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

419 Round The Clock – The Molecular System Of AtGRP7
Corinna Streitner, Jan Schoening, Selahattin Danisman, Dorothee Staiger
Molecular Cell Physiology, Bielefeld University, D-33615 Bielefeld, Germany
The Arabidopsis thaliana glycine-rich RNA binding protein AtGRP7 oscillates in a circadian manner with a peak in
protein amount at the end of the day. Daily oscillation is initiated by rhythmic transcriptional activation of the AtGRP7
gene by the circadian clock and fully developed via an autoregulatory feedback loop. In this negative feedback circuit
the accumulation of the AtGRP7 protein above a certain threshold is thought to lead to the emergence of an alternativly
spliced AtGRP7 mRNA containing a premature stop codon which prevents the translation of functional AtGRP7 protein.
Analysis of transgenic Arabidopsis lines constitutively overexpressing AtGRP7 verifies this mechanism by showing the
downregulation of the endogenous AtGRP7 mRNA level and an increased amount of the alternate transcript. Band shift
assays reveal sequences in the intron and in the 3’UTR as binding sequences for the protein. As the AtGRP7 feedback
curcuit operates downstream of the circadian clock, it may act as a slave oscillator passing circadian rhythmicity from
the central oscillator to downstream targets. Microarray technologies are used to compare wild type plants with AtGRP7
overexpressing plants and with RNAi-plants displaying an highly reduced level of AtGRP7 expression in order to find
target transcripts.
420 MEKK1 is a Negative Regulator of Stress Responses in Arabidopsis, but this Function does
not Require the Protein's Kinase Activity
Maria Cristina Suarez 1 , Lori Adams-Phillips 2 , Shih-Heng Su 1 , Peter Jester 1 , Andrew Bent 2 , Patrick Krysan 1
1
Horticulture Department and Genome Center of Wisconsin, University of Wisconsin - Madison, 1575 Linden
Drive, Madison, WI 53706, USA, 2 Department of Plant Pathology, University of Wisconsin - Madison, Madison,
WI 53706, USA.
The Arabidopsis gene MEKK1 encodes a MAP Kinase Kinase Kinase (MAP3K) that has previously been implicated
in the regulation of biotic and abiotic stress response pathways. Here we report the characterization of two independent
T-DNA mutant alleles of MEKK1 and demonstrate that plants homozygous for either of these mutations display a severe
dwarf phenotype. Genome-wide expression analysis indicated that mekk1 plants constitutively express a battery of genes
normally upregulated in response to biotic stress. The mekk1 dwarf phenotype could be rescued by transformation with
an ectopic copy of the wild-type MEKK1 gene. More surprisingly, phenotypic rescue could also be achieved by the
introduction of a kinase-inactive allele of MEKK1 (K361M) into mekk1 plants. These K361M plants were tested for their
ability to transduce signals previously thought to require MEKK1 kinase activity, and it was found that K361M plants
displayed wild-type responses to the elicitor peptide flg22 (a flagellin analog) as well as mechanical wounding. It was
also observed that K361M plants exhibit only subtle changes in their response to virulent and avirulent Pseudomonas
syringae. Our results indicate that MEKK1 acts as a negative regulator of stress response pathways in Arabidopsis, but
this functionality does not require the protein's kinase activity. We propose a model for MEKK1 function in which the
protein has a unique structural role in regulating stress response, but a dispensable kinase activity that may be compensated
for by related MAP3Ks.