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

429 Involvement of Phytohormone Signaling Pathways During Seed Germination Under Salt
and Osmotic Stress Conditions
Kun Yuan, Joanna Diller
Department of Biological Sciences, College of Sciences and Mathematics, Auburn University
Soil salinity is one of the most significant abiotic stresses limiting plant growth. The adaptation of plant cells to stress
conditions involves triggering a network of signaling events. The plant hormone abscisic acid (ABA) regulates many
important aspects of plant growth and development, and plays a critical role in stress responses. The process of seed
germination is affected by salt and osmotic stress at least partially via the ABA signaling pathway. Several components
in the GA signaling pathway are known to be involved in germination but have not been tested for their involvement in
salt and osmotic stress. We examined the responses of several mutants in GA signaling pathway to salt and osmotic stress
during seed germination and early seedling development. Several mutants in the ABA signaling pathway, previously
demonstrated to be involved in seed germination under these stress conditions, were used as positive controls. Real-time
PCR was employed to test the genes relative expression levels in several mutants and under various stress conditions to
determine whether salt and/or osmotic stress affects the seed germination via transcriptional control of the components
in GA or ABA signaling pathway. This study suggested that different genes in ABA and GA signaling pathways are
involved during different developmental stages under stress condition. We will also report on possible crosstalk between
different hormone signaling pathways under the salt and osmotic stress conditions.
430 Investigating the Role of ETA2/CAND1 in Regulating SCF Complex Activity
Wenjing Zhang, Hironori Ito, Marcel Quint, William Gray
Department of Plant Biology, University of Minnesota-Twin Cities
The eta2-1 mutant was identified in a genetic screen for enhancers of the tir1-1 auxin (eta) response defect. eta2-1
plants exhibit several phenotypes related to impaired auxin response. Molecular studies found that these phenotypes are
the result of reduced SCF-TIR1 activity in eta2-1 mutants (1). Isolation of the ETA2 gene revealed that it encodes an
Arabidopsis ortholog of human CAND1 (Cullin-Associated and Neddylation-Dissociated). Biochemical studies with
mammalian cell lines suggest that CAND1 acts as a negative regulator of SCF function by sequestering unmodified
CUL1 away from SKP1 and the F-box protein, thus preventing assembly of the SCF complex. In contrast, we find that
the eta2-1 mutation diminishes the ability of CAND1 to interact with CUL1, demonstrating that the interaction between
these two proteins is required for SCF activity and that CAND1 positively regulates SCF function. These paradoxical
findings have been explained by a model invoking CAND1 in regulating a dynamic cycle of assembly and disassembly
of the SCF complex in vivo, through association and dissociation with CUL1. Double mutant analysis with the axr6-2
and eta1/axr6-3 alleles of CUL1 reveals additional insight into the interactions between CAND1 and CUL1. Whereas
eta2-1 and axr6-3 interact synergistically, eta2-1 axr6-2 double mutants show mutual suppression of eta2-1 and axr6-2.
Although the eta2-1 mutation itself is recessive, its suppression of axr6-2 is dominant, indicating a heightened sensitivity
to CAND1 dosage level. Since the eta2-1 mutation dramatically reduces CUL1 binding activity, these genetic findings
suggest that the axr6-2 mutation may inhibit dissociation of CUL1 from the CAND1-CUL1 complex. Consistent with
this possibility, co-immunoprecipitation experiments detect a dramatic increase in CAND1-CUL1 complex abundance in
axr6-2 mutant extracts in comparison to wild-type. Phenotypic and biochemical studies of these double mutants will be
discussed in the poster. Molecular studies examining the effects of these mutations on SCF homeostasis are underway.
1. Chuang et al. (2004) Plant Cell 16, 1883-97