Transcriptional profiling of embryogenic callus produced from Medicago truncatula mesophyll protoplasts indicated upregulation of ethylene biosynthesis and ethylene response genes. Using inhibitors of ethylene biosynthesis and perception, it was shown that ethylene was necessary for somatic embryogenesis (SE) in this model legume. We chose several genes involved in ethylene biosynthesis and response for subsequent molecular analyses. One of these genes is a gene encoding a transcription factor that belongs to the AP2/ERF superfamily and ERF subfamily of transcription factors. We demonstrate that this gene, designated M. truncatula SOMATIC EMBRYO RELATED FACTOR1 (MtSERF1), is induced by ethylene and is expressed in embryogenic calli. MtSERF1 is strongly expressed in the globular somatic embryo and there is high expression in a small group of cells in the developing shoot meristem of the heart-stage embryo. RNA interference knockdown of this gene causes strong inhibition of SE. We also provide evidence that MtSERF1 is expressed in zygotic embryos. MtSERF1 appears to be essential for SE and may enable a connection between stress and development.
Mitochondrial small subunit (mtSSU) rDNA sequences elucidated phylogenetic relationships in Neonectria Wollenw. (anamorphs = Cylindrocarpon Wollenw.; Ascomycetes, Hypocreales). Twelve isolates representing seven species in five taxonomically informal groups of Neonectria were subjected to phylogenetic analysis. Fusarium inflexum R. Schneid. (teleomorph: Gibberella) and Nectria cinnabarina (Fr.) Fr. (= Nectria s.str.) were outgroups. All of the Neonectria species formed a strongly supported clade with respect to the outgroups, indicating a single ascomycete genus for the holomorphs of Cylindrocarpon. Within the Neonectria clade there were three well-supported subclades that only partially corresponded to phenotype-defined groups. DNA sequence divergence among the twelve Neonectria isolates, 2.3-7.4%, was sufficient to resolve them. The results suggest that the mtSSU rDNA region is appropriate for phylogenetic analysis of Neonectria and Cylindrocarpon. The following new combinations are proposed: Neonectria coronata, Neonectria discophora, Neonectria neomacrospora, Neonectria radicicola, Neonectria rugulosa, Neonectria veuillotiana.Résumé : Les séquences de la petite sous-unité du rADN mitochondrial (mtSSU) permettent de définir les relations phylogénétiques chez les Neonectria (anamorphe = Cylindrocarpon Wollenw.; Ascomycètes, Hypocréales). Les auteurs ont soumis 12 isolats, représentant sept espèces au sein de 5 groupes taxonomiquement informels du genre Neonectria, à l'analyse phylogénétique. Le Fusarium inflexum R. Schneid. (télémorphe : Gibberella) et le Nectria cinnabarina (Fr.) Fr. (= Nectria s.str.) sont des groupes étrangers. Toutes les espèces de Neonectria forment un clade fortement supporté par rapport aux groupes étrangers, ce qui suggère l'existence d'un genre ascomycète unique pour les holomorphes associés au Cylindrocarpon. Au sein du clade Neonectria, on retrouve trois sous-clades bien supportés qui ne correspondent que partiellement aux groupes définis par leurs phénotypes. La divergence de la séquence ADN parmi les 12 isolats du genre Neonectria étant de 2,3 à 7,4%, est suffisante pour toutes les résoudre. Les résultats suggèrent que la région mtSSU du rADN convient à l'analyse phylogénétique des Neonectria et Cylindrocarpon.
Somatic embryogenesis (SE) is a remarkable developmental process enabling nonzygotic plant cells to form embryos and ultimately, fertile plants. It is an expression of totipotency. This chapter initially considers the genotypic component and the progenitor stem cells where SE is induced to form the initial asymmetric division of the somatic embryogenesis program. These cells are part of a stem cell niche dependent on the surrounding cells. Recent evidence is discussed that before the SE pathway can be initiated a GA-modulated pathway that represses inappropriate embryogenesis needs to be derepressed. The current understanding of how stress and hormones can induce the activation of specific SE genes is examined. Important stress components are reactive oxygen species and the signalling of stress-related hormones. The action of the key developmental hormone auxin, and also cytokinin, in relation to developmental genes is considered and, based on current understanding, a model is presented for the mechanism of SE. While there are many SE applications in contemporary biotechnology, understanding the reprogramming process associated with SE remains an important question for developmental biology.
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