hair initiation and root hair elongation. To examine the effects of iar4 mutations on SCFTIR1 activity, the
HS:AXR3NT-GUS reporter was introduced into iar4-3 to examine Aux/IAA protein stability. While the basal level and
stability of the AXR3NT-GUS fusion protein are significantly increased in iar4-3 compared to wild-type, AXR3NT-GUS
degradation in auxin-supplemented media is comparable to wild-type. Remarkably, all of the iar4 mutant defects are
rescued when the seedlings are grown at high temperature (28C). Since auxin biosynthesis in planta is increased at high
temperature, the iar4-3 phenotypes may be the result of a defect in auxin homeostasis. In support of this hypothesis, the
activation-tagged allele of YUCCA, previously shown to confer elevated levels of free IAA, also rescues most of the iar4-
3 mutant phenotypes. IAA measurements detected no significant difference between iar4-3 and wild-type for free IAA,
but a significantly higher level of IAA-amino acid conjugates was observed in the iar4-3 mutant. We therefore suggest
that iar4 mutations affect basal auxin responses via altered auxin homeostasis, perhaps due to localized or transient IAA
dynamics not revealed by whole seedling analyses.