RESUMEN
The proper balance of parental genomic contributions to the fertilized embryo and endosperm is essential for their normal growth and development. The characterization of many gametophytic maternal effect (GME) mutants affecting seed development indicates that there are certain classes of genes with a predominant maternal contribution. We present a detailed analysis of the GME mutant zak ixik (zix), which displays delayed and arrested growth at the earliest stages of embryo and endosperm development. ZIX encodes an Armadillo repeat (Arm) protein highly conserved across eukaryotes. Expression studies revealed that ZIX manifests a GME through preferential maternal expression in the early embryo and endosperm. This parent-of-origin-dependent expression is regulated by neither the histone and DNA methylation nor the DNA demethylation pathways known to regulate some other GME mutants. The ZIX protein is localized in the cytoplasm and nucleus of cells in reproductive tissues and actively dividing root zones. The maternal ZIX allele is required for the maternal expression of miniseed3. Collectively, our results reveal a reproductive function of plant Arm proteins in promoting early seed growth, which is achieved through a distinct GME of ZIX that involves mechanisms for maternal allele-specific expression that are independent of the well-established pathways.
Asunto(s)
Proteínas de Arabidopsis/fisiología , Arabidopsis/crecimiento & desarrollo , Proteínas del Dominio Armadillo/fisiología , Semillas/crecimiento & desarrollo , Arabidopsis/genética , Arabidopsis/metabolismo , Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Proteínas del Dominio Armadillo/genética , Proteínas del Dominio Armadillo/metabolismo , Núcleo Celular/metabolismo , Citoplasma/metabolismo , Endospermo/genética , Endospermo/crecimiento & desarrollo , Endospermo/metabolismo , Regulación de la Expresión Génica de las Plantas , Raíces de Plantas/metabolismo , Semillas/genética , Semillas/metabolismo , Factores de Transcripción/genética , Factores de Transcripción/metabolismoRESUMEN
The eukaryotic origin recognition complex (ORC) is made up of six subunits and functions in nuclear DNA replication, chromatin structure, and gene silencing in both fungi and metazoans. We demonstrate that disruption of a plant ORC subunit homolog, AtORC2 of Arabidopsis (Arabidopsis thaliana), causes a zygotic lethal mutant phenotype (orc2). Seeds of orc2 abort early, typically producing embryos with up to eight cells. Nuclear division in the endosperm is arrested at an earlier developmental stage: only approximately four nuclei are detected in orc2 endosperm. The endosperm nuclei in orc2 are dramatically enlarged, a phenotype that is most similar to class B titan mutants, which include mutants in structural maintenance of chromosomes (SMC) cohesins. The highest levels of ORC2 gene expression were found in preglobular embryos, coinciding with the stage at which homozygous orc2 mutant seeds arrest. The homologs of the other five Arabidopsis ORC subunits are also expressed at this developmental stage. The orc2 mutant phenotype is partly suppressed by a mutation in the Polycomb group gene MEDEA. In double mutants between orc2 and medea (mea), orc2 homozygotes arrest later with a phenotype intermediate between those of mea and orc2 single mutants. Either alterations in chromatin structure or the release of cell cycle checkpoints by the mea mutation may allow more cell and nuclear divisions to occur in orc2 homozygous seeds.
Asunto(s)
Proteínas de Arabidopsis/genética , Arabidopsis/genética , Proteínas de Unión al ADN/genética , Genes de Plantas , Arabidopsis/citología , Arabidopsis/crecimiento & desarrollo , Proteínas de Arabidopsis/química , Secuencia de Bases , Ciclo Celular/genética , Cromatina/genética , ADN de Plantas/genética , Proteínas de Unión al ADN/química , Prueba de Complementación Genética , Datos de Secuencia Molecular , Mutación , Complejo de Reconocimiento del Origen , Fenotipo , Subunidades de Proteína , Semillas/crecimiento & desarrollo , Supresión GenéticaRESUMEN
SUMMARY Suppression Subtractive Hybridization (SSH) was used to search for genes of Phytophthora infestans that are induced during the infection of potato. To avoid having to distinguish the genes of the pathogen from the plant genes involved in defence responses and to isolate the genes involved in the early stages of interaction, mycelium of P. infestans was induced by contact with the host plant and then separated from the plant tissue. A differential cDNA library was generated by SSH that compared such induced mycelium with mycelium incubated in water. The expression of about 100 cDNA fragments from this differential cDNA library was analysed by hybridization of the arrayed PCR products with mRNA from control and induced mycelium. Twenty per cent of them showed increased transcript levels in mycelium within the first 24 h after exposure to a potato leaf. For six of these cDNA clones the elevated expression in response to the potato leaf could be proven by RNA gel blot analysis. Five of these cDNA clones have predicted amino acid sequence homologies to entries in the databases, including an amino acid transporter, a sucrose transporter, a spliceosome-associated factor, an ABC transporter, and a cell division control protein. We showed that the genes corresponding to these six cDNA clones are differentially regulated during their life. Reliable gene expression analysis of Phytophthora in infected leaf tissue is not possible until c. 48 h post-infection, but for two of the genes we identified, induction during in planta growth was detectable by RNA gel blot analysis. Therefore the SSH library that we have created provides a basis for the identification of P. infestans genes that are up-regulated during the interaction with the plant, which could be important for pathogenicity.
