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

253 Identification and Characterization of a Genetic Locus Controlling Ethylene-Induced Leaf
Senescence
Reza Shirzadian Khorramabad, Hai-Chun Jing, Jacques Hille, Paul Dijkwel
Molecular Biology of Plants, Groningen Biomolecular Sciences and Biotechnology Institute, University of
Groningen , The Netherlands
Leaf senescence represents the last stage of leaf development and leads to cell death, thereby limiting the life span or
longevity of a leaf. Although senescence occurs in an age-dependent manner, it is known that internal senescence-promoting
factors can regulate leaf senescence. In particular ethylene has been shown to be a potent inducer of leaf senescence. In
an attempt to better understand the mechanism of ethylene-dependent senescence, we identified and analyzed a mutation,
named onset of leaf death 101 (old101). The old101 mutation extends Arabidopsis leaf longevity and this phenotype is
not the result of a reduced sensitivity to ethylene. The segregation analysis of the mutant showed that it is a monogenic
recessive trait and the mutation is located on chromosome 5 between BACs K19B1 and MQN23. The old101 mutant
showed a delayed onset of various senescence symptoms during ethylene-dependent and age-dependent senescence but
little effect on leaf senescence, artificially induced by darkness, was found. The mutation in the OLD101 locus causes a
delay in all senescence parameters examined, including chlorophyll content, photochemical efficiency of photosystem II,
ion leakage and nutrient remobilization. Relative expression of senescence-associated genes and an ethylene responsive
marker gene were significantly lower than the wild type. Expression of photosynthesis-associated genes in old101 leaves
was higher than in the wild type. Remarkably, old101 was more resistant to Pseudomonas syringae pv tomato DC3000
as compared to the wild type, and the stay green mutant seedlings exhibited increased tolerance to oxidative stress. Thus,
the old101 phenotype involves a mutated gene that might encode a protein stimulating the leaf senescence process. An
increased understanding of the OLD101 gene function can be important for extending the shelf life of green vegetables
and for agronomic improvement and enhanced stress resistance in crop species.
254 Morning-Specific Transcription of an Arabidopsis Clock Gene, LHY
Mark Spensley, Jae-Yean Kim, Isabelle Carre
Department of Biological Sciences. University of Warwick.
LHY and CCA1 encode partially redundant Myb transcription factors that form part of a transcriptional feedback
loop which is essential for the function of the Arabidopsis circadian oscillator. Genetic studies have identified several
upstream regulators of these genes, including TOC1, ELF3 & LUX, but the mechanisms by which these factors act to
regulate the morning-specific transcription of LHY is unclear.
To understand the transcriptional regulation of LHY, we have performed a cis-analysis of the LHY promoter. Rhythmic
expression and acute light responses are mediated through a short, proximal region of the promoter, 128bp upstream of
the predicted transcriptional start site. An adjacent 103bp of upstream sequence, containing a G-box and a repressive
element, modulates the phase of LHY expression and may mediate a second, rhythmic input to the promoter. In vitro
analysis of protein-DNA interactions within these two regions has identified four classes of DNA-binding activity that
may account for these functions.
We are currently investigating the roles of these activities in the circadian and light regulated transcription of LHY
with the aim of understanding how the functions of several upstream regulators are integrated at the LHY promoter to
produce the characteristic transcriptional profile of LHY providing greater insight into the functions of LHY and its
upstream regulators in the Arabidopsis circadian clock.