The Actinorhizal Symbiosis |
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Authors: | Luis Gabriel Wall |
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Institution: | Departamento de Ciencia y Tecnologia, Universidad Nacional de Quilmes, R. Sáenz Pe?a 180, B1876BXD Bernal, Argentina, AR
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Abstract: | Abstract
The term ``actinorhiza' refers both to the filamentous bacteria Frankia, an actinomycete, and to the root location of nitrogen-fixing nodules. Actinorhizal plants are classified into four subclasses,
eight families, and 25 genera comprising more than 220 species. Although ontogenically related to lateral roots, actinorhizal
nodules are characterized by differentially expressed genes, supporting the idea of the uniqueness of this new organ. Two
pathways for root infection have been described for compatible Frankia interactions: root hair infection or intercellular penetration. Molecular phylogeny groupings of host plants correlate with
morphologic and anatomic features of actinorhizal nodules. Four clades of actinorhizal plants have been defined, whereas Frankia bacteria are classified into three major phylogenetic groups. Although the phylogenies of the symbionts are not fully congruent,
a close relationship exists between plant and bacterial groups. A model for actinorhizal specificity is proposed that includes
different levels or degrees of specificity of host-symbiont interactions, from fully compatible to incompatible. Intermediate,
compatible, but delayed or limited interactions are also discussed. Actinorhizal plants undergo feedback regulation of symbiosis
involving at least two different and consecutive signals that lead to a mechanism controlling root nodulation. These signals
mediate the opening or closing of the window of susceptibility for infection and inhibit infection and nodule development
in the growing root, independently of infection mechanism. The requirement for at least two molecular recognition steps in
the development of actinorhizal symbioses is discussed. |
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Keywords: | :Frankia Root nodules N2 fixation Actinorhiza Plant microbe interaction |
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