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

119 CLB19, a Novel Pentatricopeptide Repeat Protein that Regulates the Expression of the
Rubisco Large Subunit in Arabidopsis
Maricela Ramos 1 , Arturo Guevara-Garcia 1 , Charles Andres 2 , Maria de la Luz Gutierrez-Nava 1 , Ian Small 2 , Patricia
Leon 1
1
Instituto de Biotecnologia UNAM, 2 Unite de Recherche en Genomique Vegetale INRA
The biogenesis of chloroplasts is a complex process that is still not well understood. This differentiation occurs in response
to specific signals in coordination to the differentiation of mesophyll and palisade cells. Thus chloroplast biogenesis depends on
the coordinated expression of the nuclear- and chloroplast-encoded genes. In addition to various structural genes, the nucleus
encodes for the majority of the regulatory proteins that modulate chloroplast gene expression. Most chloroplast transcripts
are subject to extensive post-transcriptional processing and these events are essential for the expression of chloroplastic genes
and for the correct organelle biogenesis. Today only few proteins involved in chloroplast transcript processing and translation
are known. However, the identification of key elements required in this process represents not only a potential avenue for its
future manipulation, but also has permitted to uncover novel biological processes.
To identify genes and factors required for proper development of a proplastid into chloroplast various mutants that block
chloroplast development, at different stages during organelle differentiation (clb) were identified. One of these mutants severely
affect chloroplast biogenesis, the clb19, is lethal and showed pale yellow seedling as it accumulates low levels of chlorophyll
and carotenoid pigments. The molecular characterization of this mutant demonstrated that the gene responsible for its phenotype,
CLB19, corresponds to a novel pentatricopeptide repeat (PPR) protein. This protein belongs to one of the largest gene families in
plants, identified by a particular motif known as the pentatricopeptide repeat whose particular functions are basically unknown.
The analysis of CLB19 has shown that this protein this precise function of this PPR protein has demonstrated its essential role
in the post-transcriptional regulation during chloroplast development, that explains its dramatic phenotype. Our analysis has
shown that CLB19 protein affects the expression of four chloroplast genes of the chloroplast genome. Our results demonstrate
that at least one of the roles of CLB19 relates to the proper processing of both the atpE-atpB and rbcL transcripts. In particular,
CLB19 is required for the efficient processing and translation of the catalytic subunit of the RbcL gene during a particular
plant developmental stage. Together this data supports the essential role that post-transcriptional events play in the chloroplast
biogenesis and exemplifies one of the most unique functions of this family of proteins.
120 Elucidation of the Molecular Role of SCD1 in Cytokinesis and Cell Elongation through the
Identification of Binding Partners
Colleen McMichael 1 , Tanya Falbel 2 , Lisa Koch 1 , Sebastian Bednarek 1
1
Department of Biochemistry, University of Wisconsin-Madison, 2 Department of Botany, University of
Wisconsin-Madison
The timing of cell division, orientation of the division plane, and direction and extent of cell expansion are fundamental
to plant morphology. Establishment of both the orientation of the division plane and direction of cell expansion are
regulated by polarized membrane trafficking and cytoskeletal dynamics (1). Arabidopsis plants carrying mutations
within the Stomatal Cytokinesis Defective 1 (SCD1) gene display defects in both cytokinesis and polar cell expansion,
indicating that the SCD1 protein is critical to both processes (2). Based on the phenotypes of scd1 mutants and the
conserved domains of the SCD1 protein, we predict that SCD1 plays a role in vesicular trafficking and/or cytoskeletal
dynamics that is critical for cytokinesis and cell expansion. A combined genetic and biochemical approach is underway
to define the molecular interactions of SCD1 in order to elucidate its function. Identification of the role of SCD1 will
provide insight into the molecular mechanisms that link plant cytokinesis and cell expansion.
(1) Bednarek SY & Falbel TG. (2002) Membrane Trafficking During Plant Cytokinesis. Traffic 3: 621-629.
(2) Falbel TG, Koch LM, Nadeau JA, Segui-Simarro JM, Sack FD & Bednarek SY. (2003) SCD1 is required for cytokinesis and polarized cell
expansion in Arabidopsis thaliana. Development 130: 4011-4024.