Minisymposium 23: Cytoskeleton

Abs # 37003: The ""Arabidopsis"" actin-related proteins AtARP4 and AtARP7 are essential for embryo development

Presenter:	Kandasamy, Muthugapatti K, kandu@arches.uga.edu
Authors	Kandasamy, Muthugapatti K (A)   Deal, Roger B (A)   McKinney, Elizabeth C (A)   Meagher, Richard B (A)
Affiliations:	(A): University of Georgia

Actin-related proteins (ARPs), which share about 20 to 60% sequence homology and the basal structure with conventional actins, are found in a wide variety of organisms, including plants. Biochemical, genetic and functional studies in yeast, fruit fly and yeast suggest that ARPs participate in a diverse array of cellular processes including modulation of actin assembly, microtubule-based motility of vesicles, and regulation of chromatin structure and function. The model plant ""Arabidopsis"" encodes at least eight ARPs most of which have orthologs in other distant organisms. To gain insight in to the role of these ARPs in plants, we have examined the spatial expression, subcellular distribution and the genetics of some highly divergent ""Arabidopsis"" ARPs. We found that AtARP4, a homolog of human BAF53 and yeast Arp4, and AtARP7, a novel plant-specific ARP, are localized to the nucleus in a cell cycle dependent manner. In the interphase cells, both AtARP4 and AtARP7 proteins are concentrated in the nucleus, whereas in mitotic cells lacking a nuclear envelope (e.g. metaphase, anaphase and early telophase stages) they are excluded from the chromosomes and dispersed throughout the cytoplasm in all the organs and tissues. To further explore the role of these two ubiquitously expressed ARPs in plants, we have isolated T-DNA insertion mutants and generated RNAi suppression lines that are partially or completely defective in the expression of these proteins. Phenotypic and genetic analyses of ""ARP4-3"" and ""ARP7-1"" mutants, which have T-DNA insertion in the first intron and exon respectively, revealed aberrant embryo development and homozygous lethality. Immunocytochemical analysis showed that the nuclei of aborting homozygous mutant embryos did not stain positively for the corresponding actin-related protein. Partial reduction in the level of AtARP4 protein in the ""ARP4-1"" T-DNA mutant and RNAi lines resulted in male and female sterility and severe defects in flower and silique development. We suggest that both AtARP4 and AtARP7 are essential for normal embryo morphogenesis and plant development. Based on our results and data on the ortholog of AtARP4 in yeast and other organisms, we are considering possible roles for these two ARPs in chromatin remodeling-related transcriptional regulation in plants.