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

427 Integration of light and abscisic acid signaling during seed germination and seedling
growth
Hao Chen, Liming Xiong
Donald Danforth Plant Sceince Center
The phytohormone abscisic acid (ABA) plays important roles in plant growth and development including seed
dormancy, stomata movement, stress adaptation, and flowering. ABA also partially mediates drought rhizogenesis, a
novel adaptive response of roots to drought stress. In this study we identified that HY5, a well-characterized component
in the light signal transduction pathway, is also required for ABA-mediated drought rhizogenesis and ABA inhibition of
seed germination and seedling growth. In hy5 mutant seeds the transcript levels of several ABA-regulated genes such as
AtEM1, AtEM6, Rab18, and ABI5 were greatly reduced. Given that ABI5 has been shown to bind to AtEMs promoters
and activate their transcription, we hypothesized that HY5 might be able to directly regulate ABI5 expression. Indeed,
in vitro gel retardation assays showed that HY5 protein was able to bind to ABI5 promoter. This surprising discovery
prompted us to investigate whether ABI5 has any role in light signal transduction. The ABI5 gene was then ectopically
expressed in the wild type and hy5 backgrounds. Overexpression of ABI5 in hy5 conferred increased ABA sensitivity to
hy5. Furthermore, overexpression of ABI5 in the wild type resulted in enhanced light responses and shorter hypocotyls.
Our data thus indicate that HY5 functions in ABA and light signaling partially through activating ABI5. Supported by
NSF (grant #0521250) and USDA-NRI (grant #2005-35100-15275) (to L.X.)
428 Novel transcription factors involved in brassinosteroid signal transduction in Arabidopsis
Yanhai Yin 1 , Michelle Guo 1 , Li Li 1 , Lei Li 1 , Xiaofei Yu 1 , Shigeo Yoshida 2 , Tadao Asami 2 , Joanne Chory 3
1
Department of Genetics, Development and Cell Biology, Plant Science Institute, Iowa State University, Ames,
IA 50011, 2 Plant Functions Lab, RIKEN (The Institute of Physical and Chemical Research), Wako-shi, Saitama
351-098, Japan, 3 Howard Hughes Medical Institute and Plant Biology Laboratory, The Salk Institute for
Biological Studies, 10010 N. Torrey Pines Rd., La Jolla, CA 92037
Plant steroid hormone brassinosteroids (BRs) play important roles throughout growth and development. Unlike
animal steroid hormones that bind nuclear receptor superfamily transcription factors to directly regulate target gene
expression, BRs are perceived by membrane-localized receptor kinase BRI1. Multidisciplinary approaches have been
used to study how the BR signal is transduced from the cell surface receptor to nuclear target genes. By screening for
bri1 mutant suppressors, we identified a nuclear protein BES1 that functions downstream of BR receptor to mediate
target gene expression. Although BES1 does not have significant homology to known DNA-binding domains, its aminoterminal
domain is involved in DNA binding and can potentially form a novel basic-helix-loop-helix (bHLH) motif.
Indeed, BES1 is a transcription activator that binds to and activates BR-target gene promoters both in vitro and in vivo. In
addition, BES1 interacts with a typical bHLH protein, BIM1, to synergistically bind to E-box sequences present in many
BR-induced promoters. Our recent genetic, genomic and molecular studies identified several new transcription factors
that cooperate with BES1 and regulate BR target gene expression. Loss-of-function and/or gain-of-function mutants of
these BES1 partners display various BR-response phenotypes including changed resistance to BR biosynthesis inhibitor
brassinazole. Our results therefore establish that BRs signal to regulate transcription activator BES1 and BES1 functions
with other transcription factors to regulate plant growth and development.