Andrija Finka, Jean-Pierre Zryd and Didier G. Schaefer

Laboratory of Plant Cell Genetics, Institute of Ecology, University of Lausanne, CH-1015 Lausanne-Dorigny, Switzerland

Loss of function of the Arp3 gene has a pleiotropic effect on cell elongation and differentiation in the moss Physcomitrella patens

The actin cytoskeleton is an essential effector of signal transduction networks controlling cellular differentiation and development in eukaryotes. In yeast, protozoa and animals, actin dynamics is tightly controlled by the Arp2/3 complex, a complex formed by two actin-related proteins, Arp2 and Arp3 and 5 other subunits. This complex is an actin nucleating factor and choreographs the formation of branched actin networks. Little is known on the role of the Arp2/3 complex in plants but homologues to the 7 subunits of the complex have been identified in the genome of Arabidopsis. The moss Physcomitrella patens has a single Arp3 gene with genomic organization similar to the Arabidopsis Arp3 gene (9 exons). It encodes a predicted protein of 424 amino acids that shares 76% and 58% identity with the Arabidopsis and fission yeast Arp3 protein, respectively. Loss of function mutants were generated by replacing exons 4 to 8 with a hygromycin resistance cassette. The Arp3 moss mutants displayed a pleiotropic developmental phenotype characterized by the following features; (1) strong reduction of chloronema elongation (length to width ratio of 1.2 in mutants and 7 in WT), (2) absence of caulonema and rhizoides whose differentiation could not be restored by auxin treatment, (3) differentiation of meristematic buds from chloronema instead of caulonema, (4) reduced elongation of leafy shoots resulting in stunted gametophores, (5) enhanced differentiation of axillary meristems and (6) sterility. Other stages of the moss physiology and development were not significantly altered. To further investigate the relation between these phenotypes and the actin cytoskeleton, the Arp3 mutation was generated in moss strains expressing GFP-talin and displaying in vivo labeled actin cables. The actin cables were significantly disorganized in Arp3 mutants as compared to wild-type thereby establishing a direct connection between the observed phenotypes and correct organization of the actin cytoskeleton.

Our data support a complex involvement of the Arp2/3 complex in cell elongation and differentiation at several developmental stages of the moss life cycle and are comparable to the results presented in two recent studies demonstrating that mutations in the Arp2 and Arp3 genes affects cell shape development in Arabidopsis (Mathur et al. 2003, Plant Cell 15 and Le et al. 2003, Current Biology, online July 3).