Fasciation

Abstract: The shoot apical meristem of dicotyledonous plants is highly regulated both structurally and functionally, but little is known about the mechanisms involved in this regulation. Here we describe the genetic and phenotypic characterisation of recessive mutations at three loci of Arabidopsis thaliana in which meristem structure and function are disrupted. The loci are Clavata1 (Clv1), Fasciata1 (Fas1) and Fasciata2 (Fas2). Plants mutant at these loci are fasciated having broad, flat stems and disrupted phyllotaxy. In all cases, the fasciations are associated with shoot apical meristem enlargement and altered floral development. While all the mutants share some phenotypic features they can be divided into two classes. The pleiotropic fas1 and fas2 mutants are unable to initiate wild- type organs, show major alterations in meristem structure and have reduced root growth. In contrast, clv1 mutant plants show near wild-type organ phenotypes, more subtle changes in shoot apical meristem structure and wild-type root growth.

Abstract: Rhodococcus fascians is a nocardiform bacteria that induces leafy galls (fasciation) on dicotyledonous and several monocotyledonous plants The wild-type strain D188 contained a conjugative, 200 kb linear extrachromosomal element, pFiD188 Linear plasmid-cured strains were avirulent and reintroduction of this linear element restored virulence Pulsed field electrophoresis indicated that the chromosome might also be a linear molecule of 4 megabases Three loci involved in phytopathogenicity have been identified by insertion mutagenesis of this Fi plasmid Inactivation of the fas locus resulted in avirulent strains, whereas insertions in the two other loci affected the degree of virulence, yielding attenuated (att) and hypervirulent (hyp) bacteria One of the genes within the fas locus encoded an isopentenyltranferase (IPT) with low homology to analogous proteins from Gram-negative phytopathogenic bacteria IPT activity was detected after expression of this protein in Escherichia coli cells In Rfascians, ipt expression could only be detected in bacteria induced with extracts from fasciated tissue Rfascians strains without the linear plasmid but containing this fas locus alone could not provoke any phenotype on plants, indicating additional genes from the linear plasmid were also essential for virulence These studies, the first genetic analysis of the interaction of a Gram-positive bacterium with plants, suggest that a novel mechanism for plant tumour induction has evolved in Rfascians independently from the other branches of the eubacteria

Abstract: Three virulence loci (fas, att, and hyp) of Rhodococcus fascians D188 have been identified on a 200-kb conjugative linear plasmid (pFiD188). The fas locus was delimited to a 6.5-kb DNA fragment by insertion mutagenesis, single homologous disruptive recombination, and in trans complementation of different avirulent insertion mutants. The locus is arranged as a large operon containing six open reading frames whose expression is specifically induced during the interaction with host plants. One predicted protein is homologous to P-450 cytochromes from actinomycetes. The putative ferredoxin component is of a novel type containing additional domains homologous to transketolases from chemoautotrophic, photosynthetic, and methylotrophic microorganisms. Genetic analysis revealed that fas encodes, in addition to the previously identified ipt, at least two new genes that are involved in fasciation development, one of which is only required on older tobacco plants.

Abstract: In plant meristems, each cell divides and differentiates in a spatially and temporally regulated manner, and continuous organogenesis occurs using cells derived from the meristem. We report the identification of theArabidopsis thaliana TEBICHI (TEB)gene, whichisrequiredforregulated celldivision anddifferentiation in meristems. Thetebmutants show morphological defects, such as short roots, serrated leaves, and fasciation, as well as defective patterns of cell division and differentiation in the meristem. The TEB gene encodes a homolog of Drosophila MUS308 and mammalian DNA polymerase u, which prevent spontaneous or DNA damage‐induced production of DNA double strand breaks. As expected from the function of animal homologs, teb mutants show constitutively activated DNA damage responses. Unlike other fasciation mutants with activated DNA damage responses, however, teb mutants do not activate transcriptionally silenced genes. teb shows an accumulation of cells expressing cyclinB1;1:GUS in meristems, suggesting that constitutively activated DNA damage responses in teb lead to a defect in G2/M cell cycle progression. Furthermore, other fasciation mutants, such as fasciata2 and tonsoku/mgoun3/ brushy1, also show an accumulation of cells expressing cyclinB1;1:GUS in meristems. These results suggest that cell cycle progression at G2/M is important for the regulation of the pattern of cell division and of differentiation during plant